mmd
04-05-2005, 12:08 AM
Comments on another thread has goaded me into expressing some thoughts on the use of pour-in-place foam for hull flotation. In general, this is good stuff, but like every other material, it has its "dark side".
Types of Foam
There is lots of different types of pour-in-place foam. Some are good for marine use and some are not. The ones that are not are that way because they are structurally weak and crumble into dust at the least provocation, or they don't form discrete bubbles in the goo and therefore suck up water like a bathtub sponge, or they are easily dissolved by exotic substances like gasoline or suntan oil. Most commonly, marine-use foams are based on polyurethane goo.
Weight of Foam
Foam is comprised of bubbles in some type of solidified goo. Regardless of what the manufacturer says, water will eventually be absorbed into the bubbles (if water gets into the fabric of a solid fiberglass hull creating water-filled osmotic blisters, what make you think that it won't get into your airy foam?). Manufacturers combat this by formulating smaller bubbles in the goo to slow down the absorption rate, and call it "impervious to water". Unfortunately, smaller bubbles means denser foam, which means greater weight. What this means to you is that if you use a really lightweight foam, it'll suck up water sooner than if you use a heavy one. How much does this foam weigh? Light stuff is around 1.75 pounds per cubic foot and will absorb water up to 10% of its volume, making its weight at the end of a season about 8 to 10 pounds per cubic foot if used in salt water. The denser stuff weighs in at around 3.25 to 4.0 pounds per cubic foot and will absorb about 2% of its volume in water, giving it a year-end weight of around 4.5 to 5.25 pounds per cubic foot.
Perils of Foam
Pour-in-place foam is pretty easy to use, but there are a few characteristics about it that if you don't pay attention can reach out and bite you in the butt - HARD!
1.) There is the problems created by its most wonderful feature - it expands quickly and a lot. If you pour too much in an enclosed space such as the forepeak of your boat, it can and will break its way out if it doesn't have enough room to grow. Blowing out a bulkhead or popping off a deck panel or plank is not uncommon. Read the manufacturer's instructions and calculate carefully how much to mix. It's better to be a little under than a little over.
2.) The chemical reaction that creates the foam also creates heat. Small pours of a cubic foot or so (expanded) just get warm to the touch, but a big pour of several cubic feet can get hot enough to scorch surrounding structures and warp some materials. Get really enthusiastic about pouring a truckload and I'm sure you'll get chance to have a beer with your local fire department guys after they finish hosing down what's left of your shop. If you have a large volume to fill, do it in several small pours.
3.) The chemical reaction also creates out-gassing. This may only be a rather nasty, acidic assault on your probiscus (and sure to get SWMBO cranky about the smell wafting through the house from your basement workshop), but some formulations can be downright toxic. Read the manufacturer's instructions - all of 'em, even the fine print under the cute cartoon of the guy in a respirator - before you start mixing the goop. A respirator, an exhaust fan and ducting, or even an outdoor pour might be required if you want to be able to continue to write your name coherently or breathe unaided in the future.
How Much Do I Need?
Uh-oh. Here it comes - the designer's favourite answer... It depends. It depends on the size and weight of your boat, whether there is a motor, how much gear is aboard, etc., etc. The idea is to calculate how much the boat and all its gear weighs, then figure out the buoyancy of all that stuff (wood boat = very buoyant; outboard motor = not so buoyant). Subtract the buoyancy from the weight to get the displacement of the submerged boat, then add the volume of buoyancy needed to make the boat weigh less than the water it displaces. With simple boats is a simple process; the more complicated the boat, the more you pull your hair out and curse at random number combinations on the helpless pieces of paper on your desk. Here in Canada the Feds publish a book called Construction Standards for Small Vessels that explains this stuff; I'm sure Uncle Sam has a similar book. If in doubt, consult your friendly neighbourhood boat designer.
So there ya go... Enough information on frothy goo to allow you to boldly go forth and make crunchy messes all over your shop, your boat, you, even the cat. The dedicated hobbyist can even swipe the wife's muffin tin and make the mother-in-law a faux chocolate-chip muffin decorative basket using nothing other than left-over goo and the little nuggets from the bottom of the bunny cage!
