yacht hull outline

Definitions

Length overall (LOA)

Length of water line (lwl)

Length between perpendiculars (LFF)

Rated length

he hull of a yacht is a complex three-dimensional shape, which cannot be defined by any simple mathematical expression. Gross features of the hull can be described by dimensional quantities such as length, beam and draft, or non-dimensional ones like prismatic coefficient or slenderness (length/displacement) ratio. For an accurate definition of the hull the traditional lines drawing; is still a common tool, although most professional yacht designers now take advantage of the rapid developments in CAD introduced in Chapter 1.

In this chapter we start by defining a number of quantities, frequently referred to in yachting literature, describing the general features of the yacht. Thereafter, we will explain the principles of the traditional drawing and the tools required to produce it. We recommend a certain work plan for the accurate production of the drawings and, finally, we show briefly how the hull lines are generated in a modern CAD program.

The list of definitions below includes the basic geometrical quantities used in defining a yacht hull. Many more quantities are used in general ship hydrodynamics, but they arc not usually referred to in the yachting field. A complete list may be found in the International Towing Tank Conference (ITTC) Dictionary of Ship Hydrodynamics.

The maximum length of the hull from the forwardmost point on the stem to the extreme after end (see Fig 3.1). According to common practice, spars or fittings, like bowsprits, pulpits etc are not included and neither is the rudder.

The length of the designed waterline (often referred to as the DWL).

This length is not much used in yachting but is quite important for ships. The forward perpendicular (FP) is the forward end of the designed waterline, while the aft perpendicular (AP) is the centre of the rudder stock.

The single most important parameter in any rating rule. Usually L is obtained by considering the fullness of the bow and stern sections in a more or less complex way.

The maximum beam of the hull excluding fittings, like rubbing strakes.

Fig 3.1 Definitions of the main dimensions

Beam of waterline (bwl)

Displacement

The maximum beam at the designed waterline.

The maximum draft of the yacht when floating on the designed waterline. Tc is the draft of the hull without the keel (the 'canoe' body).

The vertical distance from the deepest point of the keel to the sheer line (see below). Dc is without the keel.

Could be either mass displacement (m) ie the mass of the yacht, or volume displacement (V or V), the volume of the immersed part of the yacht. mc, Vc and Vc are the corresponding notations without the keel.

Midship section For ships, this section is located midway between the fore and aft perpendiculars. For yachts it is more common to put it midway between the fore and aft ends of the waterline. The area of the midship section (submerged part) is denoted AM, with an index 'c' indicating that the keel is not included.

Maximum area section For yachts the maximum area section is usually located behind the midship section. Its area is denoted Ax (AXc).

Prismatic coefficient This is the ratio of the volume displacement and the maximum section (CP) area multiplied by the waterline length, ie CP = V/(AX • Lwl). This value is very much influenced by the keel and in most yacht applications only the canoe body is considered: CPc = Vc(AXc • Lwl). See Fig 3.2. The prismatic coefficient is representative of the fullness of the yacht. The

Circumscribed cylinder volume = v = L^ Ay

Fig 3.2 The prismatic coefficient

BOX WL WL c

Circumscribed box volume =

Fig 3.3 The block coefficient

Block coefficient ( CB)

Centre of buoyancy (B)

Centre of gravity (G)

Freeboard fuller the ends, the larger the Cp. Its optimum value depends on the speed, as explained in Chapter 5.

Although quite important in general ship hydrodynamics this coefficient is not so commonly used in yacht design . The volume displacement is now divided by the volume of a circumscribed block (only the canoe body value is of any relevance) CBc = V J(Lwl • BWL • Tc). See Fig 3.3.

The centre of gravity of the displaced volume of water, its longitudinal and vertical positions are denoted by LCB and VCB respectively.

The centre of gravity of the yacht must be on the same vertical line as the centre of buoyancy. In drawings G is often marked with a special symbol created by a circle and a cross. This is used also for marking geometric centres of gravity. See. for instance, Figs 5.27 or 8.2.

The intersection between the deck and the topside. Traditionally, the projection of this line on the symmetry plane is concave, the 'sheer* is positive. Zero and negative sheer may be found on some extreme racing yachts and powerboats.

The vertical distance between the sheer line and the waterline.

Tumble home

When the maximum beam is below the sheer line the upper part of the topsides will bend inwards (see Fig 3.4). To some extent this reduces the weight at deck level, but it also reduces the righting moment of the

Fig 3.4 Definition of tumble home and flare

Tumble home crew on the windward rail. Further, the hull becomes more vulnerable to outer skin damage in harbours.

Flare The opposite of tumble home. On the forebody in particular, the sections may bend outwards to reduce excessive pitching of the yacht and to keep it more dry when beating to windward.

Scale factor (a) This is not a geometrical parameter of the hull, but it is very important when designing a yacht. The scale factor is simply the ratio of a length (for instance the Lw,) at full scale to the corresponding length at model scale. Note that the ratio of corresponding areas (like the wetted area) is a2 and of corresponding volumes (like displacement) a3.

Lines drawing A complete lines drawing of the YD 40 is presented in Fig 3.5. The hull is shown in three views: the profile plan (top left), the body plan (top right) and half breadth plan (bottom). Note that the bow is to the right.

In principle, the hull can be defined by its intersection with two different families of planes, and these are usually taken as horizontal ones (waterlines) and vertical ones at right angles to the longitudinal axis of the hull (sections). While the number of waterlines is chosen rather arbitrarily, there are standard rules for the positioning of the sections. In yacht architecture the designed waterline is usually divided into ten equal parts and the corresponding sections are numbered from the forward perpendicular (section 0) backwards. At the ends, other equidistant sections, like # 11 and # 1 may be added, and to define rapid changes in the geometry, half or quarter sections may be introduced as well. In Fig 3.5 half sections are used throughout.

The profile is very important for the appearance of the yacht, showing the shapes of the bow and stern and the sheer line. When drawing the waterlines, displayed in the half breadth plan, it is most helpful if the lines end in a geometrically well defined way. Therefore a 'ghost" stem and a 'ghost' transom may be added. The ghost stem is the imagined sharp leading edge of the hull, which in practice often has a rounded stem, and the ghost transom is introduced because the real transom is often curved and inclined. If an imagined vertical transom is put near the real one at some convenient station, it will facilitate the fairing of the lines. The drawing of Fig 3.5 has been produced on a CAD system and no ghost stem is shown. However, a ghost transom is included.

In the body plan, the cross sections of the hull are displayed. Since the hull is usually symmetrical port and starboard, only one half needs to be shown, and this makes it possible to present the forebody to the right and the afterbody to the left. In this way mixing of the lines is avoided and the picture is clearer. Note that in the figure the half stations are drawn using thinner lines.

The above cuts through the hull are sufficient for defining the shape, but another two families of cuts are usually added, to aid in the visual perception of the body. Buttocks are introduced in the profile plan,

* * ^ "i * 2 § 2 II II II II II II II ll II

showing vertical, longitudinal cuts through the hull at positions indicated in the half breadth plan. The diagonals in the lower part of the half breadth plan are also quite important. They are obtained by cutting the hull longitudinally in different inclined planes, as indicated in the body plan. The planes should be as much as possible at right angles to the surface of the hull, thus representing its longitudinal smoothness. In practice, the flow tends to follow the diagonals, at least approximately, so that they are representative of the hull shape as "seen' by the water. Special attention should be paid to the after end of the diagonals, where knuckles, not noticcd in the other cuts, may be found, particularly on lOR yachts from the 1970s and the 1980s. Almost certainly, such unevenncss increases the resistance and reduces the speed of the yacht.

The other line in the lower part of the half breadth plan is the curve of sectional areas, representing the longitudinal distribution of the submerged volume of the yacht. The value at each section is proportional to the submerged area of that section, while the total area under the curve represents the displacement (volume). A more detailed description of the construction of the curve of sectional areas will be given in Chapter 4.

In order to define exactly the shape of the hull a table of offsets is usually provided by the designer. This is to enable the builder to lay out the lines at full size and produce his templates. Offsets are always provided for the waterlines, but the same information may be given for diagonals and/or buttocks also. Note that all measurements are to the outside of the shell.

The drawing should be made on a special plastic film, available in different thicknesses. The film is robust and will not be damaged by

Photo 3.6 Tools (triangle, plastic film, straight edge, brush, pens, pencil, erasing shield and eraser)

Photo 3.7 Tr¿\nster of measures from body plan (top) to half breadth plan (bottom) using a paper ribbon

erasing. Furthermore, it is unaffected by the humidity of the air. which may shrink ordinary paper.

Since the film is transparent the grid for the lines drawing is drawn on the back so that it will remain, even after erasing the hull lines on the front many times. Great care must be exercised when drawing the grid, making sure that the alignment and spacing are correct and that all angles arc cxactly 90°. In Fig 3.5 the grid is shown as thin horizontal and vertical lines, representing waterlines, buttocks and stations.

Black ink should be used when drawing the grid and preferably when finishing the hull lines also. However, when working on the lines a pencil and an eraser are needed. There are, in fact, special pencils and erasers for this type of work on plastic film. An erasing shield and a brush are also most useful (see Photo 3.6).

For creating the grid a long straight edge is required, together with a

Photo 3.8 Ducks and a spline used for drawing a water Iine

Photo 3.9 Templates used for drawing lines with large curvature

large 90° set square. It is very convenient to have a bunch of paper ribbons, which can be used for transferring different measures from one plan to the other. For example, when drawing a waterline the offsets of this line may be marked on the ribbon directly from the body plan and moved to the half breadth plan (Photo 3.7).

To draw the hull lines it is necessary to have a set of splines and weights or ducks. Long, smooth arcs can be created when bending the splines and supporting them by the ducks at certain intervals. Photo 3.8 shows how these tools are used when drawing a waterline. The splines should be made of plastic, somewhat longer than the hull on the drawing, and with a cross-section of about 2.5 mm2. Many different types of ducks can be found, some of them home made. Preferably,

Photo 3.10 PI an i meter they should be made of lead, and the weight should be between 1.5 and 2.5 kg. To be able to support the spline, they should have a pointed nose, as shown in Photo 3.8.

The splines are needed when drawing the lines in the profile and half breadth plans. However, the lines of the body plan are usually too curved for the splines, so it is necessary to make use of a set of templates especially developed for this purpose. The most well known ones are the so called Copenhagen ship curves, the most frequently used of which are shown in Photo 3.9.

A very convenient instrument, well known in naval architecture, is the planimeter, used for measuring areas (see Photo 3.10). The pointer of the planimeter is moved around the area to be measured, and the change in the reading of the scale when returning to the point of departure gives the area enclosed by the path followed. Considering the difficulty in following exactly any given line the accuracy is surprisingly high, more than adequate for the present purposes. The need for measuring areas will be explained in the next chapter.

Since many calculations have to be carried out when preparing the drawings, and indeed in the whole design process, an electronic calculator is essential. A simple one would be sufficient in most cases, but a programmable calculator would simplify some of the calculations, particularly if a planimeter is not available. Most scientific calculators have programs for calculating areas with acceptable accuracy, and programs are available for most of the calculations described in the next chapter.

Designing the hull is a complex process, and many requirements have to be considered. One difficulty is that important parameters, such as the displacement cannot be determined until the lines have been fixed. This calls for an iterative method. Such a method is also required in the fairing of the lines. The problem is to make the lines in one projection correspond to smooth lines in the other two projections. For an inexperienced draftsman this problem is a serious one, and many trials may be needed to produce a smooth hull.

While the preferred sequence of operations may differ slightly between yacht designers the main steps should be taken in a certain order. In the following, we propose a work plan, which has been found effective in many cases. It should be pointed out that the plan does not take into account any restrictions from measurement rules.

