I thought foam was a no no around tank.
That's a myth that's perpetuated by those that don't know any better.
If it was a no-no manufacturers wouldn't be allowed to use it and the USCG wouldn't endorse/permit it.
There are things that make foaming a fuel tank in place better, the biggest one is painting the tank before it's foamed in. Painting the tank will ensure corrosion resistance should the foam ever become saturated. The paint is a barrier between the foam and the metal tank.
This circular by the USCG and Underwriters Laboratories explains it:
http://www.uscgboating.org/assets/pdf/recalls/BSC79.pdf
Also, 2 part expanding urethane foam doesn't saturate overnight. It takes a long time of constant exposure to saturate expanding foam of 2 lb. density and even longer for 4 lb. density. A lot of people think that foam sucks up water like a sponge, and that's just not true:
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14. Is this foam water resistant?
Yes, but with the following caveat. The foams that we sell are considered closed-cell, which means that each cell that makes up the foam structure is completely closed off from surrounding cells which prevents it from acting like a sponge. It is completely safe for this foam to be in contact with water for hours/days/weeks and even months with no adverse effects. However, it should never be submerged in contact with water permanently. Over a period of years the water contact can begin to soften the foam and cause it to lose its closed-cell status. This foam is designed primarily to be used as an insurance policy in case of damage/holes that could cause a vessel to lose buoyancy. Pinhole sized openings would essentially have no effect on the foam since the amount of exposure is so minimal but you should always make repairs as soon as possible to keep the foam effectiveness as good as possible. This will be the case with all after market closed-cell polyurethane foams and even manufacturer installed foams.
From the FAQ section of USComposites urethane foam section.
What people don't realize when they're digging the wet foam out of their 10-30 yr. old boat is that the boat has been neglected for years and the foam has been constantly exposed to water for years.
I have 16 yr. old foam in my boat that's light as a feather and bone dry, it gets wet every time I hose down and scrub the inside of my boat and every time I forget to put the plug in when I launch my boat... but other than that, my boat is kept covered with the plug out outside in the weather and always has been.
It's your boat and you can build it any way you want, I'm just trying to help... and if it was my boat that tank would be foamed in with 4 lb. density foam and I wouldn't ever worry about it.
