After a lot of years of installing 2 part expanding foam for both flotation and to lock in place fuel tanks, I’m looking into different methods. This is NOT to say that jobs done to this point aren’t right or will suffer rot because I’m a fanatic at saturating any wood, foam or materials to keep water intrusion to a minimum, but after a lot of trial and research, I’m looking at reducing the use of foam used in securing fuel tanks and using it solely for its flotation benefits.
The 2 part polyurethane foam commonly used is most often a 4 pound density- it takes up a lot of space without weighing too much which makes it a good choice for building in floatation. I get asked why the sealed voids and cavities in a boat aren’t sufficient or why foam has to be used instead of something else like styrofoam blocks or ping pong balls cant take the place. In fact both chunks of styrofoam and closed air containing pieces like ping pong balls and swim noodles ARE used but most often in smaller lake craft. The 2 part polyurethane foam though takes up absolutely every cubic quarter of an inch when you pour it correctly into a prepped cavity so you end up getting more float for the space than something loose. The USCG doesn’t recognize a sealed up airspace as counting as flotation because if you punch a hole in it, the space fills up and you’re #screwed. So as far as providing flotation, its great stuff and you’ll know its in various areas of your Highliner. Why the owner of said boat below has foam under the decks and why it isnt taking up all the space, I leave that to other sage knowitalls to comment. Id be more interested to find out how the thing got so wet…
So I’m looking at using less foam around your fuel tank(s). Why? See below…
Tasty, huh? Whats going on here is a mess caused not by the foam but by a junk application- the void space wasn’t completely filled with the foam and so when condensation, inflow from holes being cut and holes being drilled and generally pretending it’s all good led to the wood around the area to soak up the water and rot ensues.
It’s not impossible to be certain that the spaces you pour the foam in are completely filled but its not easy either. The space has to be physically available, the holes for filling and the holes to vent the escaping air and excess foam have to be provided, the foam has to be up to temperature so you get a good yield, and on and on. Problem is that when you use the foam to both provide flotation around and lock in a fuel tank, the air spaces along the interface of the tank are potential sites for condensation and now you got issues. As I use aluminum tanks, I don’t want ANY moisture next to the tank and so I lay in heavy gauge plastic sheeting in the cavity that the tank is going to be installed in. Below the plastic are raised shims that support the tank off the hull and direct any condensation to a low spot or sump. We’re not talking about a lot of moisture but any moisture as far as Im concerned needs to be away from the aluminum tank.
Even before then, I scuff up the tank with some 80 grit and wipe it down with a aluminum friendly etching acid wash that really cleans and scours the tank in prep for a coating of some stuff called coal tar epoxy. This marvel in a couple cans is just what it sounds like- its a thick, black, sticky 2 part coating that looks like what you see spilling out of a roofer’s tar kettle. Just bombproof as far as waterproofing. So we have a tank coated in hardened tar, wrapped in plastic sheeting and sitting on raised, padded strips. Seems like that’d be good enough and I agree, IF the tank takes up the space completely. Thing is, the tank and its fuel load combine to add to the total weight of the boat that has to be countered with at least an equal amount of flotation so your pride and joy floats. To that end you have to add some space around the tank for foam or whatever and build in spaces elesewhere nearby in the hull to place the foam and I call that an insulated fish hold and or composite sandwich panels.
So there’s an inch of foam all around, under and over the tank and thats it. What I’m doing to lock the tank in is providing built in straps or clamps made of solid laminate fiberglass over the tank top and blocking the tank at the ends. The foam isn’t keeping the tank in place, well it is sorta, but not as it has been used. The tanks are mechanically fastened. The result is more work for moi but what it gets you is a moisture damage-free tank install so that years down the road you can look the new buyer straight in the eye and say your tank is in great shape. That you will never have to worry about a tank panel rotting and blowing out, mid channel is a freebie.