Step 1: Fix the main dimensions These should be based on the general considerations discussed in Chapter 2, using information on other yachts of a similar size, designed for similar purposes. This way of working is classical in naval architecture, where the development proceeds relatively slowly by evolution of previous designs. It is therefore very important, after deciding on the size of the yacht, to find as much information as possible on other similar designs. Drawings of new yachts may be found in many of the leading yachting magazines from all over the world.

The dimensions to fix at this stage are: length overall, length of the waterline, maximum beam, draft, displacement, sail area, ballast ratio, prismatic, coefficient and longitudinal centre of buoyancy. One of the aims of this book is to help in the choice of these parameters and to enable the reader to evaluate older designs when trying to find the optimum for his own special demands.

Step 2: Draw the profile As pointed out above, this step takes much consideration, since the aesthetics of the yacht are, to a large extent, determined by tBfe pi^ffle-

Step 3: Draw the midship section The midship section can be drawn at this stage, or, alternatively, the maximum section if it is supposed to be much different. This may occur if the centre of buoyancy is far aft. The shape of the first section drawn is important, since it determines the character of the other sections.

Step 4: Check the displacement To find the hull displacement calculate (or measure) the submerged area of the section just drawn and multiply by the waterline length and the prismatic coefficient chosen for the hull. From the ballast ratio, the keel mass can be computed and the volume can be found, dividing by the density of the material (about 7200 kg/m3 for iron and 11300 kg/m- for lead). Assume that the rudder displacement is 10% of that of the keel and add all three volumes. If the displacement thus obtained is different from the prescribed one, return to step 3 and change accordingly.

The procedure described is for a fin-keel yacht. For a hull with an integrated keel, as on more traditional yachts, the prismatic coefficient usually includes both the keel and the rudder.

Step 5: Draw the designed waterline One point at or near the midship station is now known, together with the two end points from the profile, so now a first attempt can be made to draw the designed waterline.

Step 6: Draw stations 3, 7 and the transom The waterline breadth is now known, as well as the hull draft, and the sections should have a family

resemblance to the midship section. Often it is helpful to draw a ghost transom behind the hull.

Step 7: Draw new waterlines Two or three waterlines can now be drawn above and below the DWL. If the appearance is not satisfactory, go back to step 6 and change.

Steps 8 and 9: Add new sections and waterlines

Once this is done, sections I-9 should be completed as well as 7-10 waterlines. Constant adjustments, have to be made in order to create smooth lines in the body plan, as well as in the half breadth plan.

Step 10: Recheck the displacement and the longitudinal centre of buoyancy The curve of sec tional areas can now be constructed. Its area gives the displacement (excluding that of keel and rudder) and its centre of gravity corresponds to the longitudinal position of the centre of buoyancy. If not correct, adjustments have to be made from steps 5 or 6,

Step 11: Draw diagonals Inspect the smoothness, particularly near the stern. Adjust if necessary.

Step 12: Draw buttocks This is the final check on the smoothness. Usually only very minor corrections have to be made at this stage.

Computer aided design of hulls

As mentioned in Chapter 1, most CAD programs use master curves for generating the hull surface. Each curve is defined by a number of points, called vertices. Photo 3.11 shows, in a plan view, the grid of master curves used for generating the YD-40 hull. One of the transverse curves has been selected in Photo 3.12 and it can be seen how the smooth hull surface is generated inside the curve, which is shown as piece-wise linear between the vertices.

Photo 3.11 Grid of master curves used for the YD-40 (the vertical line to the right marks the origin of the coordinate system)

Photo 3.12 A section with vertices (crosses), master curve (between the crosses), hull surface and cuwature (outermost line)

The task of the designer is to specify the vertices in such a way that the desired hull shape is created.There are different ways of achieving this. Some programs start from a long cylindrical body or a box, while others start from a flat rectangular patch, defined by an orthogonal grid. These original shapes are then distorted by moving the vertices around, and it is relatively easy to produce a yacht-like body. However, it takes experience and experimentation to obtain a shape that satisfies criteria set up beforehand. In practice, designers very seldom start from scratch, but work from earlier designs, which already have a desirable shape and a known grid of master curves surrounding it. Since most new designs are evolutions of previous ones this approach is very natural.

A problem encountered when the first CAD programs for yachts appeared was that the scale on the screen was too small, and the resolution too low to enable the designer to create fair lines. Small bumps on the surface could not be detected 011 the screen, and it sometimes happened that the bumps were noticed only after the start of the hull construction . Therefore the CAD program developers introduced plots of the curvature of lines on the hull. Such a plot is shown.in Photo 3.12. The curvature of the line, which essentially corresponds to a section, is almost constant, except at the ends where it goes to zero.

Photo 3.13 illustrates the sensitivity of the curvature to small changes of the surface. The sheer line is shown in a plan view. In the top photo (the real design) the curvature is smooth and relatively constant along the hull. In the bottom photo one vertex point has been moved 10 mm at full scale perpendicular to the surface. The resulting change in the sheer line is so small that it cannot be detected by eye, but the curvature exhibits a considerable bump and some smaller fluctuations, showing that the line is not smooth. By looking at the curvature, lines may thus be generated that look fair even at full scale.

Photo 3.13 Sheer line with vertices and curvature. (top) Real design. (bottom) One vertex point moved 10 mm

Photo 3,14 Perspective view A great advantage of most CAD programs is that the hull may be of the YD-40 shown in perspective. As pointed out in Chapter 1 it is important to study the sheer line in particular from different angles, since the impression of the hull contour in reality is also influenced by the beam distribution, which is not visible if only the profile view is studied. Fig 3.14 shows the YD-40 in perspective, and a good impression can be obtained of the shape. "

By using a CAD program a fair hull can be produced rapidly and different requirements may be satisfied without too much work, such as a given prismatic coefficient or longitudinal centre of buoyancy. Meeting such requirements accurately in a manual process is extremely time consuming, so it is understandable that CAD techniques are always used nowadays by professional designers. However, due to the considerable cost of a CAD system, most amateur designers will still have to use the manual approach described above.

Continue reading here: Hydrostatics And Stability

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Readers' Questions

What do you understand by drawing defines the shapes of the hull.?

How to figure the width to height to length of a yacht?

To figure out the width, height, and length of a yacht, you typically need to refer to the yacht's specifications provided by the manufacturer, yacht designer, or owner. These specifications should include the appropriate measurements. Consult the yacht's specifications: Look for the official documentation or technical information provided for the yacht. This documentation usually includes the length, width, and height of the yacht, referred to as LOA (Length Overall), Beam, and Draft, respectively. The specifications are usually available in brochures, user manuals, or on the official website of the yacht manufacturer. Seek professional advice: If you cannot find the specifications yourself or need more specific information, consider reaching out to yacht brokers, yacht builders, naval architects, or other professionals in the yachting industry. They have extensive knowledge and can guide you with accurate measurements or provide information by using the yacht's model or brand. Measure the yacht yourself: If you have physical access to the yacht and cannot find the specifications through other means, you can measure it directly. However, this method is less accurate and should only be used as a last resort. Use a measuring tape or other appropriate tools to measure the overall length, width or beam, and height. Ensure to measure from fixed reference points for consistency and accuracy. Remember that yachts come in various sizes, designs, and layouts. The width or beam, for example, may be different at different points along the vessel's length due to design variations. It is essential to refer to the official specifications or seek professional advice for the most precise and reliable measurements.

Can you use geometry on boats?

Yes, geometry can be applied to various aspects of boats, particularly in the design and construction phase. Here are a few examples: Hull Design: Geometry is crucial in designing the shape and dimensions of a boat's hull. The angles, curves, and mathematical calculations are used to ensure stability, hydrodynamics, and buoyancy. Stability Analysis: Geometry is used to determine the center of buoyancy, center of gravity, and metacenter of a vessel. These calculations are essential for assessing a boat's stability, both at rest and in motion. Navigation and Bearings: Geometric concepts such as angles, triangles, and trigonometry are used to calculate headings, course corrections, and bearings while navigating a boat. Sail Measurement and Adjustments: Sailboats utilize various geometric principles to determine sail sizes, aspect ratios, and shapes. The geometry of sail adjustments, such as tightening or loosening the sail, can affect the boat's speed and performance. Nautical Charts: Geometry plays a vital role in nautical charting, which involves representing the Earth's curved surface on a flat chart. Projections, grid systems, and coordinate systems are employed to accurately depict and navigate waterways. These are just a few examples of how geometry can be applied to boats. Overall, geometry is critical in ensuring boat design, navigation, and performance, making it an important aspect of the boating industry.

How to find ship displacement from submerged area?

To find the ship displacement from submerged area, you can follow these steps: Determine the underwater or submerged area of the ship. This can be done by calculating the area of the ship's hull that is below the waterline when it is fully submerged. Convert the area into a volume by multiplying it by the ship's beam (width) or mean draft (depth). Multiply the volume by the density of the water. The density of water varies slightly depending on temperature and salinity, but a typical value is around 1,000 kilograms per cubic meter. The result of this calculation will be the ship's displacement. It represents the weight of the water displaced by the ship when it is fully submerged. Note: This method assumes that the ship's hull has a constant shape below the waterline. In reality, the shape may vary, especially towards the ends of the ship.

How to draw a simple ship?

To draw a simple ship, follow these step-by-step instructions: Start by drawing a horizontal line slightly curved at the ends to create the ship's hull. Add a smaller curved line above the hull to outline the ship's deck. At the front of the ship, draw a triangular shape for the bow. On top of the deck, draw a small rectangular structure or cabin. Add a flagpole at the back of the deck by drawing a long, thin rectangle. Draw a small rectangle or square at the top of the flagpole for the flag. Next, add a slightly curved line near the waterline for the keel of the ship. On both sides of the hull, draw a series of diagonal lines to create the ship's planking. To indicate windows or portholes, draw small oval or circular shapes along the cabin. Add a couple of mast poles on the deck. To do this, draw two vertical lines with a horizontal line connecting them at the top. On top of the mast poles, add triangular or rectangular shapes for the sails. Finally, erase any unnecessary guidelines, and you can add more details like waves or seagulls to complete your simple ship drawing. Remember, this is just one way to draw a simple ship. Feel free to modify the design or add additional elements to make it your own!

How to draw a pin keeldrawing tutorialtop keel?

To draw a pin keel, follow these steps: Begin by drawing a slightly curved horizontal line. This line will serve as the water surface. Next, draw a long vertical line that will represent the keel. The keel should start at the bottom of the water surface line and extend downward. At the bottom of the vertical line, draw a slightly curved horizontal line. This line will represent the lower part of the keel. On the left side of the keel, draw a diagonal line extending outward. This line will represent the forward part of the keel. Repeat the previous step on the right side of the keel, drawing a diagonal line to represent the aft part. Connect the ends of the diagonal lines with a curved line, forming the bottom part of the keel. Add additional detail to the keel by drawing a small horizontal line near the top. This line represents the top part of the keel. Finally, erase any unnecessary lines and add shading to give the keel more depth and dimension. Remember to take your time and practice as much as needed to improve your drawing skills.

How do you draw a ship?