Have at 'er boys! (But play safely, OK?)
[ 04-05-2005, 01:18 AM: Message edited by: mmd ]
Types of Foam
There is lots of different types of pour-in-place foam. Some are good for marine use and some are not. The ones that are not are that way because they are structurally weak and crumble into dust at the least provocation, or they don't form discrete bubbles in the goo and therefore suck up water like a bathtub sponge, or they are easily dissolved by exotic substances like gasoline or suntan oil. Most commonly, marine-use foams are based on polyurethane goo.
Weight of Foam
Foam is comprised of bubbles in some type of solidified goo. Regardless of what the manufacturer says, water will eventually be absorbed into the bubbles (if water gets into the fabric of a solid fiberglass hull creating water-filled osmotic blisters, what make you think that it won't get into your airy foam?). Manufacturers combat this by formulating smaller bubbles in the goo to slow down the absorption rate, and call it "impervious to water". Unfortunately, smaller bubbles means denser foam, which means greater weight. What this means to you is that if you use a really lightweight foam, it'll suck up water sooner than if you use a heavy one. How much does this foam weigh? Light stuff is around 1.75 pounds per cubic foot and will absorb water up to 10% of its volume, making its weight at the end of a season about 8 to 10 pounds per cubic foot if used in salt water. The denser stuff weighs in at around 3.25 to 4.0 pounds per cubic foot and will absorb about 2% of its volume in water, giving it a year-end weight of around 4.5 to 5.25 pounds per cubic foot.
Perils of Foam
Pour-in-place foam is pretty easy to use, but there are a few characteristics about it that if you don't pay attention can reach out and bite you in the butt - HARD!
1.) There is the problems created by its most wonderful feature - it expands quickly and a lot. If you pour too much in an enclosed space such as the forepeak of your boat, it can and will break its way out if it doesn't have enough room to grow. Blowing out a bulkhead or popping off a deck panel or plank is not uncommon. Read the manufacturer's instructions and calculate carefully how much to mix. It's better to be a little under than a little over.
2.) The chemical reaction that creates the foam also creates heat. Small pours of a cubic foot or so (expanded) just get warm to the touch, but a big pour of several cubic feet can get hot enough to scorch surrounding structures and warp some materials. Get really enthusiastic about pouring a truckload and I'm sure you'll get chance to have a beer with your local fire department guys after they finish hosing down what's left of your shop. If you have a large volume to fill, do it in several small pours.
3.) The chemical reaction also creates out-gassing. This may only be a rather nasty, acidic assault on your probiscus (and sure to get SWMBO cranky about the smell wafting through the house from your basement workshop), but some formulations can be downright toxic. Read the manufacturer's instructions - all of 'em, even the fine print under the cute cartoon of the guy in a respirator - before you start mixing the goop. A respirator, an exhaust fan and ducting, or even an outdoor pour might be required if you want to be able to continue to write your name coherently or breathe unaided in the future.
How Much Do I Need?
Uh-oh. Here it comes - the designer's favourite answer... It depends. It depends on the size and weight of your boat, whether there is a motor, how much gear is aboard, etc., etc. The idea is to calculate how much the boat and all its gear weighs, then figure out the buoyancy of all that stuff (wood boat = very buoyant; outboard motor = not so buoyant). Subtract the buoyancy from the weight to get the displacement of the submerged boat, then add the volume of buoyancy needed to make the boat weigh less than the water it displaces. With simple boats is a simple process; the more complicated the boat, the more you pull your hair out and curse at random number combinations on the helpless pieces of paper on your desk. Here in Canada the Feds publish a book called Construction Standards for Small Vessels that explains this stuff; I'm sure Uncle Sam has a similar book. If in doubt, consult your friendly neighbourhood boat designer.
So there ya go... Enough information on frothy goo to allow you to boldly go forth and make crunchy messes all over your shop, your boat, you, even the cat. The dedicated hobbyist can even swipe the wife's muffin tin and make the mother-in-law a faux chocolate-chip muffin decorative basket using nothing other than left-over goo and the little nuggets from the bottom of the bunny cage!
Have at 'er boys! (But play safely, OK?)
[ 04-05-2005, 01:18 AM: Message edited by: mmd ]