foam for positive bouyancy

i have foam filled a boat in the past with 2 part expanding foam. it worked well but after a while water found its way in and the foam retained it. i would like to foam fill a 4 meter grp runabout i have just fixed up. my question is : what is the most suitable type/brand of foam to buy and is there some sort of drainage sytem i could fit so water will flow out when the boat is tilted up on the trailer. i was thinking of putting a length of slotted pvc pipe along the keel for drainage.

 

Do you need the foram for structural strength or just for buoyancy? Two part foam is generally open cell or easy to break the membranes. True closed cell foam with adaquate membranes is fairly expensive. You don't even want to think about the price of pre-formed macro sphere syntactic and it is most likely too late now anyway. If the spaces are large enough, use empty PETE water bottles with the caps epoxied on...really...less weight, more buoyancy than most foams with zero water retention and easy draining. Make sure they and the air are cold when you glue the caps on and they will not crush.

 

When you pour your foam, line the compartment with a trash bag or or sheet plastic material. this will let you pop out the hunk of foam after it's setup. Remove the plastic sheeting and coat the foam with a few coats of epoxy, then reinstall in the compartment.

If doing this on a wooden boat, you'll want to make some channels so condensation can collect in the bilge. When you pop out the foam and remove the plastic, carve vertical groove, at least a 1/3" (6 mm) deep, preferably 1/2" (13 mm). Carve enough material from the foam block so it will rest on just a handful of foamed "ribs" which will act as "stand offs" to permit moisture drainage, yet not allow much water to fill the voids. Of course, after carving the foam, you'll want to epoxy coat it, to seal the foam up good.

Conversely, you can glue stand offs to the foam and reinstall. The stand offs should be inert, maybe some more of the foam.

Get a USCG approved, two part, pourable polyurethane foam. These are typically 2 pound, non-structural foams, but that's what buoyancy foam is.

 

Here is another option. Use foam beads that are coated in epoxy.

Using a large container put in a little epoxy then add beads and stir so all beads get coated. Dont use any more epoxy than is needed to wet the beads. Place plastic in the cavity and pour in the beads. You can press it down and they will fill any shape.

As PAR said be sure and allow for condensation drainage.

http://www.foamerica.com/SearchResults.asp?Cat=48

 

Good advice from PAR but here is an alternative that I have used on three boats. This only works if the compartments are open where you will be putting the foam

Go to Home Depot or Lowes or any other home improvement store. Buy sheet insulation foam. It is polystyrene, but it is not styrofam. It has much smaller and denser cells and is closed cell foam. It comes in sheets that are 8 ft by 2 ft by 2 inches. There are different brands. Some is pink, some is blue, some is white. It is all the same stuff. It Is also 2 lb density.

Do as PAR said, lay down sheet plastic in the compartment but make sure there is a large amount that can be folded over the foam. Cut the foam into pieces and fit them into the space. Then close up the plastic sheet and seal it shut with a very good tape. I use plastic packing tape with embedded nylon threads.

This puts your foam in a bag. Water can't get in. Neither can gas, bilge cleaners or other things that would normally dissolve styrene foams.

Here is a photo of this on my 18 foot Sea Ray

 

Foam beads, even squished down really good would still leave voids to harbor mold, dirt, mildew and moisture, though would be a cheaper method, given the risks.

 

thanks for the great replys, the foam is only for bouyancy not strength. i think i will go with a mix of the ideas above. i have drums of single pack polyurethane sealer i use at work, i could seal the finished foam blocks with that.

 

What "single pack" will you use? I've yet seen a single part polyurethane paint that was actually waterproof. If it's not, you'll eventually have moisture gain in the foam.

 

It is not paint. It is the waterproofing membrane we use to seal wet areas inour waterproofing business. Great stuff.

 

Coat a piece of balsa say a 3" cube, available at most hobby supply stores with your single pack, let it cure then weigh it accurately. Immerse it in a few feet of water for 30 days, then weigh it again. Base line samples should be included, plus maybe one coated with three coats of BoatCote and see where you stand after 30 days.

 

good idea. our product is made by the company we work for and has passed stringent csiro testing. another test we do when required is to flood test wet areas after we seal them . they must lose no water in a 24 hour period indoors.

 

I like the immersion test as it places the coating under pressure, which simulates long term immersion. I'm currently testing some low cyanoacetate polyurethanes for long term durability for one of the major formulators. The immersion test separates the men from the boys, usually pretty quickly.

 

i have a drum out the back, i will do the immersion test and see how it fairs.