Drawing a ship can be a fun and creative process. Here's a step-by-step guide on how to draw a ship: Start by drawing a long, slightly curved horizontal line in the center of your paper. This line will serve as the ship's waterline. From one end of the waterline, draw a slanted rectangle shape, slightly wider at the bottom than the top. This will be the ship's hull. At the other end of the waterline, draw a smaller rectangle shape, slightly tilted upward. This will be the ship's bow. Connect the bow and the hull with two diagonal lines, creating the ship's front structure. Add a large, slightly curved rectangle shape at the top of the hull. This will be the main deck of the ship. Draw a smaller rectangle shape above the main deck to represent the ship's superstructure. Sketch two parallel, slanted lines on the front of the ship's superstructure to create the pilot house. On the main deck, draw a few rectangular shapes to indicate windows or portholes. Add details like railing, stairs, and lifeboats on the sides and top of the ship as desired. Extend the hull below the waterline using a curved line to give the ship depth. For the finishing touches, you can draw some waves around the ship, seagulls in the sky, or a flag on top. Remember to be creative and modify the design as you like. Don't worry if your drawing doesn't turn out perfect at first; practice makes perfect!

What hull curves do yachts fallow is it x squared?

The hull curves of yachts can vary depending on the design and purpose of the yacht. While some yacht hulls may follow a curve that resembles the function of x squared, others may follow different curves such as parabolic curves, ellipses, catenary curves, or other mathematical shapes. The specific curvature of a yacht's hull is determined by factors such as the desired speed, stability, maneuverability, and hydrodynamic efficiency of the vessel. It is typically designed by naval architects and engineers who consider various factors including the size and weight distribution of the yacht, the intended use (e.g., racing, cruising, etc.), and materials used in construction. In summary, while some yachts may have hull curves similar to x squared, there is no universal standard hull curve for all yachts. The hull design depends on various factors and can incorporate different mathematical curves to achieve specific performance characteristics.

How to calculate the curvature of a boat?

To calculate the curvature of a boat, you would need to determine the radius of its curvature. The curvature refers to the degree of how much the boat's hull curves or bends. Gather the measurements: You will need the length and width measurements of the boat. These measurements can be obtained from the boat's specifications or by physically measuring it. Determine the midpoint: Locate the midpoint of the boat's length. This can be done by dividing the boat's length measurement by 2. Measure the rise: Starting from the midpoint, measure the distance between the bottom of the boat's hull and a straight line connecting the bow and stern (i.e., the rise). Measure the run: Measure the distance between the midpoint and the bottom of the boat's hull at the bow and stern. Calculate the radius of curvature: The radius of curvature can be calculated using the following formula: Radius = (run^2 + rise^2) / (8 x rise). The curvature: The curvature is calculated as the reciprocal of the radius of curvature. It's important to note that this calculation assumes a boat's hull shape can be represented by a simple section of a circle. More complex hull shapes, such as those with multiple curves or irregular shapes, may require different mathematical models or numerical methods to accurately determine curvature.

How to measure the curveture of a boat hull?

There are several methods to measure the curvature of a boat hull. Here are three common techniques: Profiling: This method involves taking measurements at specific points along the hull's surface to understand the curvature. You can use a flexible measuring tape or string to measure the distance from the hull to a straight reference line at different points along the boat's length. These measurements can then be plotted on a graph to depict the curvature of the hull. Reflection Method: For this technique, you need a laser level and a measuring tape. Firstly, position the laser level at a fixed distance from the boat hull and horizontally direct the laser beam towards the hull. The laser beam will be reflected back from the hull surface. Measure the distance from the laser level to the hull at different points along the boat's surface. These measurements can be used to calculate the curvature of the hull. 3D Scanning: Utilizing modern technology, you can use a 3D scanner to create a digital model of the boat hull. The scanner emits laser beams or projects structured light patterns onto the hull, capturing its shape in detail. The resulting 3D model can then be used to measure the curvature of the hull accurately. It is important to note that measuring the curvature of a boat hull may require specific tools and expertise. Hence, it is advised to consult with industry professionals or specialists for accurate measurements.

How to draw a yacht keel?

To draw a yacht keel, you can follow these steps: Start by drawing a horizontal line on your paper. This line will serve as the waterline. From the center point of the waterline, draw two vertical lines going downward to create the main part of the keel. These lines should taper towards the bottom. At the bottom of the keel, draw a horizontal line connecting the two vertical lines. This will form the bottom edge of the keel. Now, draw a diagonal line on each side of the keel, starting from the top and curving slightly outward. These lines will form the shape of the keel as it narrows towards the top. Connect the ends of the diagonal lines at the top with a smooth curve to create the rounded shape of the keel. Next, draw horizontal lines across the keel to represent the different sections or layers. These lines can be evenly spaced or closer together at the top and gradually getting wider towards the bottom. Add details such as ribbing or reinforcements by drawing diagonal lines across the keel, intersecting the horizontal lines. To give the keel a more realistic look, you can shade the bottom part and add some shadow where it meets the waterline. Finally, you can add additional details such as a bulbous bow or a fin at the bottom of the keel based on the specific design of the yacht you are drawing. Remember to sketch lightly at first and gradually darken your lines as you refine the shape. And don't forget to have fun and experiment with different styles and variations to make your drawing unique!

How to draw a boat into transverse stations?

Drawing a boat into transverse stations can be done by following these steps: Start by selecting a suitable scale for the drawing. This will depend on the size of the boat you want to draw and the size of the paper or canvas you are using. Begin by drawing a horizontal line across the paper, representing the waterline. Next, draw vertical lines representing the transverse stations at regular intervals along the waterline. These lines should be evenly spaced and represent the cross-sections of the boat at different points along its length. Use reference drawings or images of the boat to guide your drawing. Start by drawing the outline of the boat's hull within each station. Pay attention to the curvature and tapering of the hull as it moves towards the bow and stern. After drawing the outline, add any additional details such as deck lines, windows, hatches, and other features of the boat. Use shading techniques to add depth and dimension to the drawing. Pay attention to the light source and add shadows accordingly to create a realistic representation of the boat. Finally, go over your drawing and make any necessary adjustments or corrections to ensure accuracy.

What is nonprismatic hull?

A nonprismatic hull is a type of hull shape in naval architecture that does not conform to the standard prismatic shape of traditional sailing vessels. Nonprismatic hulls are designed to increase performance in certain areas such as speed and efficiency, as well as to reduce drag and enhance maneuverability. Nonprismatic hulls are also often used as part of a wave piercing design to cut through wave crests, thus reducing the size of the wake behind the ship.

How to design a schooner hull?

Research the history of schooner hulls and their design features. This will help you understand the shipbuilding principles and methods used in their construction. Consider the type of schooner you want to design. Is it a racing vessel or a cruising boat? This will help you determine the size, weight and other characteristics of the hull. Consider the type of material you will use for the schooner. Traditionally, schooner hulls have been made of wood or fiberglass, but there are other materials that can be used as well. You need to choose a material that meets your needs and budget. Work with an experienced maritime designer or drafter to create a 3D model of the schooner hull. This will help you visualize the hull and make sure it meets your specifications. Have a qualified shipwright or boat builder construct your schooner. Ensure that the schooner is tested and certified by a naval architect before you take it out on the water.

How to draw hull lines plan from boat existing images in reverse engineering?

Take a picture of the boat's existing lines plan. Import the image into a vector graphic program such as Inkscape, Adobe Illustrator, or Corel Draw. Trace the contours of the boat's hull using the Pen Tool or other trace tool in the program. Adjust the lines to make sure they accurately represent the boat's shape and contours. Once the lines plan is complete, use a ruler to draw perpendicular lines from the boat's existing lines plan as a reference for the hull. Use the curved line tool to refine the shape of the hull and make sure everything is in proportion and accurate. Double-check to make sure the hull lines plan is correct, and save the file for future reference.

What does half a sideways figure eight mean on a ship drawing?

Half a sideways figure eight on a ship drawing typically denotes the ship's waterline—the line where the ship sits in the water.

How to work out the shape and profile of a yatch datum line?

Establish the design criteria and parameters of the yacht. This should include the length, width, height and any other characteristics relevant to the design of the yacht. Define the design goals and objectives of the yacht, including the purpose and function of the yacht, how it will be used, and what type of sailing or other activities will take place on it. Choose an appropriate hull shape and size for the yacht based on the design criteria, goals and objectives. Create a 3D computer model of the yacht design, incorporating the appropriate hull shape and size. Use the model to define a datum line for the yacht, which will help to accurately measure the craft's performance and characteristics. The datum line should run from the center of the waterline around the hull to the transom. Using the 3D model, define the profile of the yacht by “lofting” the curves of the hull and the deck. Refine the design by adjusting the curves of the hull and deck to ensure that the yacht's performance characteristics are maximized. Use the computer model to run “virtual wind tunnel” tests on the design, to ensure that its performance characteristics are optimized.

How to draw a boat on water?

Start by sketching the basic shape of the boat. Start with a long, rectangular shape to form the hull of the boat. Add a slight curve to the top of the boat to give it an authentic boat shape. Draw a smaller rectangular shape for the cabin of the boat. Sketch two triangular shapes on the left and right side of the cabin for the sails. Draw a series of small circles along the bottom of the boat to create the waterline. Now add the details to your boat: windows, doors, life preservers, etc. Finally, draw some small waves around the boat to create the illusion of the boat sailing on water.

What are fair lines and sheer lines of a yacht?

Fair lines are the contours of the yacht's hull. Sheer lines are the long, gradual arch of the deck, starting at the bow and extending to the stern.

How to draw a hardshine boat hull quickly?

To draw a hardshine boat hull quickly, you can follow these steps: Start by drawing a horizontal line to represent the waterline. This line will serve as the base for the boat hull. Sketch a rough outline of the boat hull shape above the waterline. Keep in mind that hardshine boat hulls are typically streamlined and have a sharp, angular shape. Add a slightly curved line below the waterline to depict the bottom part of the hull. The curve should be gentle and gradually merge into the horizontal waterline. Extend two diagonal lines downward from the front end of the boat hull to create the bow. The bow should be pointed and sharp to cut through the water efficiently. Add a small transom at the rear end of the boat hull. The transom is usually flat or slightly curved upward. Sketch two straight lines from the bow to the stern to represent the deck of the boat. Draw a horizontal line across the middle section of the hull to indicate a separation or border between the upper and lower parts. Add details to the hull, such as chines (angled lines along the sides of the hull) and spray rails (small fins or ridges). These elements contribute to the boat's stability and improve its performance in the water. Shade the lower portion of the hull with a darker tone to emphasize the hardshine effect. Use quick and light strokes to achieve a glossy appearance. Finally, erase any unnecessary guidelines and refine the drawing as needed. Remember, practicing and experimenting with different techniques will help you improve your drawing skills and speed over time.

How to measure a ships hull shape from inside?

One way to measure a ship's hull shape from inside is by using 3D laser scanning. This technique uses lasers to take precise measurements of a ship's inner hull shape. The lasers scan around the interior of the ship and create a 3D image of the ship's shape. This data can then be used to create a precise and accurate measure of the ship's hull shape.

How to lay out a lines drawing for displacement hulls?

Start by drawing the waterline at the mid-point of the vessel. Draw the bow from the top of the waterline to the nose of the vessel. Draw the stern from the bottom of the waterline to the end of the vessel. Draw in all of the chines of the vessel, the curved lines along the bottom of the sides of the boat, at the waterline. Draw in any other details such as the upturned bow, the tail, or any other details that the vessel may have. Draw the sheer line and the sheer forward, running along the top of the vessel and curving inwards and downwards in the center. Add in any additional lines needed to complete the displacement hull. Use a protractor to make sure all of the angles are correct. Use a ruler to draw the exact lines and make sure the lines are the correct length.

Boat Hulls 101: Complete Guide to Boat Hull Types, Shapes, and Designs

Table of Contents

Last Updated on August 17, 2023 by Boatsetter Team

If you’re new to boating, then you may not have even considered a boat’s hull , its importance, and the way that it affects your time on the water. With the  hull  being the part of the boat in the water, it is perhaps the most important part as it gives your boat the ability to float. Not only that, but it affects every single characteristic of your boat and the smoothness of your ride. This article on boat hulls will equip you with the technical knowledge and expertise necessary to understand hulls and the way they work.

