naticen
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So in my quest to decide how to do foam in a new ski, I got a little carried away with some investigating. I have been in different situations where foam helped as well as where it caused problems, so I wanted to come up with the right answer as if it exists.
I weighed some of my skis ranging from my FX to my 750SX and they average about 330 with an empty tank and shop vacced out. You would think that since a cubic foot of foam displaces a cubic foot of water and straight water weighs 62.4 lbs per cubic foot that you would need well over 5 cubic feet of foam to float a ski, or more accurately, neutrally buoyant. But the motor and even the fiberglass displace water as well.
Fiberglass weighs 115 lbs per cubic foot (looked it up in multiple places), so it would displace 62.4 lbs of water and have 52.6 lbs still sinking it. This means that you only need to displace 52.6 lbs of water to keep it buoyant. This ratio is .46 lbs of flotation needed for each pound of hull.
I'll pretend the rest of the boat is aluminum, which is 171 lbs per cubic foot. That leaves 108.6 lbs to float and gives a ratio of .64.
I hate rounding numbers when the difference between 3 and 5 cubic feet is just the tray, or the tray, nose, and pole foam, but I will do it here. I'm going to say the hull is 120 and the rest is aluminum and weighs 210. I know there are components like the crank, driveshaft, and prop, but I'll say you gotta have air pockets in the motor and hopefully some in the exhaust or hood or somewhere to bring it down to at least aluminum ratios. Multiply those numbers by the ratios and that means that 55lbs of the hull needs additional buoyancy and 134 lbs of aluminum needs it. 134+55=189 lbs that needs flotation. Divide that by 62.4 pounds per cubic foot of water that needs displaced and you get 3 cubic feet of non waterlogged foam. That converts to about 23 gallons of water to keep it near the surface.
I kinda sank a ski that stayed afloat and am glad that it was an easy tow. The thing I hated was when I sank one next to the jetty and it wasn't quite buoyant or a full out sinker and it was a scary situation with waves, currents, a huge investment, and lives all fighting to be #1.
I personally am going to do some more precise numbers for my aftermarket hulls and use water to fill the tray which has footholds and a wide tray and see how much foam it will hold. Then I will decide if I can make it a true floater based on if I need additional foam and can get it in the nose or pole. I'm not sure what to do if it proves impossible, but I think that I can do it.
It may be a little over the top, but hopefully it can help somebody else out too. I never want to be in that situation again of holding a tow rope and having my whole life pulled under/into the jetty. Here's the ratios if it helps anybody else: Steel @ .87, Aluminum @ .64, Fiberglass @ .46. I personally am going to buy some polypropylene, figure out its cubic feet and then just fit it wherever it goes so that it floats. I'll probably also put a drainplug for the engine compartment and one for the tray and try to keep them both sealed.
I weighed some of my skis ranging from my FX to my 750SX and they average about 330 with an empty tank and shop vacced out. You would think that since a cubic foot of foam displaces a cubic foot of water and straight water weighs 62.4 lbs per cubic foot that you would need well over 5 cubic feet of foam to float a ski, or more accurately, neutrally buoyant. But the motor and even the fiberglass displace water as well.
Fiberglass weighs 115 lbs per cubic foot (looked it up in multiple places), so it would displace 62.4 lbs of water and have 52.6 lbs still sinking it. This means that you only need to displace 52.6 lbs of water to keep it buoyant. This ratio is .46 lbs of flotation needed for each pound of hull.
I'll pretend the rest of the boat is aluminum, which is 171 lbs per cubic foot. That leaves 108.6 lbs to float and gives a ratio of .64.
I hate rounding numbers when the difference between 3 and 5 cubic feet is just the tray, or the tray, nose, and pole foam, but I will do it here. I'm going to say the hull is 120 and the rest is aluminum and weighs 210. I know there are components like the crank, driveshaft, and prop, but I'll say you gotta have air pockets in the motor and hopefully some in the exhaust or hood or somewhere to bring it down to at least aluminum ratios. Multiply those numbers by the ratios and that means that 55lbs of the hull needs additional buoyancy and 134 lbs of aluminum needs it. 134+55=189 lbs that needs flotation. Divide that by 62.4 pounds per cubic foot of water that needs displaced and you get 3 cubic feet of non waterlogged foam. That converts to about 23 gallons of water to keep it near the surface.
I kinda sank a ski that stayed afloat and am glad that it was an easy tow. The thing I hated was when I sank one next to the jetty and it wasn't quite buoyant or a full out sinker and it was a scary situation with waves, currents, a huge investment, and lives all fighting to be #1.
I personally am going to do some more precise numbers for my aftermarket hulls and use water to fill the tray which has footholds and a wide tray and see how much foam it will hold. Then I will decide if I can make it a true floater based on if I need additional foam and can get it in the nose or pole. I'm not sure what to do if it proves impossible, but I think that I can do it.
It may be a little over the top, but hopefully it can help somebody else out too. I never want to be in that situation again of holding a tow rope and having my whole life pulled under/into the jetty. Here's the ratios if it helps anybody else: Steel @ .87, Aluminum @ .64, Fiberglass @ .46. I personally am going to buy some polypropylene, figure out its cubic feet and then just fit it wherever it goes so that it floats. I'll probably also put a drainplug for the engine compartment and one for the tray and try to keep them both sealed.