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Cabin Trunk

Building a Cabin Trunk.

If there is one thing worse than a cabin that leaks, it is one that looks like a box.

While it is worth taking pains over the caulking of joints to make them watertight it is just as important to take care over the shape and looks

The structure also needs to be strong enough to withstand whatever the elements throw at it and to take the weight of crew members walking on top.

Oh, I'm just remembered, there is one type that is the worst of all and that's a nasty, fiberglass cuddy dripping with condensation.

Cabin-to-deck joints

Corner posts.

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Cabin to Deck joints

There are two methods of fixing the cabin-trunk to the deck .

Inside the carlines , in which case the callings will need to be shaped or canted to conform to any tumblehome.
On top of the deck, fixed to a sill which is through bolted to the carline .

There are many variations on both methods.

The first method will make for a neater finish on the inside but will be more prone to leaks and the carlings will require shaping before fitting.

The second method should make for a dryer interior and be simpler to construct.

A simple box shaped trunk is simplest form to construct.

However, that box will ruin the look of even the most elegant hull.

A well shaped cabin-trunk will on the other hand enhance the appearance of any hull shape.

Much of the shaping is needed to counter the optical illusions created by building on a shapely hull and deck.

Sides which are vertically built on a sloping deck will appear to slant outwards.

So the sides need to be built slanting inwards towards the top, just a few degrees is all that is needed to offset this optical illusion.

Many boats however are built with a more pronounced tumblehome.

A flat horizontal roof will appear to be hogged, and look hump backed.

So a small amount of sheer curve is needed and the top will look much better if it slopes slightly up towards the stern.

And a curved top to the cabin will not only look good but drain water away much more quickly than a flat top.

The sides also need to curve in a horizontal plane to reflect the curve of the hole not only for looks but to allow for usable side decks.

Some of the most elegantly beautiful designs have only very subtitle amounts of sheer, tumblehome and curve.

It does make for extra work but the finished look will be very much worth the effort.

The sides, front and back of deck-houses were traditionally built from solid planks of wood.

This is an excellent method particularly if you want a bright natural finish.

However, it does have its drawbacks.

First there is the problem of finding suitable full length planks of sufficient width.

Then, there is the problem of bending the plank to fit the fore and aft curve of the carlines .

Because there is the possibility of warping any wide planks will require vertical stiffeners or drifts run through the width of the plank.

When I came to renovate Mignonne's cabin-trunk, the solid mahogany sides had not only bowed but there were one or two longitudinal splits.

The splits were treated by forcing a gap filling glue into them with a fine blade fortunately they were not wide enough to need filling.

To correct the bowing I through bolted 2inch by 1 inch decorative uprights on the inside with thinner slats on the outsides.

Panels can be made up using tongue and groove or splined, but the joints will have to be perfect and well glued to avoid the danger of splitting.

Another method which has successfully been used to build trunk sides is strip planking.

This can look rather good if done properly.

However, getting it right is rather complicated, and the port-light openings cannot be cut until afterwords.

Plywood is the easiest material with which to build and can be most satisfactory.

Just remember that if you spend a lot of time and effort building that cabin you want it to last, so use Marine grade ply and seal the edges especially around any openings such as portlights.

The thickness of the ply you can use will depend on the amount of bending you need to do.

Trying to bend plywood thicker than ½ inch over a short distance will be difficult and it will induce stresses which could result in damage.

Laminating the sides from ¼ inch ply will be much easier and result in a stronger panel.

The next problem is how to join the corners.

It is possible to make a 'box' with simple butted corners but this wont be very strong, any exposed end grain will be prone to rot and it will look cheap.

The preferred method is with a nicely radiused, rabbeted corner post.

The best posts are made from a solid piece but this can be tricky as you will have to allow for any desired tumblehome.

It will be much easier to laminate them up from two separate pieces.

The lower end of the post will have to fit the carling and beam and be beveled to allow for the tumblehome.

The top will need shaping to conform to the slope of the top.

Leave some extra material on the outer radius so that it can be fared to the sides after construction.

Roof.

The cabin-top is another area where the traditional cambered look combines looks withfunction.

But again a certain amount of subtlety and compromise is required.

Too much camber while it will allow more standing headroom below will tend to look too much like a barrel.

And any force pushing down, such as foot traffic will have a tendency to force the cabin sides outwards. This is why church builders had to resort to the flying buttress.

So, a gentle curve well supported at the ends and with supporting beams in-between.

It is unlikely that you will have access to sufficient lumber with the grain running in the correct curve so, forget about sawn beams.

Well made laminated beams can be an extremely attractive overhead feature.

And a cabin-top laminated up from layers of ¼ inch marine grade plywood will have great multi-directional strength.

Mignonne's main cabin had originally been covered with tongue and groove boarding which, had then been covers with canvas.

As the boards, once they were cleaned up looked rather attractive on the inside I retained them but covered the whole on top with painted plywood.

On the aft part which, I had to rebuild I again used tongue and groove as the first layer, for its decorative effect then laminated plywood on top.