What is a Boat Hull?

First of all, we’ll go into a bit of detail on what a boat hull is. The hull is the body of the boat. It is sealed to prevent water from transmitting its way through and keeping your boat afloat. A hull can be open where you sit in it, such as a small dinghy, or a deck may cover it as you would find on a yacht.

When there is a deck placed on top of a hull, it opens up many more options for utilizing the space on your boat more appropriately as it is raised to the top of the hull, where more space is apparent. For example, on a deck, you can place a cabin -like you would find on a center console or even a mast and sail rigs to create a sailboat.

When the hull is open, options to use your space effectively are reduced as you sit at the bottom of the bowl shape. In addition to having less space, you also feel the rock of the water in a more pronounced manner as it is just the keel of the boat (the bottom) separating you from the water. Therefore, every wave and lurch in the water that rocks the boat is felt, which may cause you discomfort if you haven’t quite found your sea legs.

Why Are Hulls Important?

The knowledge of how a boat floats is fundamental if you are looking to get into boating. Without actually knowing, you put yourself at risk of compromising your boating activities and creating a danger that you cause your boat to sink. The key line to this knowledge is that the air encapsulating your boat must be denser than the water it sits upon. This not only includes the air but the items on your boat as they contribute towards the pressure that your boat’s hull puts upon the water.

The greater the amount of weight your boat holds, the further it pushes itself into the water, lowering or raising the level that your hull sits in the water. This force displaces the water to a level that is equal to the boat. If the average density of the boat is greater than the water, then the boat shall sink. You can see this in action if you have a small dinghy; the more people you place on it, you’ll notice that your boat edges itself ever so slightly more into the water as the boat’s weight is rising.

Different types of Boat Hulls

We’ll now walk you through the different types of boat hulls that you come across. The design of the boat’s hull changes the type of boat that you have. If you are browsing through our boat rentals, you’ll notice the various types of boats. Each of these boats has a different type of hull design. For example, a pontoon boat rental is designed for calm waters, whereas a giant yacht is designed for taking on the rough seas, meaning that their hulls vary greatly.

There are two main types of hull: displacement and planing. We’ll give you the rundown of both of these types and the other sub-varieties within them.

Displacement Hulls

The first variety of hulls that we shall examine are displacement hulls. These hulls are typically found on boats that need to carry a heavy load, such as a large fishing boat and big yachts. The hull sits deeper into the water, and the boat is supported by buoyancy, as opposed to its thrust.

Due to the boat sitting deeper in the water, it might be slower, but it will ride steadier. These larger boats are particularly good for the sea as they can handle stronger waves and currents as the boat can stabilize themselves better. This is why you’ll see container ships and other varieties that need to bear a heavy load using these types of hulls.

When it comes to boat rentals, you are most likely to find a sailing boat with a displacement boat hull. The hull is rounded at the bottom, allowing the sailboat to lurch strongly to one side while turning without any danger of capsizing. Thus, we can see the impact that the hull has on your boat rental as it gives your sailboat the extra capacity to lurch around sharp turns and enjoy some exhilarating fun.

Planing Hulls

The other main type of hull is the planing hull. This hull’s design allows the boat to accelerate to higher speeds due to less hull being placed in the water. When a boat with a planing hull is cruising at lower speeds, it operates similarly to a boat with a displacement. When it starts to hit around 15 knots, things start to change depending on the weight of the boat’s load. The flatter surface of the planing hull allows the boat to propel itself upwards to skiff itself across the water. This is what causes boats with planing hulls to obtain higher speeds. In addition, because the bulk of the hull is not placed below the water’s surface, there is less tension from the water holding the boat back, meaning that it can move through the water faster and using less power to obtain a speed that a boat with a displacement hull can.

There is not only one type of planing hull but instead many different varieties. We’ll look through these varieties to examine how it affects your boating experience so that you can make a more informed decision when choosing your next boat rental.

Flat Bottomed Hulls

As the name suggests, these hulls do not have the traditional curved hull that reaches a point at the bottom but has a flat surface instead. These tend to be small skiffs or fishing boats where you cast out from. Due to them having a flat hull, they are excellent for getting into shallow water where some of your favorite catches may lie. These boats don’t need much power for the planing power to come into action and reach quick speeds in no time. They also tend to handle well not just on the flats but also on the sea, with choppy water not being a big issue. So, if you’re looking for some gentle fishing on the flats or maybe out in some nearshore waters, check out our range of small flat bottomed hull boats to truly enjoy some great fishing experiences.

Pontoons  are one of the great boat rentals for cruising around and enjoying time with friends because the design of their hulls allows for more space to be created. Pontoons have two-cylinder hulls that sit parallel to each other on the surface of the water. The deck is placed atop these two cylinders, and because they are placed on cylinders, the deck can expand beyond the cylinders, creating more space. This allows for a comfortable seating/social area to be created on the boat, allowing you to use it for parties and some relaxed exploring with the wider family. These boats are best used on inland and flat waters. This is due to waves rocking them a lot more, and a storm at sea can even put them at risk of capsizing. For some fun on a lake, however, pontoons are hard to beat.

In recent years many tritoons have started to crop up on the boat rental market. These are similar to pontoons, but they have a third cylinder that gives them some extra stability. It also means that they can handle a more powerful engine that can bring them up to higher speeds than a pontoon boat. If a pontoon has an engine that is too powerful, then its planing hull can lift it too far above the water’s surface, causing great instability. However, when it comes to tritoons, the greater speeds that you can reach allow you to expand upon other activities and add in some wakeboarding or tubing action onto your party on the water!

V Bottom Hull

The shape of a v bottom hull has a sharper decline that accumulates in having a meeting point at the bottom, creating a v shape, as stated in the name. Because of the honed hull, one of these boats can cut through the water at decent speeds and are particularly good when out on seawater. However, they require a powerful engine for the boat to go into a planing mode. One of the most common types of v bottom hull boat rentals is center consoles. These are great vessels for going for some nearshore or offshore fishing or some general saltwater exploring. Their v bottom hull allows them to cut through the waves so that you can rush to the best fishing grounds in no time at all.

The tri-hull design is a variation of the v bottom hull. It has a v-shaped hull in the center and two parallel smaller hulls on either side of the main central one. This gives the tri-hull boat some extra stability when going forward . Additionally, this also allows the boat to have more deck space as the hull covers a wider range. One of the big drawbacks of the tri-hull – also known as a cathedral hull – is that the bat rocks more when it is in choppier water because the hull is wider. Nevertheless, tri-hulls make for a great option for fishing or exploring on lakes or calm coves.

Catamaran: A Multi-Hulled Boat

Perhaps the most popular multi-hulled boat is the catamaran. This type of boat has two separate hulls that run parallel to each other. These hulls sit on either side of the boat and the deck connects them. This type of design allows forecast amounts of space onboard . Many catamarans are luxury boats that can have the space to hold swimming pools and even helipads. Because they have dual hulls, catamarans can get themselves in shallow waters and lagoons where other luxury boats cannot. This makes them the perfect boat rental if you plan to visit a location where there are multiple small islands such as Hawaii or The Bahamas. The multi-hull system also provides a lot more stability and comfort, so they are perfect boat rentals if you are prone to suffering from seasickness . Catamarans are not only luxury liners as smaller versions with a trampoline-designed deck can also be found that make for great day adventures.

As we hope you have been able to discover in this blog post, the type of hull that your boat has affects everything about your boat. By having a little bit of knowledge on how the design of a boat’s hull has an impact on your boating experience, you can begin to make more informed decisions on which boat rental is best for you. To reinforce this information a little bit further, check out this  video !

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Basics of Sailboat Hull Design – EXPLAINED For Owners

There are a lot of different sailboats in the world. In fact, they’ve been making sailboats for thousands of years. And over that time, mankind and naval architects (okay, mostly the naval architects!) have learned a thing or two.

If you’re wondering what makes one sailboat different from another, consider this article a primer. It certainly doesn’t contain everything you’d need to know to build a sailboat, but it gives the novice boater some ideas of what goes on behind the curtain. It will also provide some tips to help you compare different boats on the water, and hopefully, it will guide you towards the sort of boat you could call home one day.

Table of Contents

Displacement hulls, semi displacement hulls, planing hulls, history of sailboat hull design, greater waterline length, distinctive hull shape and fin keel designs, ratios in hull design, the hull truth and nothing but the truth, sail boat hull design faqs.

Basics of Hull Design

When you think about a sailboat hull and how it is built, you might start thinking about the shape of a keel. This has certainly spurred a lot of different designs over the years, but the hull of a sailboat today is designed almost independently of the keel. 

In fact, if you look at a particular make and model of sailboat, you’ll notice that the makers often offer it with a variety of keel options. For example, this new Jeanneau Sun Odyssey comes with either a full fin bulb keel, shallow draft bulb fin, or very shallow draft swing keel. Where older long keel designs had the keel included in the hull mold, today’s bolt-on fin keel designs allow the manufacturers more leeway in customizing a yacht to your specifications.

What you’re left with is a hull, and boat hulls take three basic forms.

• Displacement hull
• Semi-displacement hulls
• Planing hulls

Most times, the hull of a sailboat will be a displacement hull. To float, a boat must displace a volume of water equal in weight to that of the yacht. This is Archimedes Principle , and it’s how displacement hulled boats get their name.

The displacement hull sailboat has dominated the Maritimes for thousands of years. It has only been in the last century that other designs have caught on, thanks to advances in engine technologies. In short, sailboats and sail-powered ships are nearly always displacement cruisers because they lack the power to do anything else.

A displacement hull rides low in the water and continuously displaces its weight in water. That means that all of that water must be pushed out of the vessel’s way, and this creates some operating limitations. As it pushes the water, water is built up ahead of the boat in a bow wave. This wave creates a trough along the side of the boat, and the wave goes up again at the stern. The distance between the two waves is a limiting factor because the wave trough between them creates a suction. 

This suction pulls the boat down and creates drag as the vessel moves through the water. So in effect, no matter how much power is applied to a displacement hulled vessel, it cannot go faster than a certain speed. That speed is referred to as the hull speed, and it’s a factor of a boat’s length and width. 

For an average 38 foot sailboat, the hull speed is around 8.3 knots. This is why shipping companies competed to have the fastest ship for many years by building larger and larger ships.

While they might sound old-school and boring, displacement hulls are very efficient because they require very little power—and therefore very little fuel—to get them up to hull speed. This is one reason enormous container ships operate so efficiently. 

Of course, living in the 21st century, you undoubtedly have seen boats go faster than their hull speed. Going faster is simply a matter of defeating the bow wave in one way or another.

One way is to build the boat so that it can step up onto and ride the bow wave like a surfer. This is basically what a semi-displacement hull does. With enough power, this type of boat can surf its bow wave, break the suction it creates and beat its displacement hull speed.

With even more power, a boat can leave its bow wave in the dust and zoom past it. This requires the boat’s bottom to channel water away and sit on the surface. Once it is out of the water, any speed is achievable with enough power. 

But it takes enormous amounts of power to get a boat on plane, so planing hulls are hardly efficient. But they are fast. Speedboats are planing hulls, so if you require speed, go ahead and research the cost of a speedboat . 

The most stable and forgiving planing hull designs have a deep v hull. A very shallow draft, flat bottomed boat can plane too, but it provides an unforgiving and rough ride in any sort of chop.

If you compare the shapes of the sailboats of today with the cruising boat designs of the 1960s and 70s, you’ll notice that quite a lot has changed in the last 50-plus years. Of course, the old designs are still popular among sailors, but it’s not easy to find a boat like that being built today.

