I have a 14 foot aluminum boat and I plan to build a fishing deck onto it.  I noticed that under each one of the aluminum bench seats there is some foam. I am assuming it is a buoyancy thing, but is it really necessary?  Does it really effect the amount of weight the boat can handle? If so, could I fill some of the dead space under the deck I plan to build with foam? Thanks  

Yes it's necessary, it makes it a lot easier to find your boat when you forget to put the plug in. That foam isn't there to allow you to carry more weight, it's there in case you have some type of accident your boat won't SINK. Well at least not completely. And to answer your other question you can put extra foam in. When I put the floor in my boat I used the styrofoam sheets between the ribs, then the floor and decks, behind the driver's and passenger seats I installed small compartments (from Cabelas) for tackle and packed small pieces of the styrofoam around them, held in place with spray foam insulation, this not only helped with noise, but helped keep the floor ridgid and one of the compartments serves as a cooler (if using one of the compartments as a cooler install a drain hose before sealing it down with spray foam).

Thanks Flatfish for the info...  

Nothing like being chased back to the boat landing by a storm with rollers coming over the bow. Even with a good bilge pump going it is some comfort to know you are riding in a cork, not in a stone.

Most flotation foams support about 65 pounds per cubic foot.   There is not enough in the average jon to make it float level if has a motor or heavy battery in the back.  It will float, but probably bow up with the motor under water.  If you get true flotation foam it comes as a two part liquid.   You mix them in equal parts by weight, blend  them very quickly (you only have about 10 seconds before that start expanding) and pour them.  The hotter the the area is and the better it's mixed makes it expand more.  They expand 10 fold so if you pour too much into an inclosed area, like under a deck, they will expand and bust something out, the deck, the hulls rivets etc, but something is going to give.  Added weight I probably about two pounds per cubic foot.  It's better to use too little and have to add more a couple of times than to try getting it right the first time and getting too much, then you're in trouble

In my old saltwater boat, we moved the center console 6' to the stern for a tuna fishing set up.  When we did, I had also decided this would be a good time to re-deck the whole thing with 1".  This was a great decision as when we removed the deck, about 1/2 way back we found a broken rib.  Needless to say, we removed every inch of foam from the hull of a 21 ft boat with shovels, knives, you name it.

A. God awful smell

B. stuff makes you itch

C. Just plain nasty work.

Anyway, when it came time to refill the hull with foam, I brought in a guy from a local Marina here in RI to do it on the side after work.

Ben is sooo right.  It is amazing to see how an expanding foam can be so strong.  You would think,ok, it needs somewhere to go but when it's full it will stop expanding.  NOPE!

I guarentee if I had tried to foam the hull myself, I would have blown my boat to pieces.  Literally.

The most important thing to remember when using the expanding foam is to leave it some place to go.  Drill several 2 to 3 inch holes in the deck.  You can always trim off the extra foam if need be, it is real hard to fix busted boat parts.

Good luck,

Tom D.

DON'T depend on holes to relieve the pressure, this stuff doesn't do like a liquid or gas once it starts expanding and relieve it's pressure through a couple of holes.  It's more like an expanding semi-solid, the only pressure a hole is going to relieve is for that that's directly under the hole.  What's not under the hole is still going to push against what ever's restricting it until the chemical action has finished.  If that's three inches more than the space it had to epxand in, some part of the boat is going to be out of place that three inches.  You can pour too much in area under your deck with half the other side of the deck not having any, it can still bust the floor out when it expands up to it and still needs to epxand in that direction, even though theres a couple of feet it can expand to the other side of the deck.   This stuff is wicked if you don't pay attention to what your doing with it.