Whether you are modifying, restoring or building from scratch, do take pains over the design of the superstructure, dog house or whatever you build on top.

An ugly, boxy, dog kennel stuck on top of an elegant hull will draw everyone's eye including the owners.

I have seen too many that I would love to take a chain saw to.

A well designed cabin will enhance the beauty of any wooden boat and enhance the owner's pride.

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09-12-2011, 12:46
Boat: Shell Swifty 14, & 24' Culler / Bolger cat ketch sharpie	, with a 1/4 ply top. Looks like the braces are from exterior ply. There are I think 3 across the top. They help the shape just fine. But, they sure don't feel beefy to stand on. With the just in front of the , I know my weight will be on it a few times at least. So, I'm thinking a couple cross braces inside, laminated into place, epoxied, blocked at the ends, a few screws. The entire cabin's only about 5' long, and 2-1/2 to 3' wide. I think a couple braces maybe 1" wide x 1-1/2" tall would be good. What should I use, and in what thickness, so that I could simply bend the layer to shape, clamp it, and add another layer? I'd rather not have to steam bend these...

09-12-2011, 13:33
Boat: 1983 Gulfstar 36	. 3 or 4 should do. Laminate in place and brace up with some more of the same to keep the tension while curing. Like a spring holding them up. Just cover the ends of the braces with 3 mill poly tarp pieces. The thicker, the stronger but you will end up with a divit in you Knogin if too thick!

09-12-2011, 13:35
Boat: 1983 Gulfstar 36

09-12-2011, 16:22
Boat: Nauticat	top. For pieces that short I'd start with 3/8" solid stock. If it has a really pronounced camber you might have to down to 1/4", if it's pretty flat you might be able to do 1/2". I would not use ply for this, as half the plies will have the grain facing the wrong way for this application and so will be half as strong for a given thickness. Ply for structural beams is just wrong. I would use a nice sticker dried local timber, not kiln dried garbage from the local store. Don't know what kind of this is, but based on post's of the OP's I have seen I'd guess it's a woody. Even more reason to use decent timber. If it will be finished bright you can do fun things like alternating layers of light and dark timber when laminating the beams, or finishing with a final layer of birds eye or burl or whatever you like that's only 1/4". I' prefer something light but strong for beams and house beams. Spruce, alaskan yellow, port orford, nice VG fir which has been carefully selected and sticker dried, etc. In an application like this you could have fun and go with Alder or Cypress or whatever you think will look good with the present . Here's some laminated beams in my last boat-

09-12-2011, 16:34
Boat: 1983 Gulfstar 36	only because it is easy. The laminated cross section gives it enough (Not the greatest) strength and I assumed it would be painted over as it was an said to be an add on cuddy, My bad because I know how to spell assume and shouold not have.

09-12-2011, 16:37

Boat: Nauticat

09-12-2011, 17:58
Boat: Shell Swifty 14, & 24' Culler / Bolger cat ketch sharpie

09-12-2011, 18:00
Boat: Shell Swifty 14, & 24' Culler / Bolger cat ketch sharpie

09-12-2011, 18:26
Boat: Nauticat	. Also if you aren't using very expensive timber and are happy with regular kiln dried VG fir, which seems perfectly appropriate here to me, you might consider a sawn beam in an application this small. Wastage wouldn't be too severe, and the material is , though the end result is not quite as strong. That camber is severe, you might need to go all the way back to 3/16" for the aft end of the house. Thats a lot of lams and will take time, though it's no big deal. Fasteners will be a pain on the first few lams since you are attaching to 1/4" ply and wont be able to use fasteners, just braces. This means you need to wait for cure times for each lam until you have enough meat to use a t-nailer or stapler for succesive lams, then you can stack them all on at once, using braces again to apply pressure for the final lam to cover up any fasteners. Looks like fun!

09-12-2011, 18:30
Boat: 1983 Gulfstar 36	. Then a clear, bright 1/4 X 1 1/2 strip/batton for an easy fashion statement. The "Add on" cuddy looks like it was original... nice.

09-12-2011, 19:16
Boat: Shell Swifty 14, & 24' Culler / Bolger cat ketch sharpie	and a few improvements. There are things I would have done differently. Alas, it's in my garage now. I've done, seems it would be an even bigger bear to attempt that. I like the trim strip idea, SVA. May need to consider that. If I went sawn, 1" white oak would probably be sturdy enough (judging from the dozens of plans I've studied), especially for such a short beam.

09-12-2011, 20:42
Boat: Shell Swifty 14, & 24' Culler / Bolger cat ketch sharpie

03-06-2013, 21:40
Boat: Shell Swifty 14, & 24' Culler / Bolger cat ketch sharpie	, allows for the operational window frames, provides solid mounting so that the grab rails won't give way from me using them, and in addition, provides a few nice hand-holds and tie-downs inside. By the way, the inside of this thing isn't much more than a pup tent, or a good place to store on a wet day sail. So the minor loss of may be offset by the pull-up bars.

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