Today’s boats are sleeker. They have wide transoms and flat bottoms. They’re more likely to support fin keels and spade rudders. Rigs have also changed, with the fractional sloop being the preferred setup for most modern production boats.

Why have boats changed so much? And why did boats look so different back then?

One reason was the racing standards of the day. Boats in the 1960s were built to the IOR (International Offshore Rule). Since many owners raced their boats, the IOR handicaps standardized things to make fair play between different makes and models on the racecourse.

The IOR rule book was dense and complicated. But as manufacturers started building yachts, or as they looked at the competition and tried to do better, they all took a basic form. The IOR rule wasn’t the only one around . There were also the Universal Rule, International Rule, Yacht Racing Association Rul, Bermuda Rule, and a slew of others. 

Part of this similarity was the rule, and part of it was simply the collective knowledge and tradition of yacht building. But at that time, there was much less distance between the yachts you could buy from the manufacturers and those setting off on long-distance races.

Today, those wishing to compete in serious racing a building boat’s purpose-built for the task. As a result, one-design racing is now more popular. And similarly, pleasure boats designed for leisurely coastal and offshore hops are likewise built for the task at hand. No longer are the lines blurred between the two, and no longer are one set of sailors “making do” with the requirements set by the other set. 

Modern Features of Sailboat Hull Design

So, what exactly sets today’s cruising and liveaboard boats apart from those built-in decades past? 

Today’s designs usually feature plumb bows and the maximum beam carried to the aft end. The broad transom allows for a walk-through swim platform and sometimes even storage for the dinghy in a “garage.”

The other significant advantage of this layout is that it maximizes waterline length, which makes a faster boat. Unfortunately, while the boats of yesteryear might have had lovely graceful overhangs, their waterline lengths are generally no match for newer boats. 

The wide beam carried aft also provides an enormous amount of living space. The surface area of modern cockpits is nothing short of astounding when it comes to living and entertaining.

If you look at the hull lines or can catch a glimpse of these boats out of the water, you’ll notice their underwater profiles are radically different too. It’s hard to find a full keel design boat today. Instead, fin keels dominate, along with high aspect ratio spade rudders. 

The flat bottom boats of today mean a more stable boat that rides flatter. These boats can really move without heeling over like past designs. Additionally, their designs make it possible in some cases for these boats to surf their bow waves, meaning that with enough power, they can easily achieve and sometimes exceed—at least for short bursts—their hull speeds. Many of these features have been found on race boats for decades.

There are downsides to these designs, of course. The flat bottom boats often tend to pound when sailing upwind , but most sailors like the extra speed when heading downwind.

How Do You Make a Stable Hull

Ultimately, the job of a sailboat hull is to keep the boat afloat and create stability. These are the fundamentals of a seaworthy vessel. 

There are two types of stability that a design addresses . The first is the initial stability, which is how resistant to heeling the design is. For example, compare a classic, narrow-beamed monohull and a wide catamaran for a moment. The monohull has very little initial stability because it heels over in even light winds. That doesn’t mean it tips over, but it is relatively easy to make heel. 

A catamaran, on the other hand, has very high initial stability. It resists the heel and remains level. Designers call this type of stability form stability.

There is also secondary stability, or ultimate stability. This is how resistant the boat is to a total capsize. Monohull sailboats have an immense amount of ballast low in their keels, which means they have very high ultimate stability. A narrow monohull has low form stability but very high ultimate stability. A sailor would likely describe this boat as “tender,” but they would never doubt its ability to right itself after a knock-down or capsize.

On the other hand, the catamaran has extremely high form stability, but once the boat heels, it has little ultimate stability. In other words, beyond a certain point, there is nothing to prevent it from capsizing. 

Both catamarans and modern monohulls’ hull shapes use their beams to reduce the amount of ballast and weight . A lighter boat can sail fast, but to make it more stable, naval architects increase the beam to increase the form stability.

If you’d like to know more about how stable a hull is, you’ll want to learn about the Gz Curve , which is the mathematical calculation you can make based on a hull’s form and ultimate stabilities. 

How does a lowly sailor make heads or tails out of this? You don’t have to be a naval architect when comparing different designs to understand the basics. Two ratios can help you predict how stable a design will be .

The first is the displacement to length ratio . The formula to calculate it is D / (0.01L)^3 , where D is displacement in tons and L is waterline length in feet. But most sailboat specifications, like those found on  sailboatdata.com , list the D/L Ratio.

This ratio helps understand how heavy a boat is for its length. Heavier boats must move more water to make way, so a heavy boat is more likely to be slower. But, for the ocean-going cruiser, a heavy boat means a stable boat that requires much force to jostle or toss about. A light displacement boat might pound in a seaway, and a heavy one is likely to provide a softer ride.

The second ratio of interest is the sail area to displacement ratio. To calculate, take SA / (D)^0.67 , where SA is the sail area in square feet and D is displacement in cubic feet. Again, many online sites provide the ratio calculated for specific makes and models.

This ratio tells you how much power a boat has. A lower ratio means that the boat doesn’t have much power to move its weight, while a bigger number means it has more “get up and go.” Of course, if you really want to sail fast, you’d want the boat to have a low displacement/length and a high sail area/displacement. 

Multihull Sailboat Hulls

Multihull sailboats are more popular than ever before. While many people quote catamaran speed as their primary interest, the fact is that multihulls have a lot to offer cruising and traveling boaters. These vessels are not limited to coastal cruising, as was once believed. Most sizable cats and trimarans are ocean certified.

Both catamarans and trimaran hull designs allow for fast sailing. Their wide beam allows them to sail flat while having extreme form stability. 

Catamarans have two hulls connected by a large bridge deck. The best part for cruisers is that their big surface area is full of living space. The bridge deck usually features large, open cockpits with connecting salons. Wrap around windows let in tons of light and fresh air.

Trimarans are basically monohulls with an outrigger hull on each side. Their designs are generally less spacious than catamarans, but they sail even faster. In addition, the outer hulls eliminate the need for heavy ballast, significantly reducing the wetted area of the hulls. 

Boaters and cruising sailors don’t need to be experts in yacht design, but having a rough understanding of the basics can help you pick the right boat. Boat design is a series of compromises, and knowing the ones that designers and builders take will help you understand what the boat is for and how it should be used. 

What is the most efficient boat hull design?

The most efficient hull design is the displacement hull. This type of boat sits low in the water and pushes the water out of its way. It is limited to its designed hull speed, a factor of its length. But cruising at hull speed or less requires very little energy and can be done very efficiently. 

By way of example, most sailboats have very small engines. A typical 40-foot sailboat has a 50 horsepower motor that burns around one gallon of diesel every hour. In contrast, a 40-foot planing speedboat may have 1,000 horsepower (or more). Its multiple motors would likely be consuming more than 100 gallons per hour (or more). Using these rough numbers, the sailboat achieves about 8 miles per gallon, while the speedboat gets around 2 mpg.

What are sail boat hulls made of?

Nearly all modern sailboats are made of fiberglass. 

Traditionally, boats were made of wood, and many traditional vessels still are today. There are also metal boats made of steel or aluminum, but these designs are less common. Metal boats are more common in expedition yachts or those used in high-latitude sailing.

Matt has been boating around Florida for over 25 years in everything from small powerboats to large cruising catamarans. He currently lives aboard a 38-foot Cabo Rico sailboat with his wife Lucy and adventure dog Chelsea. Together, they cruise between winters in The Bahamas and summers in the Chesapeake Bay.

The Illustrated Guide To Boat Hull Types (11 Examples)

I didn't understand anything about boat hull types. So I've researched what hulls I need for different conditions. Here's a complete list of the most common hulls.

What are the different boat hull types? There are three boat hull categories: displacement hulls, which displace water when moving; planing hulls, which create lift at high speeds; and semi-displacement hulls, which displace water and generate lift at low speeds. The most common hull types are round-bottomed, flat-bottomed, multi, V-shaped, and pontoon hulls.

But that's all pretty abstract if you ask me, so below I'll give a simple overview of what it all means. After that, I'll give a list with pictures of all the different designs.

A Simple Overview of Boat Hull Types

Your boat hull will be the biggest factor in how your boat handles or sails, how wet it is, how bumpy - absolutely everything is determined by the hull shape. So it's important to understand what different hulls will do for you, and what each hull is best for. First, let's slice it up into rough categories.

Roughly, you can divide boat hulls into three categories:

• Displacement hulls - Lie inside the water and push it away when they move
• Planing hulls - Lie on top of the water and don't push it away
• Semi-displacement hulls - Lie inside the water and push it away, but can generate lift

Everything I'll be mentioning below is one of those three, or something in between.

There are five common boat hull types:

• Round-bottomed hulls - handle well in rough water: sailboats
• Flat-bottomed hulls - very stable for calm inland waters: fishing boats
• Multihulls - very stable and buoyant: catamarans
• V-Shaped Hulls - fast and comfortable in chop: powerboats
• Pontoon hulls - fast and stable: pontoon boats

And then there's everything in-between.

Here's a quick and handy overview of the different hull types

In each category, we find different designs and styles that have different characteristics. There isn't a real clear distinction between categories and styles: there are semi-displacement hulls and so on. So I thought the best way to learn you the different hull types is by simply creating a list with lots of pictures, instead of getting all theoretical about it.

So below I've listed all the different hull styles I could possibly think of, mention what category and type it is, the pros and cons of each one, and give you examples and illustrations for each one.

On this page:

Displacement hulls, round-bottom hull, catamaran hull, trimaran hull, planing hulls, flat-bottom hull, deep v-hull, modified-v hull, stepped hull, pontoon hull, semi-displacement hulls.

Examples: Sailboats, trawlers, fishing boats

Displacement hulls displace water when moving. These hulls lie in the water, instead of on top of it. The amount of water they displace is equal to the boat's weight. Displacement hulls handle way better in rough waters than flat-bottom hulls. That's why most cruisers have some sort of displacement hulls. There are actually all kinds, shapes, and forms of the displacement hull design, which we'll go over later.

The most important thing to understand about the displacement hull, is that it operates on buoyancy. This means that most of the boat's weight is supported by its capacity to float . Planing hulls, on the other hand, operate on lift instead, but we'll dive into that later.

Sailboats typically have displacement hulls, but also fishing boats, trawlers and crabbers. All in all, it's used for each boat that needs to handle well in rough conditions.

Learn everything there is to know about displacement hulls in this article . It lists all the pros and cons and really goes into detail on the nitty-gritty about how displacement hulls actually work .

But they are also slower than flat and planing hulls because the boat creates more resistance when moving. It has to push the water aside. In fact, this type of hull has a built-in upper-speed limit.

This upper-speed limit is called maximum hull speed . It means that the length of a displacement hull directly determines the maximum speed. It can't go faster, because the water-resistance increases with the boat's speed. To learn everything about calculating maximum hull speed , please check out my previous article here.

A round-bottomed hull is a type of displacement hull - it lies in the water and has to power through it. But since it's rounded, it creates little resistance and is effortless to move through the water. It's a very smooth ride and typical for any sailboat that sort of glides through the waves. In contrast, powerboats really have to eat their way through the water.

Examples: Canoes, sailboats

They are also one of the least stable. Since the bottom is rounded, your boat or canoe will rock plenty when boarding or moving around. They are also easy to capsize. That's why pro canoers learn to do a 360 in their canoes. I've never did a roll myself but came close enough a couple of times.

Almost all sailboats use a round bilge as well. This provides it its buoyancy and makes sure it handles well in waves. But since a rounded bilge is easy to capsize, a lot of sailboats have some sort of keel, which stabilizes the roll.

