Help!! Foam or Ply Bulkheads

There's nothing wrong with a foam bulkhead if it is the right kind of foam (that doesn't absorb water). A closed cell PVC foam will work wonders and if you slab each side with a layer or two of fiberglass, you'll have modern bulkheads. Plywood is cheap, stronger, easy to acquire, but rots - it will rot especially fast if not COMPLETELY sealed. I would use some good closed cell foam to construct replacement bulkheads and fiberglass them in place.

Thanks for the quick reply Jake I have some foam from a coolroom that is 6 inches thick I can cut it down to whatever width. The current bulheads are about 2 inches, in your experience what width would you go with?
regards <img src=

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2 inches is a LOT. I would probably go 1/4

foam with a fiberglass skin on each side. Note that the foam needs to be very stiff - even brittle. Regular insulating foam probably does not have the most desireable strength features and would be too flexible to provide proper structure as the center of a composite lamination.

In a sandwich construction like this, the foam performs a similar function as the webbing in an

I" beam. You can think of it like this - Fiberglass is strongest when it is in tension ... i.e. being pulled. Like most materials, it's not very strong when being pushed together from it's ends (compression). If you have a sandwich construction, and try to bend the assembly, the inner fiberglass layer is in compression where it is unable to resist very well. However, the foam helps transfer this loading to the outer skin and because of the sandwich arrangement, the outer skin is in tension - it's strongest point. The outer skin will support the entire assembly. If you bend it in the opposite direction, the loading on the skins is opposite and the unit is stiff in both directions.

The properties of the foam are important for this transfer of stress to occur properly. The foam's duty is to handle a good bit of shear stress as it prevents the two skins of fiberglass from sliding in relation to each other. If they slide, the tension of one side is no longer supporting the compression of the other and the compression side will buckle leading to failure of the entire thing. In order to adequately handle this shear, the foam needs to be stiff...and waterproof to keep from getting soggy.

Jeff,
In a typical ply hull the choice of ply or foam usually depends where the bulkheads are in the boat.
Usually the bridle (on a Stringray it would be where the forward beam is), Main beam, Rear beam and around the centrecase are ply.
Only use gaboon or Okume ply to BS1088 standards and coat in two coats of epoxy and the bulkheads will not rot. When treated properly with expoxy ply bulkheads will last forever inside a hull. I had a set of hulls filled with water for nearly 7 years and when I took the decks off the inside was still like brand new.

As far as the other bulkheads, they are usually polystryrene foam and 50mm thick. This is beacause the polystryrene foam weighs 17kg per cubic metre and is the lightest by far of the foams. The reason 50mm is used is to provide some floatation. THe polystryrene foam will absorb water over time but if the inside of the hulls are kept ventilated that should not be a problem.

As far as the foam your hulls are built from:- just to give you an idea of weights probably the most common is 80kg per cubic metre although it comes in a range of densities) it can be used in load bearing bulkheads same positions as ply already mentioned but would need a proper glass layup (preferabely under vacuum) to make them strong enough to do the job. So you don't want 50mmm thick foam in that density.

Having said all that I have not gone through a set of Stingray plans but I don't think there is much at all in the way of bulkheads. Partially because with the 3 beam platform some of the hull loading is taken by the forward beam.

I expect your plan to replicate the faom in both hulls will do the job of assisting with some of the compression experinced by the hull sides in rougher conditions and the foam will be lighter than the ply. Sealing the foam with epoxy is a bit of a two edge sword because if water does make it's way in you will find it harder to dry out. It's easy to seal ply properly with epoxy but not so sure about the polystyrene foam you are considering because of the way it is made.

Personally I'd probably check the keel and maybe add another layer of tape if that in an issue and then use foam and make sure the rear hatches are left off when not sailing to keep the hulls ventilated.
Just the way I see it.

Regards,
Phill

Thanks guys both really good replies and I appreciate the input I am planning to tape the hull on the inside then put the bulkheads back in. The foam I took out has been in a warm environment for a week and still feel damp to touch so I wont be reusing that Im really tempted to go with sealed foam. I can keep the hulls perfectly dry as my boats are always stored in the shed with hulls vented .
regards

I have used both foam and ply for bulkheads. After a couple of years I have developed a preference for Ply or carbon covered ply.

EPS foam or Styrofoam is light, cheap, and very strong in compression. It also must be kept dry. It absorbs water and once it absorbs water, its tough to get rid of it. Some sailboarders actually built a giant centrifuge to dry flooded cores. EPS is hard to seal against water because it is soft and so many things dissolve it. Epoxy and the lightest fiberglass you can find works well. 2-3 inch (50mm-75mm) thick foam is about right

PVC or polyurethane structural foam like Jake was talking about will work. It will be heavier an and much more expensive but it will not absorb water 1-2 inch (25-50mm) thick structural foam not core foam is about right

Plywood is light, cheap and strong in all directions. Sealed with epoxy and kept away from UV, it will last a long time. 4mm plywood with medium weight carbon/kevlar vacuum bagged to both side is a hell of a structure.

I vote for plywood.

Thanks Carl youve put forward a good case for the ply it would certainly be a lot easier for me as I already have the marine ply, polyurethane enamel paint, fiberglass resin and tape. I imagine if I did it your way it would be fractionally heavier but I shouldnt need to lift the decks again which is really what Im looking for. Once the gelcoat goes on Im not doing it again. <img src=

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regards