Floatation Foam in Closed Space

[76SeaRay]

The factory originally left all the stringers pretty much floating on the inside. They relied upon the foam to just flow under all the stringers and fill all the spaces. That allowed the floor to bow up under the pressure from the foam. I am bedding all of the new ones.

In the motor mount area I have brought the stringer down to the hull for better strength for the floor and motor mounts but that creates an enclosed space under the motor mounts. What is the best way to fill the space with foam, that is, best way to drill holes as fill ports? I don't like the idea of drilling from the top or the inside step. Thinking about just drilling a hole from the outboard side of the stringer and in the front and back to fill from each. I would drill a vent hole in the middle to make sure it fills.

Two other questions, what is the best foam to use and how much would be needed to fill the cavity 4 inches by 6 inches by 42 inches long?

Thanks...

 

[solar7647]

The best way to fill a cavity is to use a hole saw. Drill 2 holes with the hole saw and keep the plugs. One is a vent one is to poor. Once filled and dry, cut away any foam that came through the holes so that you can put the plug back in the hole. Make up some peanut-butter, that's resin mixed with cabosil fumed silica it will get thick like peanut-butter. Then apply it to fill in the space around the plug and the hole. Once it dries sand smooth and put a small fibergass patch over it for good measure.ake sure you do not breath in the silica, it's very light and fluffy and will damage young lungs.

The best foam to use is going to be 2 part 2lb density floatation foam. A quart kit will fill 2 cubic feet a gallon kit will fill 8 cubic feet but that can vary because of humidity. Higher humidity makes it set up fast and doesn't allow it to expand. Alot of marine stores will carry the Total boat brand of foam.

Hope that helps.

 

[todhunter]

Why don't you want to drill fill/vent holes from the top? That seems like the easiest way to me. Honestly you'd probably be OK leaving that small volume under the engine mounts without foam.

I used the US Composites 2 lb density expanding urethane foam in my boat. If you use it at ambient temps above 80F, it's supposed to expand 25-30 times the liquid volume. 6x4x42 is ~0.6 cu-ft, which is ~575 fl-oz. Assuming you get 25x expansion, you'd need 23 fl-oz of liquid foam (11.5 oz Part A and 11.5 oz Part B). I did mine in the cold months, but in a heated garage which I tried to get as hot as I could (>90F). I think I got about 20x expansion out of mine. Maybe the cavity I was pouring into wasn't as hot as the ambient air temp, which would hinder my expansion.

 

[JASinIL2006]

I think with the two-part urethane foam (like that from US Composites), temp is more of a factor than humidity. I did my foaming when temps were cool, so I heated the cavity to be filled and the hull, to make sure I got good yield of the foam mixture.

 

[76SeaRay]

Thanks for all the technical info. That helps a lot. Thinking of just drilling a holes in the inside of the box stringer to minimize the number of holes from the top. The fill holes are how the water got into the boat originally, but then, the sealing job from the factory was pretty sloppy work.

 

[76SeaRay]

Additional question relative to the fill holes. How far apart do you typically drill them? For example, I have a cavity that is 52 inches long and15 inches wide. The height on the inside stringer side is about 10 inches and the on the outside stringer is about 4 inches. I understand the part about drilling the holes on the "uphill side of the cavity so that the foam flows down to the center and expands up. I also understand about elevating the bow so the foam flows to the back of the cavity. Do I put a hole at the aft end of the cavity and one at the fore end of the cavity or one front, middle and back?

Thanks...

 

[kcassells]

One hole at each end. Place boat on a tilt to help gravity flow. @1.5-2" holes will work.