Nearly all ocean-going vessels use some sort of displacement hull, and the round bottom is the most common one. But our next guest is very popular as well.

The catamaran is similar to the pontoon hull (read on to learn more on that one), but it is a displacement multihull instead of a planing one. So it has two hulls, that lie inside the water and displace it. Like the pontoon, you will have to try really hard to capsize this design (and it won't work).

Examples: well, catamaran sailboats. But also this cool catamaran trawler:

Catamarans are extremely popular ocean cruisers. Their biggest pro is their extreme stability and buoyancy. And they have a very shallow draft for a displacement hull, making them very popular for sailing reefs and shallow waters, like the Caribbean.

Some cons for the catamaran are less agile than monohulls. They have a large turning radius, making them less maneuverable. Also, expect to pay high marina fees with this one.

Speaking of marina fees, our next one can go either way.

I think trimarans are incredibly cool, and especially the second type.

There are two types of trimarans:

• a catamaran with three hulls instead of two,
• or a displacement monohull with two floaters.

The first has the same characteristics as the catamaran: it's a displacement multihull, but now with three hulls:

The second can be a regular displacement monohull, with two pontoon-type floaters that provide extra buoyancy, making the total thing a hybrid between pontoon and displacement:

This last one has all the pros of a catamaran in terms of stability, but: you can simply wheel in those floaters whenever you head for port. That saves you a lot of money. And you can trailer her! Imagine that, a towing a trimaran home.

So those were the most common displacement hulls, aka what lives in the water. Let's move on to the planing hulls, aka what lives on the water.

Planing hulls are a hybrid between the flat-bottom and displacement hulls. Planing hulls displace water at low speeds , but create lift at higher speeds . The shape of their hull + speed lifts them out of the water, making them glide on top of the water. Most powerboats look like flat-bottom boats but use a shallow V-shape that helps the boat to handle better at higher speeds.

Examples: Water sports boat, powerboats

The most important thing to understand about planing hulls is that they operate mainly on lift instead of buoyancy. This means the weight of the boat is mainly supported by dynamic forces 1 . With the right amount of power, this design generates lift, which results in less resistance. This is why they are a lot faster than boats with displacement hulls, but also a lot rougher, even with mild chop.

A lot of powerboats use some sort of planing hull. Again, there are many designs and variations on the planing hull, and I'll try to mention as many as I can below.

Because the wedge of the hull runs into the water, it is much easier to handle at high speeds. At lower speeds, it is able to keep its course, even with a bit of wind. However, whenever the boat starts planing, it is prone to wind gusts, since the wedge shape no longer stabilizes the boat.

The flatter the hull, the faster it will go, but also the more poorly it will handle. Other powerboats use deep V-hulls, which I'll discuss below. But first, let's take a look at the flattest hulls you'll ever see.

A flat-bottom hull lies on top of the water and doesn't displace water (okay, very little) as it moves. Since there is no displacement, there is also little to no friction when moving. This makes it potentially fast, but it handles pretty poorly. It is one of the most stable hull design.

Examples: rowboats, (old) high-performance powerboats, small skiffs, small fishing boats, tug boats

They aren't just incredibly stable, they're also very practical. Because the bottom is practically flat, they maximize boat surface. But they are also extremely choppy in rough weather and waves. They will handle very poorly with stiff winds, as the wind can simply catch them and blow them across the water surface. That's why this design is almost exclusively used for calm, small, inland waters.

This type of hull operates mainly on buoyancy , like the displacement hull, but it doesn't require the same amount of power to propel, which is why it's faster.

Because of the uncomfortable ride, not a lot of boats use a perfectly flat bottom. Most boats nowadays use some sort of v-hull or hybrid design, like a semi-displacement hull; especially larger boats. So not a lot of boats have a real flat bottom. However, we do call a lot of boats flat-bottomed. How come?

There are two types of hulls we call flat-bottoms:

• Of course boats with an actual flat bottom
• Boats with almost no deadrise

What is the hull's deadrise? The deadrise is the angle of the front of the hull to the horizontal waterline.

As you can see, the green sailing dinghy in the picture above has a deadrise that's barely noticeable.

Let's move on to other variations of the planing hull. One of the most popular hull design for modern-day powerboats is the Deep Vee hull. And that's as cool as it sounds.

This is a type of planing hull that combines the best of both worlds.

These types of hulls are very popular on modern-day powerboats, and no wonder. With a V-shape that runs from bow to stern, deep into the water, you can handle this boat even in offshore conditions. It handles a lot better than flat-bottomed hulls, while it's at the same time extremely fast.

Examples: Most modern powerboats.

The Deep V-shape acts as a tiny keel of sorts, stabilizing the boat and making it more reliable and maneuverable. The rest of the hull acts as a planing hull, giving the boat its fast edge. Even at high speeds, the Deep V will cut into the water, making it more handleable.

The deep-V design is just one of many variants on the V-hull. Below we'll talk over another, the modified V hull.

The modified V hull is the ultimate crossover of all planing hull types. It's a mix of the flat-bottom and Deep V hull. It is one of the most popular hull designs for small motorboats. It's flat in the back and then runs into a narrow V-shape to the front. The flat back makes it more stable, and adds a little speed, while the V-shape front ensures good handling.

It is, in short, kind of the compromise-family-sedan of boat hulls. It's the fastest design that's also stable, that's also safe, and that also handles well. But it's not the best in any of those things.

Most powerboats you've seen will have some sort of Vee or Modified-V hull.

Stepped hulls are used on high-performance powerboats. It's a type of planing hull that reduces the hull surface by adding steps, or indents in the hull below the waterline. It looks something like this:

It is said to work extremely well at high speed (60 knots and up) and adds up to 10 knots to your top speed.

On to our next design. There are also planing multihulls, and they might even look like catamarans to you. Meet the pontoon hull.

Pontoon hulls float on top of the water using pontoons or floaters that create lift. It's a type of planing multihull that doesn't lie in the water, so it doesn't displace a lot of water. They don't really handle well. As with any multihull, they aren't agile - they're not great at maneuvering. They also have a very large turning radius. But they are extremely stable: there's no chance you'll capsize this.

Examples: Cruisers, modern trawlers, motor yachts, Maine lobster boats

Semi-displacement hulls are smack bang in the center of planning and displacement hulls. They are a bit better for speed than displacement hulls are. They are a bit better for handling rough waters than planing hulls are. This makes them very versatile.

You can see these a bit like being 'half-planing' hulls. These hulls are designed to plane at lower speeds than normal planing hulls - somewhere in the range of 15 - 20 knots, depending on the length of the boat. It also requires less power. When the hull lifts, it reduces drag (water resistance), making it faster and more efficient.

Semi-displacement hulls are perfect for boats that need to be steady and seaworthy but fast at the same time.

For more information about semi-displacement hulls, please check out my in-depth guide to semi-displacement hulls here . It has a diagram and lists all the pros and cons.

So those were my 11 examples, and my step by step explanation of the different types of boat hulls and functions. You now have a solid basic understanding of boat hulls, and can recognize the most common ones. I hope it was helpful, and if you want more good sailing information, be sure to check out my other articles below.

https://www.soundingsonline.com/boats/how-different-hull-types-react-in-rough-water .  ↩

I was wondering what your opinion would be on the ship uss Texas as far as hull type and bow type. I think it has a plumb bow and it looks to have a displacement or flat bottom hull. Im doing some research and a better trained eye would be of great help. I used images “bb-35 dry dock” to help see the hull shape. Thank you

Shawn Buckles

Hi Kirk, I don’t know about trained but here we go. I’ve checked the picture, it’s definitely a displacement hull I’d also say it’s a plumb bow.

Hahahahaa imagine liking boats hehehehehe Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water

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Hull of a Ship – Understanding Design and Characteristics

The hull of a ship is the most notable structural entity of the ship. To define the hull, it can be said that it is the watertight enclosure of the ship, which protects the cargo, machinery, and accommodation spaces of the ship from the weather, flooding, and structural damage. But this alone does not suffice our requirements of understanding all the aspects of a ship’s hull.

In this article, we will see how the hull of a ship is designed for various factors taken into consideration during the entire lifetime of the ship, and how the design of a ship’s hull plays the most important role in the entire ship design and shipbuilding project.

Hull Related Nomenclature

The above figure shows the schematic profile of a conventional ship’s hull. Understanding the meaning and applications of the nomenclatures related to it forms the basics of understanding ship design and shipbuilding technology.

Bow and Stern : The forward most contour of the ship’s hull is called the bow, and the aft-most, its stern. The stem is the forward most contour part of the bow.

Forward Perpendicular : If a perpendicular is drawn at the point where the bow intersects the waterline, this imaginary perpendicular line is called the forward perpendicular. For most of the hydrostatic calculations, the forward perpendicular is used as the forward reference of the hull.

Aft Perpendicular : Depending on the designer, the aft perpendicular can be the perpendicular drawn through the aft side of the rudder post or through the center-line of the rudder pintles. The aft perpendicular is the aft reference line for all hydrostatic calculations.

Length between Perpendiculars : The length between the forward and aft perpendiculars is the length between perpendiculars. The LBP is a very important parameter in all stability calculations, hence calculation of the LBP at various drafts becomes an important step in carrying out stability analyses.

Sheer : The upward curve formed by the main deck with reference to the level of the deck at the midship, is called sheer. It is usually given to allow flow of green water from the forward and aft ends to the midship and allow drainage to the bilges. The forward sheer is usually more than the aft sheer to protect the forward anchoring machinery from the waves.

Summer Load Line : The summer load line is the waterline of the ship at sea water when it is at its design weight and ballast conditions. It is also called the design draft; this forms the reference for all other load lines of the ship.

Length of Waterline : The length of the ship’s hull at the summer load line is the length of waterline for the ship. This length plays an important role in the calculation of hydrostatics of the ship, as well as propeller design calculations.

Length Overall : The length between the forward-most and aft-most point of the ship’s hull is its overall length. This length plays a major role in designing the docking and undocking plans of the ship. In shipyards where multiple building docks are available, the overall length, beam, and depth of the ship is a deciding factor in choosing a suitable building block for the ship.

Hull Lines and Shape

The first step in designing a hull of a ship is designing its shape and form. The form of the ship’s hull is estimated by means of various form coefficients, discussed as follows:

Block Coefficient: Block coefficient is the ratio of the ship’s underwater volume to the volume of the imaginary rectangle enclosing the underwater portion of the hull. Since the length, breadth, and height of this enclosing rectangle would be the length between perpendiculars, Maximum Beam, and Draft of the ship, the block coefficient is expressed as follows:

The value of block coefficient is one for a ship with the rectangular cross-section. Hence, for a typical ship’s hull form, it would be less than one. The higher the block coefficient, the fuller is the hull form (e.g. oil tankers, bulk carriers). Finer hull-forms have lower block coefficients (e.g. container ships , warships).

Midship Coefficient : The midship coefficient is the ratio of the submerged area of the midship section to the enclosing rectangle. It is hence expressed as:

There are a number of other form coefficients like prismatic Coefficient, Volumetric Coefficient, etc. which are basically the parameters used to define the volumetric distribution of the ship’s hull along its length. Once these coefficients are arrived at, from statistical studies, the hull lines are developed.

The lines plan of a ship’s hull comprises of three views. To understand the lines plan, we first need to know what are buttocks and waterlines.

When the hull of a ship is cut into multiple sections longitudinally, that is, if you slice the ship’s hull at every two meters starting from port to starboard, you would produce longitudinal sections at every two meters. The contour of each longitudinal section is called a buttock line, and this is exactly what is represented in the profile plan, as shown below. The reference lines for the profile view are the stations (vertical grid lines, which denote the longitudinal position) and waterlines (horizontal lines, which denote vertical positions).

