Buoyancy foam - what are the options?

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Would value the group's perspective on a little project. I have an older RIB style dinghy with a double floor. The upper floor has some cracks from the boat to bilge, some structural flex around the cracks and I believe water is able to pass through the floor into the bilge area between the upper and the hull. The boat is still able to operated and floats fine even with a bilge full of water, but I would like to figure out a repair to slow/stop any further cracking/deterioration of the floor. If I could get into the bilge, I would add a few supports to prop up the floor from below but looking at an approach that doesnt require cutting out the cracked floor, putting in supports and then re-glassing the whole boat.

My thinking was adding some expanding foam or other additive through the bilge drain hole that would serve as a structural component to support the cracked and slightly sagging floor. If the foam consumed the full area of the bilge that would seem to be an advantage as well from a buoyancy perspective. Not a huge amount of weight that needs to be supported and bilge doesnt have to be 100% waterproof based on current situation.

-What products would be liquid enough to be injected through the bilge garbord plug hole
-Are there products which could provide some structural integrity--the "height" of the bilge is only an inch or two
-Will water flow around the foam or be absorbed by it assuming that I still have cracks that will allow encroachment by water
-Is there a better way to repair this

Thanks in advance for your advice and happy boating to you!

 

Jimjimjim
Welcome to the forum.

All expansion foams absorb water. The expansion foam will NOT flow far from a discreet opening and will expand blocking the opening. Therefore expansion foam will not accomplish your goal.

DO NOT cut the sole(floor) out. You will NOT be able to reinstall it.

Possible solutions
One
Grind off paint/ gelcoat and add several layers of glass to the top of the existing sole.
Two
Grind off paint/galcoat
Bond plywood to existing sole.
Three
Do both and add plywood between layers of glass.

 

@Blueknarr -Thank you for the clarity about how the material--sounds like it doesnt have any of the right characteristics. I don't love the labor to age of boat equation but if it gets any worse that the grind/reglass is the likely path. Figure that the resin will seal the cracks, fill in the low spot and the glass/resin will hopefully provide a bit more structure above to prevent further cave in.

Meanwhile I will keep dreaming of a quick fix that doesn't exist like a balloon that can be inserted and then inflated to pressure to lift the floor.....

 

Intriguing problem that got my imagination working.

My first thought was a coarse threaded fibreglass "screw", like those kitchen cabinet adjustment feet, through a threaded fibreglass "biscuit".




You could pop the "biscuit" through the slot, rotate it to provide interference, and then screw down the coarse thread against the bottom layer.
You could even use a plastic cabinet adjustment leg, to create a mould for the fibreglass "screws".

Then chop the fibreglass screw off level, and epoxy the biscuit and screw solid with the upper layer.

 

Appreciate that you’re activating your brain for my project and love the creativity! Like your biscuit/expander idea and am looking to home improvement as a source of a solution short of a “proper” repair.

I need something like the masts of a ship in a bottle where something mechanical and folded could be slid in and then erected once in the hull.

Or a giant stent.

I’ll fiberglass in the spring if I can’t come up with a “better” solution.

 

with all the "foam knowledge" here:
any comments & recommendations to my floatation foam (extruded polystyrene) installation? The bunktop going over it will be filleted & tabbed to the hull, access later only through a circular 6" & a square opening ~15"x15" each siede (or jigsaw). Should I wrap the foam in thick polythene sheeting before closing compartment?
for the curious: the plastic tube on the stb side is for the log/sounder transducer. The "water-games" when withdrawing necessitating subsequent bilge drying always got on my nerves so much I thought...)

 

I presume the bunktop will be flush with the foam, so I would glue everything in using thickened epoxy. Foam to hull and bunktop to foam. This way there are no airpockets left to expand or condensate and create problems, it's all a solid block.

 

If its going to be exposed to bilge water (and all the other junk, oil, etc, in the bilge) then it needs to be sealed or sheathed. Polystyrene foams can be degraded by petroleum products, bilge cleaners, detergents, etc.

 

Airpockets & condensation - a valid point. Creating tight fits was impossible though, so glueing in with epoxi would need lots of it (heavy & exotherm risk). I might do it with small doses of expanding PU foam though. What do you think?
No bilge junk/water/etc. possible (until hull is holed in this compartment, & then the **** has hit the fan anyway & the foam comes into it's own)

 

I’d probably run some limbers and make a pathway to a dry sump in case any water gets in. Most of the guys on the ply forum put down 6mil poly before the foam in case any petro gets in, lest the foam melt.

I’d skip foam or epoxy. I like a clean install. You might also consider putting in some submersible vents somewhere…they would at least allow some air movement. The problem you WILL have is a small air hole will inevitably allow air into the chamber with moisture. Night will fall and it will condense the moisture and this cycle repeats until the dry chamber has puddles that never evaporate.

 

PU foam is a possibility but it's extremely dangerous, if it doesn't have a clear path to escape it can burst the plywood if you spray it into a void, or it can lift the foam sheet. If you do it, use a closed cell foam (for example Brunnenschaum).

For epoxy I expect the foam has a reasonable fit to the bottom and the voids are mainly around the sides. This means you can glue each sheet individually using a notched trowel. For the voids I would take all the foam offcutts, shred them into small pieces with a knife and mix them with some thickened epoxy (just enough to coat them). Stuff this into the voids, most of the space will be shredded foam not epoxy, it shouldn't be more dangerous then a big fillet in regard to exothermic reaction. You can do it in small batches, no need to fill a void at once.

 

Expanding foam IS a nuisance, and hard to apply around other objects without creating voids and hard patches..
Personally, if the interior of the space is epoxied, and perhaps gets a coat of paint, I would just leave the foam billets in there. They are only of use in case of a capsize, and if that happens, permanently bonded objects with air/water entrapment pockets are a pain.
Just blow some compressed air on there from time to time, to dry out any moisture. You could even use some of those laundry moisture absorption packs.

The problem of a waterproof, fully filled chamber is a real nuisance for small craft. A mate of mine came up with a fantastic solution.
Get yourself a box of expanded polystyrene beads like you use in waterproof cushions, and mix them into Epoxy.
I thought that it would just make a messy conglomerate of sticky beads, but he swears it becomes a light, impervious mixture that just pours where you want it.
I often rely on his past expertise, but a small experiment to make sure it works for you might be in order.