Okay, I see all the time on here where people are using 20 ounce bottles to "stretch" their flotation foam in their rebuilds. I have even seen several times where people use Alka Seltzer in the bottles to repressurize them before foaming around them. And I even see people who use the single part household expanding foam (like Great Stuff) even though it is not approved for, nor intended for, marine use because it is "cheaper".<br /><br />Well, I decided to do a little calculating to see if any of these things were a good idea. I will put the majority of the calculations at the end...if you believe my statements you can read through the arguments easily...if you don't believe them, then skip down to the bottom and you can check my math. Nobody is perfect, but I think I checked everything pretty close.<br /><br /> Great Stuff??? <br />So, first I will address the Great Stuff foam. A 16 ounce can will make about 1/2 cubic foot of foam (see calculations below) and costs about $5. Therefore, Great Stuff costs about $10 per cubic foot. For ease of comparison, we can say that it will cost $80 to make 8 cubic feet of foam. Also, we can see that it takes about 2 pounds of Great Stuff to make a cubic foot of foam, so it can be compared to 2 pound marine foam. Well, except for the fact that it is neither approved for nor intended for marine use. Also, Great Stuff is a one part foam that uses moisture in the air to cure. It will not cure properly if used to fill large voids unless applied in layers. In fact, it is not intended for voids larger than ½ inch and is not intended to provide structural support. It is water resistant but it is not waterproof and will eventually absorb water.<br /><br /><br /> Marine Foam <br />On the US Composites web site, you can get an 8 cubic foot kit of two part 2 pound marine foam for $48.00. This works out to $6 per cubic foot. Shipping adds about $1.50 per cubic foot for a total of $7.50 per cubic foot. <br /><br />You can get two four cubic foot kits, for a total of 8 cubic feet, of two part 4 pound marine foam for $96. This works out to $12 per cubic foot. Shipping adds about $3 per cubic foot for a total of $15 per cubic foot.<br /><br />Both of these foams will cure properly in large voids because they are two part foams. The 2 pound foam, though water resistant, will eventually absorb water. The 4 pound foam is much denser and is considered to be waterproof.<br /><br />In addition, the 4 pound foam is MUCH stronger and provides nearly as much flotation compared to the normal 2 pound foam. Check out the web site and they give all the tensile and compressive strengths.<br /><br /> In any case, we can see that it is actually cheaper to use a proper 2 pound marine foam than the Home Depot stuff. It is only a little more expensive to use the much stronger 4 pound foam. <br /><br /><br /> Bottles to "save money" <br />Now we come to the widespread practice of putting bottles in with the foam to "save money" on foam. There are about 7.5 gallons in a cubic foot. Even if we assume that each 20 ounce bottle actually displaces 24 ounces (since they are never completely full), it still takes 5.5 of them to make a gallon and about 41 to make a cubic foot.<br /><br />If you use the Alka Seltzer method of placing a bit of water and an Alka Seltzer in each bottle to repressurize them it really starts to not make sense.<br /><br />Alka Seltzer cost about 5.49 for a 36 count box which works out to a little over 15 cents each. Therefore, to pressurize the 41 bottles would cost $6.26. In other words, it would be cheaper to use 2 pound marine foam than to pressurize bottles with Alka Seltzer (not counting shipping).<br /><br />Not to mention the fact that a resealed bottle probably isnt going to hold pressure anyway. If it doesn't hold pressure, then the bottle provides very little structural support, and could lead to structural failure of the foam around it.<br /><br />Even if it does hold pressure, then you have to worry about the bottle expanding and contracting with changes in temperature. This expansion and contraction would cause voids in the surrounding foam which could collect water and also cause a loss of structural support. Even a bottle that was not repressurized would expand and contract if it had a good seal from the air inside.<br /><br />Generally people use liquid nails or the equivalent to hold the bottles in place prior to foaming around them. Liquid nails costs $2 a tube and it just so happens that it would take almost exactly a tube worth to put two quarter inch beads on each of the 41 bottles (I can prove it if I have to, but the math is getting a little boring). What? You dont believe me
well fine...32 feet of ¼ inch bead per 10.5 ounce tube. 32 X 12 = 384 inches. 384 inches divided by 41 bottles yields 9.36 inches per bottle
which is just about exactly how many inches it would take to put two beads down the sides of the bottles.<br /><br /> So it ends up costing $8.26 per cubic foot to use repressurized bottles held in place with liquid nails. This is more than the cost to use a 2 pound marine foam. <br /><br />==================================================<br /><br />Okay, I apologize for this being so long winded, but there were a lot of calculations and it couldn't be condensed too much more and cover all the points that needed to be addressed. The conclusions that I reached are just my opinion, but at least I have figures to back that opinion up. I, for one, will be using 4 pound foam without any bottles, in case you haven't already guessed. 
<br /><br />Just my $0.02 worth,<br />Joe <br /><br /><br />=================================================<br />Great Stuff calculations<br />A 16 ounce can of Great Stuff produces 335 feet of a 3/8 by ½ bead (this information came from the manufacturer's web site Great Stuff FAQ ). Therefore, .375 X .5 X 335 X 12 = 753.75 cubic inches.<br /><br />1728 cubic inches in a cubic foot. 12 X 12 X 12 = 1728.<br /><br />753.75 divided by 1728 yields the fact that a can of Great Stuff produces .44 cubic feet. So lets give them the benefit of the doubt (and make the math easier) and say that a can of Great stuff will make one half of a cubic foot of foam.<br /><br />A 16 ounce can of Great Stuff costs $4.97 a can, lets call it $5 for ease of calculation.<br /><br />Therefore, Great Stuff costs $10 per cubic foot of foam.<br />==================================================<br /><br />1728 cubic inches in a cubic foot<br />231 cubic inches in a gallon<br />7.48 gallons per cubic foot ( I know it sounds like a lot but it is true)<br />128 ounces per gallon<br /><br />128 / 24 = 5.5 bottle to make a gallon<br /><br />5.5 bottles times 7.5 gallons = 41 bottles per cubic foot
<br /><br />Just my $0.02 worth,<br />Joe <br /><br /><br />=================================================<br />Great Stuff calculations<br />A 16 ounce can of Great Stuff produces 335 feet of a 3/8 by ½ bead (this information came from the manufacturer's web site Great Stuff FAQ ). Therefore, .375 X .5 X 335 X 12 = 753.75 cubic inches.<br /><br />1728 cubic inches in a cubic foot. 12 X 12 X 12 = 1728.<br /><br />753.75 divided by 1728 yields the fact that a can of Great Stuff produces .44 cubic feet. So lets give them the benefit of the doubt (and make the math easier) and say that a can of Great stuff will make one half of a cubic foot of foam.<br /><br />A 16 ounce can of Great Stuff costs $4.97 a can, lets call it $5 for ease of calculation.<br /><br />Therefore, Great Stuff costs $10 per cubic foot of foam.<br />==================================================<br /><br />1728 cubic inches in a cubic foot<br />231 cubic inches in a gallon<br />7.48 gallons per cubic foot ( I know it sounds like a lot but it is true)<br />128 ounces per gallon<br /><br />128 / 24 = 5.5 bottle to make a gallon<br /><br />5.5 bottles times 7.5 gallons = 41 bottles per cubic foot