If the ship’s hull is sliced along each waterline, then every waterline produces a distinct curve. Since a ship’s hull is symmetric about the centerline, a common practice prevails in which the curve is drawn on either side of the centerline, and this view is called the body plan or the half breadth plan of the ship.

Important Tip: The shape of the waterlines (in the half breadth plan) play a deciding role in the shape of the stern and the efficiency of the propeller. In the above figure, the waterlines move away from the ship’s centerline with the increase in height above baseline. That is, the innermost curve is the lowermost waterline. Take note of how the waterlines straighten at the aft as we move upward from the keel. This shows that the ship has a transom stern. So why is transom stern preferred? The answer lies in the shape of the waterlines at the stern. The longitudinal direction was taken by the waterlines at the stern ensure the flow of water at the stern in a direction almost perpendicular to the propeller disc. This ensures minimum crossflow at the propeller, therefore ensuring maximum propeller efficiency.

If the ship’s hull is sliced to form a section at every station, we obtain the body plan, as shown below. The typical practice of drawing the body plan is to denote all the half sections (due to the hull’s symmetricity). The sections forward of the midship are drawn on the right side of the center line, and all the sections from the midship to the stern are drawn on the left side.

The body plan is the most useful representation of the ship’s hull lines. The reference lines in the body plan are the buttocks (vertical grid-lines), and the waterlines (horizontal grid lines). The body plan, along with the reference lines can be self sufficiently used to develop the profile plan and half breadth plan of the ship. It is also useful in developing the sectional area curve, and bonjean curves of the ship.

The complete lines plan of a ship is arranged by placing the profile view on top, with the half breadth plan just below it, and the body plan to its right, as shown below. The lines plan provides for the foundation of developing not only the three-dimensional hull model, but also developing frame-wise structural drawings, general arrangement, and loft drawings at the shipyard.

Hull Structure and Strength

The structural design of the hull of a ship amounts to approximately 70 percent of the total structural design of the ship. The stages in designing the hull structure are as follows:

Step 1: Calculation of Loads on the Hull: This is where the classification society rules come into play. Rulebooks have specialized formulae for calculation of wave loads on the ship’s hull. The still water bending moment, wave bending moment, and shear forces are to be calculated using these formulae. These load values act as set points in the entire structural design process.

Step 2: Scantling Calculations for Midship: The dimensions of all the structural members of the ship (plates, stiffeners, girders, beams, pillars, etc.) are collectively called scantlings. The loads calculated in Step 1 are used to arrive at the scantlings, and this is calculated for structural members at every frame.

Step 3: Midship Section Modulus: The midship section structural drawing is prepared according to the calculated scantlings. This is followed by locating the neutral axis of the midship section and calculating the section modulus of the midship section. Two criteria are to be satisfied at this stage:

• The obtained midship section must be equal to or more than the minimum section modulus value obtained by the empirical formula in the rule book.
• The bending stress at the deck and the keel are calculated, and it is checked if the stress values are within the required factor of safety.

In the above midship section drawing, the blue line (NA) is the neutral axis of the section. The bending stress graph is drawn with the neutral axis with the reference (origin), and the top-most and lowermost ends of the graph would denote the stress values at the deck and keel respectively, as shown in the stress graph below.

Why do you think it is important to design the structure of the midship section of the ship before any other sections? Read this article to find out what makes midship the structurally most important region of the ship.

Step 4: Frame-wise Scantling Calculation: Once the midship scantlings satisfy the criteria, the scantlings for structural members at each frame are calculated, and corresponding frame-wise structural drawings are prepared. Special formulae are applied for the forward and aft sections, and bulkheads, and drawings are prepared for the same.

Step 5: Calculation of Steel Weight: The obtained scantlings are the used to calculate the steel weight of the ship. This is where the iteration begins. If the calculated steel weight lies outside the empirically and statistically obtained values, the designer might have to look at using lighter weight steel at suitable regions or take other decisions to keep the lightship weight within limits.

Step 6: Development of 3D Structural Model and FEA Analyses: With the structural drawings at each frame, a three dimensional structural model is prepared for the entire hull. This process takes the longest time because the accuracy of this model would directly impact the results of the finite element analyses that are to follow. Three-dimensional meshing is carried out on the 3D model, followed by finite element analyses for various conditions. It is based on the results of these analyses that classification societies today approve a ship’s structural design, as they produce more reliable data than those produced by linear calculations.

Course Stability of the Hull

The other important aspect of the ship’s hull is its directional or course-keeping performance at sea. In other words, its manoeuvrability. In order to evaluate the manoeuvrability of the bare hull, we evaluate the following aspects:

• Straight Line Stability : If a ship moving in a straight line is subjected to an external disturbance, and it changes its direction but continues to move in a straight line along the new direction, without the help of the rudder, then the hull is said to have straight-line stability.
• Directional Stability: If a ship moving in a straight line is subjected to an external disturbance, and it continues to move along a new path which is parallel to the initial direction, the ship is said to possess directional stability. Directional stability is not possible without the aid of a control surface (e.g. rudder), but having straight-line stability makes it easy to attain directional stability.
• Path Stability: If a ship moving in a straight line is disturbed externally, and it continues to move along the same path (after a few oscillations), it is said to have path stability. Path stability, like directional stability, can only be attained if straight-line stability is achieved.

The design goal during the development of a ship’s hull, is, therefore, to attain straight line stability. For this various tank, tests are carried out at model basins and the hydrodynamic coefficients are measured for the bare hull. These hydrodynamic coefficients are the characteristic properties of the hull’s course keeping abilities, and in case of unwanted results, changes in the hull’s shape or geometry are decided upon. For example, a skeg is often added to the hull in later stages of design to improve on its straight line stability, after the model basin test results are obtained.

Hull-Superstructure Interaction

It has been observed that the presence of a superstructure on the main deck reduces the bending stress at the deck from the stress value predicted by the beam bending theory. This is because of the interaction of the shear stresses with the bending stress at the ends of the superstructures. However, this leads to deformations at the superstructure ends. So, in other words, if a superstructure is an efficient one, is must be able to absorb a certain portion of the bending stress at the deck. The extent to which it takes up the bending stress determines its efficiency, which comes designers prefer to call Superstructure Efficiency. It can be expressed as:

It depends on the designer whether to design a superstructure that would take up the bending stress from the hull, or whether to design one that is free from any interaction with the hull. Designing a 100 percent efficient superstructure would be possible, but would come at the cost of heavy weight deep bulkheads at the superstructure ends to prevent severe distortions due to shear. However, to increase the superstructure efficiency, most ships have superstructures connected to the hull by means of bulkheads transverse bulkheads under the deck, and webs that run continuously from the hull to the superstructures at its forward and aft ends.

Other Aspects of a Ship’s Hull Design

There are other aspects of a ship’s hull that play a major role in the performance of the ship at sea. Calculation of bare hull resistance is an important step in determining the energy efficiency of the hull. Methods to calculate the bare hull resistance have been discussed in detail in this article .

Another vital aspect of the ship’s hull is its watertight integrity. To ensure this, the designer must ensure the intact and damaged stability of the ship. To know about the stability aspects of a ship’s hull, you are advised to read the articles on intact stability and damaged stability of a ship. The article on subdivision of a ship’s hull discusses how the number and position of watertight bulkheads are decided during the design of a ship’s hull.

Vibration and dynamic response of the ship’s hull is a factor that determines not only the performance of the ship but also its longevity at sea. Out of all the different vibrations on a ship, vibration of a ship’s hull girder is of major concern. A ship with unwanted levels of vibration could possibly be a scrapped project right in its initial years. Read this article to know more about the types of vibrations on a ship’s hull, the sources of excitation, and design measures taken to minimize the level of hull vibrations on board.

Disclaimer:  The authors’ views expressed in this article do not necessarily reflect the views of Marine Insight.  Data and charts, if used, in the article have been sourced from available information and have not been authenticated by any statutory authority. The author and Marine Insight do not claim it to be accurate nor accept any responsibility for the same. The views constitute only the opinions and do not constitute any guidelines or recommendation on any course of action to be followed by the reader.

The article or images cannot be reproduced, copied, shared or used in any form without the permission of the author and Marine Insight. 

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Soumya is pursuing Naval Architecture and Ocean Engineering at IMU, Visakhapatnam, India. Passionate about marine design, he believes in the importance of sharing maritime technical knowhow among industry personnel and students. He is also the Co-Founder and Editor-in-Chief of Learn Ship Design- A Student Initiative.

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10 comments.

What is the ship’s six degree of rotations ? Please explain with graphical representation.

Explain about Ship’s Six degree of rotation with graphical representation.

same as an aircraft (it’s easier to find graphical representations). Roll (port-starboard lean), pitch (fore-aft vertical movement), yaw (port-starboard turn, or horizontal movement)

Dear Sir I’m QA Engineer trying to organize PPT Presentation for New QC Team joined our Shipyard to familiarize with various ships/vessels and platforms Hull and main components. Appreciate if you can lend your help.

Thanks Ganesan Singapore

Representing the ship’s body in a system of immobile coordinates (Oxyz), exist 6 degree of liberty, in witch the vessel will move only in flotation stage, as follow: a)- 3 translation moves on the axis X;Y;Z, b)- 3 rotation moves on the axis X;Y;Z, noted as (x); (y) and (z). Considering movements (X);(Y) and (Z), observe that vessel is in indifferent equilibrium, because: – around axis (Z) is vertical move which represent medium draught variation, and a rotation move which represent the ship’s gyration; situation of stable equilibrium; -around axis (X) and (Y) are manifesting the pitching and rolling moves, when the ship is in unstable equilibrium.

For the students of the first year of teaching? Too narrow point of view///^(

Hi sir. I need resources for research about fore and aft bodies plane. Could you help me please? Thanks Mehrdad Iran

Hi could you please update the figure 5 replacing the right picture title with “body plan” instead of “half breath plan”. Thanks.

frame numbering of structure how to name aft of aftwe-perpendiculer

Thank you Sir for sharing your quite obvious passion of ships.

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Ship Hull Construction – Main Plans And Drawings

One aspect of a ship that needs additional consideration during design and construction is the hull. In the development of naval architecture, hull designs have gone from cylindrical wooden shanks to steel columns over time. To provide higher structural strength, engineers have been improving hull designs regularly.

A variety of forces are working simultaneously on the hull since it is constantly in contact with water. Additionally, a hull needs to be very resilient and durable to avoid structural damage in the event of a collision or grounding. One aspect of a ship that needs additional consideration during design and construction is the hull.

In order to construct a ship’s hull, naval architects use a variety of techniques depending on the ship’s function and design. We shall examine the fundamental ship hull designs that are frequently employed in this post.

Types Of Drawings Used For Hull Construction

From the initial design to the final production phase, naval architects and engineers use different sets of drawings to model the complex 3D hull shape such that each and every aspect of it is available as a plan with which the design can be brought into reality.

We shall look into some of the most important drawings used for the construction of a hull.

The Lines Plan

The three simplest drawings of a ship are its lines plan, which depicts the main shape of its hull. It consists of the body plan, the waterlines or half breadth plan, and the profile or sheer plan. The line plan only represents the 3D shape of the shape and does not include any construction details.

The line drawing is made up of projections of the points where the hull and several planes cross. In each of the three dimensions, the planes are equally spaced (x, y, z). The planes in one dimension will be parallel to the planes in the other two dimensions. The sets of planes are referred to as being orthogonal or mutually perpendicular planes.

Placing the ship inside a hypothetical rectangular box with sides that just touch the ship’s keel (baseline) and sides can help you understand how a “lines drawing” works. Three orthogonal projection screens will be based on the box’s front, side, and bottom.

