Replacing foam buoyancy CL16 dinghy

Hello there,
I’m replacing the waterlogged foam in my CL 16 dinghy.It not only provides buoyancy, but it’s also semi structural, it’s between the hull skin and the underside of the cockpit sole. I removed a section of the hull skin. either side and excavated the wet foam.
the existing foam looks like a pour foam but must have been made in a mold, and then installed. I’m going to install some plywood dams to keep a new pour foam core in place, create a central bilge and prevent it running too far forward then reinstall the removed hull skin sections and pour in the foam through some strategically drilled holes.
I’m comfortable with the procedure, but my question is what weight foam to put back in.
Total boat sells a 6 pound foam and from their website,
220psi shear strength
55-80 psi compressive
500 psi tensile
Foams that are above 6 pounds are harder to find and a lot more expensive.
I calculated 6 ft.³ of volume and I removed a wet 76 pounds of foam, 12.6 lbs per cubic foot, but to reiterate, that is wet.
I appreciate this is not a very good way to build a boat, but that’s the way it is and to re-engineer it some other way would be difficult and costly and probably not worth it.
And so, my question is can I get away with 6 pound poor foam. Given the volume a 10 pound foam would weigh 60 pounds and that seems like a lot of weight,
Thoughts?

 

I would think so. Even that seems excessive to me.

What are all the other CL 16 owners replacing it with?

 

I don’t know, I can’t find anything online about it.

 

This might get you started:
https://forums.sailinganarchy.com/threads/cl-16-cockpit.221154/

 

Thank you, I’ve seen that thread. he talks about installing blocks of foam in heat shrunk plastic in order to keep it dry. I’m not sure how that would work because, at least in my case, the foam needs to fit snuggly between the underside of the cockpit sole and the hull as it supports both.
In second picture you can see the underside of the cockpit sole.
The first picture show shows the foam filling between between the hole and the cockpit.

 

Most builders use 2 lb foam or less, many 1.65lb. 4lb foam is considered overkill in the vast majority of boats. The weight of the boat goes up dramatically as you increase the density of the foam.


Unless the foam is used extensively for actual support, the lighter the better, sort of. Lighter foams will absord water quicker, so if you can't seal the foam off thoroughly from water, slightly heavier foam might be better.


Placing a heavy plastic garbage bag in the space and filling it with foam will provide the same support, but also help protect the foam from water. It's not perfect, but it's cheap and could add years to it's useful life.

 

Thank you Ondarvr. in this application the foam is structural. I removed 76 pounds of wet foam, if I put back 36 pounds of dry foam that might be stronger and less prone to water absorption, I’ll be happy. I like the idea of pouring it into a bag although I’m not sure how I could execute that in this application,

 

I've sold foam to, and done tech service for all the major players in the marine industry for the last 30 years that I've been on this side of the business. I haven't seen even a single manufacturer use 4lb foam, let alone 6lb or heavier. There may be a couple of applications where a heavier foam was used for a specific reason, but not in a hull for a situation like this.

That doesn't look any more structural than the typical ski boat, and a sailing dingy doesn't take near the abuse of a power boat.

Do you have specific knowledge that they used a higher density foam in these hulls?

You put the bag in the space or compartment, then pour the foam in it, it fills the space as it expands. It's no different than just filling the cavity.

 

On older Sunfish sailboats, the pour foam glued in the Styrofoam (white expanded polystyrene) blocks between the deck and bottom hull.

 

Which is it?

 

I have never seen the hull cut to replace foam. I have done many repairs and the usual method is to cut the deck or sole. The foam looks like it was made in a mold, because the hull acted like a mold. Is your plan to pour foam and then laminate over it? In that case, make sure that the foam and resin are compatible.

 

Bluebell,
It’s both, I saw the thread, but it doesn’t answer the question and I don’t really understand how his repair method would tie the cockpit sole and the hull together in a structural way.
Gonzo,
I turned the boat upside down to address some osmotic blisters and various scratches and dings. It then became apparent that the foam was waterlogged. Having it upside down, makes it ergonomically much easier to work on, I’m standing on the floor, everything is accessible and at the right height.

 

You just might rethink that approach when you are in the middle of refinishing the outside surface.

 

I might, I’ll have to do a good job putting it back together.

 

I know y'all love your pour foam, but me, seeing you have cut this nice big access holes in the boat, I would go a different way. First I would laminate myself a nice flange all around the hole, on the inside, with some csm and tape. Then I'd go down to the hardware store and get some XPS sheets and a can or two of closed cell sprayfoam (it's used on wells for example). What's left to do is a big puzzle, cut the XPS in as big pieces as can reeasonably fit those spaces, and glue it in with the sprayfoam, making sure to fill everything. Sand the top of the foam with some sandpaper on a stick flush with the hole, then put the cut panels back on the flange with some thickened resin. Feather the outside joint, add some glass tape. Sand and fill to the desired finish level ("not even the fish will see it under the antifouling" to "baby behind, mirror worthy racing stuff").

Why would I go to the trouble of puzzling together all that foam you ask? Because factory sheet XPS is guaranteed to not absorb more then 1% water by volume. Even if the sprayfoam proves to be not as closed cell as advertised and gets waterlogged over time, there's only so much of it in the boat, and the weight increase will be minor, so you'll never have to repeat this repair again.