Foam underfloor floatation

**waterboymuz** · 05-03-2016, 06:15 AM

I was wondering what everyones thoughts were on filling up the voids under the floor of my Allison GT20 with pour in 2pack expanding foam? I know that the foam will absorb moisture over time if there is a small leak or access for water to get in under the floor but after sinking the boat recently(1st time ever in 43 years of boating) I do not want it to happen again.
I have rectified the other issues that contributed to the sinking, much bigger bilge pump, the motor well now drains instead of being a pond at the transom and moving the fuel tank forward. I will also fill up under the sides and bow of the boat with polystyrene to give the boat some extra buoyancy.Any suggestions would be greatly appreciated, thanks.

**olboatman** · 05-03-2016, 06:31 AM

I would never add foam under deck (I remove it during all my rebuilds) Instead I would consider putting inflatable bladders in the large voids. I have had good luck with the bags from commerical milk machines after they are cleaned out. Gary

**XstreamVking** · 05-03-2016, 06:34 AM

Don't do it.......

**FMP** · 05-03-2016, 06:35 AM

I've asked the same , not with an Allison and not many like foam. It does add support and strength to an extent and as well adds sound qualities.

**pyro** · 05-03-2016, 06:45 AM

Some basic math can tell you how much foam volume you'll need to keep it floating while swamped. You'd find that foam under the sides is likely enough to keep it on top of the water.

Always put the bow "out" into oncoming waves when you tie off or anchor.

**olboatman** · 05-03-2016, 06:48 AM

Originally Posted by FMP

I've asked the same , not with an Allison and not many like foam. It does add support and strength to an extent and as well adds sound qualities.

I agree with the strength and support but I would add a grid of light weight composite under the floor for that instead. JMO Gary

**olboatman** · 05-03-2016, 06:58 AM

Always put the bow "out" into oncoming waves when you tie off or anchor.[/QUOTE]

When I was 14 (long long ago!) I tied my dads Glastron 153 I/O bow in on our dock after commin in late one nite. The next day the wind came up and he told me he turned it around at lunch. Then he said "when ya get home tomorrow grab a friend and put your boat(small skiff with 12hp Scott) in the back yard" I said "Great dad when are we gonna paint it" He said " We aren't your off the water for the rest of the season!!!" I have NEVER docked a boat bow in since that day! Gary

**Bodhi** · 05-03-2016, 07:52 AM

The foam would be a bad idea due to the above mentioned problems, however, why not use a trick from the old sea rays, they used to have hollow flotation boxes in the stern on bow riders. I've also seen this technique glassed into bows as well. Keeps the weight down, no absorbtion, and gives the flotation when needed

**transomstand** · 05-03-2016, 10:58 AM

Not a fan of foam either, I prefer to put more effort into "sinking avoidance"

**electricjohn** · 05-03-2016, 11:23 AM

My 1970 boat has buoyancy bottles in the floor and gunnels. Nothing better than air to provide buoyancy. During my on going rebuild of the floor and transom, I told my restoration guy to put the bottles back in the floor (and take a picture). By my displacement calculation the bottles total 670 lbs of buoyancy or about 11 cubic feet. This is more than enough to counter the weight of the motor and battery which are the only two things that would sink. Water weighs 64.3 lbs a cubic foot.