These screens will be used to project lines onto. The intersection of the hull with planes parallel to each of the three orthogonal planes results in the projection of the lines.

Let us learn more about the three main drawings of the lines plan to understand more.

The Half Breadth Plan Or The Waterline Plan

Consider a hypothetical box. A base plane is a reference plane that is located at the bottom of the box. Typically, the base plane and keel are parallel.

Now, Imagine at regular intervals a set of planes parallel and above the base plane. Each plane will cross the hull of the ship at its intersections, forming a line. These lines, which are referred to as “waterlines,” are all projected onto the “Half Breadth Plan,” a single plane.

Every waterline displays the true shape of the hull from the top perspective at a slight elevation above the base plane, making it possible to use this line as a guide when building the ship’s framing.

The waterlines being discussed here have nothing to do with where the ship floats in reality. These waterlines are where a hypothetical plane above the base plane meets the ship’s hull.

This waterline, also known as the “Design Water Line or Loaded Waterline,” will be one plane above the base plane and correspond with the ship’s usual draft (or draught) . Actually, the ship floats above this waterline. In designs, the design water line is frequently denoted by the letters “DWL,” and “LWL”.

Stations or sections are the planes that run port to starboard parallel to the front and back of the hypothetical box. Typically, a ship is divided into 11, 21, 31, or 41 uniformly spaced stations. There will be more stations built as the ship gets bigger. An even number of equal blocks are created between an odd number of stations.

The symmetry of the ship is used to the body plan’s advantage. Because the opposite half of each section is identical, just half of each section is drawn.

By custom, the sections in front of the middle of the ship are drawn on the right side, while the sections in the back are drawn on the left. The amidships region of the body design is often depicted on both sides. The centerline is the vertical line in the middle that divides the ship’s left and right halves.

Each section line depicts the hull’s true shape from the front view at a specific longitudinal position on the ship, making it possible to use that line as a guide while building the ship’s transverse framing. At the points where each station plane intersects the ship’s hull, a curved line is created.

These lines are collectively referred to as “sectional lines” or “sections,” and they are all projected onto the “Body Plan” plane.

Sheer Plan Or Profile Plan

A centerline plane is a plane that passes right down the middle of the ship, parallel to the sides of the fictitious box. It travels from bow to stern. Imagined at regular intervals from the centerline are a number of planes parallel to one side of the centerline plane.

Each plane will cross the hull of the ship and create curved lines when it does. These lines are collectively referred to as “buttocks” and are all projected onto the “Sheer Plan.”

When viewed from the side and offset from the ship’s centerline, each buttock line reveals the true contour of the hull. This enables them to act as a template for the longitudinal structure of the ship.

The “profile” of the ship can be seen in the centerline plane as a unique buttock. The sheer line aboard a ship is the inspiration behind the name of the sheer plan. The upward longitudinal curvature of a ship’s deck is known as the sheer line. The vessel’s attractive aesthetic quality is due to its sheer line.

Shell Expansion Plan

This shell expansion plan provides details about the shell platings and how they are to be arranged so as to form the 3D shape of the hull. The ship’s hull shape is depicted as a three-dimensional surface in two dimensions. It is created using the ship’s line plan, with the contour lines built straight on the baseline to reflect the length of the ship.

On the length of the baseline, the contour lines on the lines plan are positioned at corresponding stations denoted by corresponding frame numbers. The shell expansion outline is created when a continuous line connects the ends of the vertical lines on the baseline.

The surface is then appropriately marked with parallel lines that are both vertical and horizontal, and they are aligned such that they exactly match the number of strakes that make up one-half of the hull surface.

Construction/production plan

The production plan is used to provide the production engineers and fabricators with the full details with which they can make the design a reality.

The production drawings help in enabling the full construction of the hull in a sequential format that best aligns with the build processes. The production drawings and information are basically a detailed explanation of the 3D Hull model. This comprises 2D drawings, welding schedules, structural assembly, plumbing, mechanical, hull outfitting, electrical, and HVAC 3D and CNC data.

Hull construction is both a design and production-oriented complex process which is one of the main functional parts of a ship that determines its various characteristics such as stability, sea keeping, and so on. The design and construction drawings provide the engineers and naval architects with a detailed plan with which they can make the design a reality.

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About the author

I worked as an officer in the deck department on various types of vessels, including oil and chemical tankers, LPG carriers, and even reefer and TSHD in the early years. Currently employed as Marine Surveyor carrying cargo, draft, bunker, and warranty survey.

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Design process

Getting to know you and what is important for you is the foundation on which we build your dream boat.

The process of yacht design

Drawing is just a small part of turning your ideas into a final design. Most of it is talking, asking questions, thinking, more talking and coffee. A lot of coffee.

Yacht exterior design

Using existing hull designs can shorten the lead time of your build. Hulls we’ve built before can be perfectly suited for your ideas. Drawings can simply be ordered form the naval architect and building can commence relatively quick.

Yacht interior design

Knowing the hull type, we can take your wishes and put them in a preliminary design. This will be a rough outline of the general arrangement (interior layout), a sail plan, a lateral view of the underwater hull and a deck plan. Together with you and the architect we can tweak this plan into a final design.

Building a yacht: step by step plan

After a first contact, usually by phone or email, we make an appointment to meet. Face to face, digital or in the flesh, whatever works best for you. We discuss initial ideas, wishes and demands. We want to get to know you and your sailing intentions, so we understand what kind of boat fits you. When we have a general idea of the intended use, the size, hull shape, riging wishes and budget, that’s when we can start sketching.

We draw our initial designs. Top view, side views. These sketches form the basis of a process that can take anything from a couple of months to more than a year. There are many choices to make. Shape, size, exterior, interior, materials, number of masts, huts, bunks, engine type to name but a few. After each round of schetches we discuss the design choices. If they meet your demands and how they have consequences for other wishes you might have. You can imagine how for example the choice for a lifting keel influences the interior.

Prepare for building

When the final drawings are approved, we can prepare for hull building. This means breaking the whole design down to a list of all parts and materials and checking their availability. Once we know when we can have all necessary materials, we can start planning the build.

Start building

The day we start the actual build is a special day. If in any way possible the owners are present and get the honour of performing the first weld, much like laying the first brick when building a new house.

When the hull is done and it is time to build the interior, we ideally make a mockup of the yacht so we can walk you through it and make sure we’re still on the right track.

Finishing touch

Now it’s time for finishing, electricity, plumbing and then we’re ready for testing.

Before we can launch, we test everything inside. Water, plumbing, lights, gas and if everything works as planned, it time to launch.

Launch time

The moment everybody has been waiting for. Launch time. But the yacht isn’t finished yet. The next step still holds some essential parts.

Getting ready

Mast, rigging and sails are installed. Quite important for a sail boat.

Now your yacht is ready to be tested in the field, or in our case in the lake, the IJsselmeer.

And then, after what usually has been more than 12 months, we can hand the ownership officially over to you.

A custom built yacht is unique. There is no way of knowing beforehand that what looked like an excellent idea on paper, will perform exactly as expected. So after a few weeks of sailing, you might want to come back and have some options changed or added. This is normal and we still have to build our first yacht that doesn’t need a few tweaks here and there before its new owners are perfectly happy.

Questions about the design process?

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A Quick Guide to Boat Hull Shapes

Are you new to the boating scene and in the market for a boat? Then you’ve probably heard that you need to consider the types of boat hulls. All boat hulls can either ride on top of the water or displace it while traveling. However, there’s more to them than simply how they move vessels on the water. When making a decision  to buy a boat , you can choose from many different shapes that all have their own advantages.

Learn more about the different types of hulls on a boat with this quick guide and how to choose the best one for your needs.

Most Common Types of Hulls

The different boat hull types will determine the amount of available storage on board, the way the boat handles the water and the boat’s speed and stability. Boat hulls can be divided into three main categories:

• Displacement hulls:  A displacement hull is any hull that helps boats cut and move through the water by pushing it aside. The weight of the boat is its displacement in the water, making those with displacement hulls slow but steady while being able to carry a lot of weight. Boats with displacement hulls include large ships, like cruise ships and sailboats.
• Planing hulls:  A planing hull is designed to rise up and ride on top of the water rather than push through it like displacement hulls. It allows vessels to glide smoothly over the water surface, enabling them to reach higher speeds. This hull shape is typically found in speedboats and powerboats.
• Semi-displacement hulls:  This type of hull gives the boat the advantages of both a displacement and planing hull. A semi-displacement hull has both round and flat sections. The round sections allow for additional storage while the flat areas raise the boat’s bow out of the water, reducing drag at higher speeds. Some cruising motor yachts have hulls like this and they typically require a high-powered engine.

Below are the most common hull shapes and how they affect performance. They may fit into one or a combination of the three categories.

Round-Bottom Hulls

Hulls with a round bottom are a type of displacement hull. They are designed to roll with the waves. Canoes  have round-bottom hulls , as do most sailboats. The rounded shape creates little resistance, allowing boats to move smoothly through calm and rough water at slower speeds. However, they can be unstable and easily roll over.

Flat-Bottom Hulls

A flat-bottom hull is a type of planing hull, which means it can glide on top of the water. Water vessels with this boat hull design are very stable and only need a small engine to start planing. However, they are very rough in inclement weather and choppy waves — they are best for calm waters like lakes, ponds and shallow rivers.

Multihulls are recognizable as two or more distinct separate hulls on one boat. These hull types fall within the displacement or planing hull category depending on their shape and the size of their engines. Because of its wide beams, multihull boats like catamarans and some sailboats are faster and more stable than monohull types. Although they handle rough water well, they have a large turning radius.

V-Shaped Hulls

V-shaped or V-bottom hulls fall within the planing category. They’re a common hull design for speed boats, especially those meant for recreational activities. V-shaped hulls allow for smooth rides at high speeds and perform well in rough or choppy water conditions. These boats do require larger engines and are less fuel-efficient than other hull designs.

People have modified V-shaped hulls to compensate for their instability while floating:

• Modified-V hulls:  The modified design provides more stability without losing too much speed. It combines the best characteristics of the deep V-shaped hull with a flat-bottom section toward the stern.
• Stepped hulls:  These  contain steps or indentations  that force water flow to separate from the hull. This design reduces water surface contact the faster it goes, creating less drag.

Pontoon boats float and ride on long buoyant tubes that usually consist of aluminum. Sometimes considered a multihull type, these planing boat hull shapes maximize the available deck space. Pontoon boats are stable and operate at low speeds on calm water. However, they have a large turning radius, like other multihull boats.

Choosing the Right Hull Shape

It’s important to match the hull shape to your next boat’s purpose and preferences. Here are some questions to consider:

• What type of planing hull handles rough water the best?  V-shaped hull boats are the best for navigating smoothly through rough water conditions.
• What type of boat is most cost-effective for fuel?  Boats with flatter hulls are more fuel-efficient at lower speeds, making them suitable for long-distance cruising.
• Which hull type will give me the most speed?  V-shaped hull boats are best for the waves and rough waters out on the ocean, while flat-bottom boats work well on calm water bodies.
• What boat type is best for any type of water?  Boats with V-shaped hulls offer the most versatility and practicality. 

EZ Dock Boat Docks Can Make Docking Any Watercraft Easy

You can dock any boat easily by using  one of our EZ BoatPorts  — whether for recreational or business purposes. Our floating docks are durable, slip-resistant and modular and require little to no maintenance. The  Optimus BoatPort is customizable  to fit different boats and can easily accommodate stepped hull boats with our optional Stepped Hull EZ Slide.

Request a quote online  for details on the products you’re interested in. Not sure what is best for you? Our dock experts can help you find a configuration that fits your needs and budget.

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