Boat Flotation

My friend and I are in Boy Scouts and we are buidling a boat for the Bathtub Rigatta. We were planning on using 2" thick sheet foam to make twin pontoons for a catamaran. Does anyone know how many cubic feet of foam we would need to sufficiently support 400 pounds?

 

A 4' x 8' sheet of foam is 32 square feet. 2" thick foam, cut into 12" wide strips, 6' long will support 10 pounds (with no margin). This means 330 pounds of buoyancy per sheet, not accounting for the weight of the foam and no reserve margin. Assuming a safe 2:1 safety margin (800 pounds) you need about 2 and 1/2 sheets of foam.

So, if you have 2 and 1/2 sheets of 2" foam and you cut them into 20" wide strips (8' long of course), you'll have a triple decker foam sandwich of 3 eight foot long, 2" foam sheets, per hull. Naturally one sheet will have to be pieced together to make the 20" width, but this isn't a big deal and duck tape should easily handle it.

With two, 6" tall, 20" wide and 8' long stacks of foam, you now have materials for a pontoon boat.

Might I recommend you not make a pontoon boat (if possible). The reason being, you have to make two hulls, then join them rigidly enough to put people on, which may not be as easy as it seems, plus they don't steer or row well.

A single foam hull (same amount of materials) will be faster under oar or paddle, you only have to make one hull and length equals speed, if my assumsions of what you're doing are correct.

Maybe, 6 layers of 20" wide, 8' long foam, with staggered joints to make a long surfboard (12 to 16') would be a better way to go. No hull joining, easier to steer and paddle and duck tape cures all, well at least long enough to win.

 

Par has given good advice. Do not make a catamaran. I would make the boat 24 inches wide for the sake of simplicity and a little added stability. Make the boat 12 to 16 feet long as suggested. Glue the foam together as a 3 or 4 ply laminate. Use glue such as Titebond. Contact cement will not do because it will dissolve the foam.

You can estimate the load bearing capacity by doing a bit of arithmetic. A cubic foot of water will displace about 62.4 pounds. Less the weight of the foam, you can estimate perhaps 58 pounds of flotation per cubic foot of foam more or less. Do use at least twice as much foam as you will need to support the weight of the paddlers and the boat itself. You have called for 400 pounds of total load. If there are two paddlers then they are going to be really big scouts. The boat itself will not weigh much. Somewhere around 40 pounds if you use three complete sheets of 2 inch foam. Three sheets of foam will support nearly a thousand pounds. You will want to make both ends of the boat pointy at the ends, so you will lose some bouyancy when that material is removed. You will still have plenty of flotation so not to worry.

When doing the arithmetic; one cubic foot is equal to 1728 cubic inches. If you do not wish to use cubic feet in the calculations, then consider that one cubic inch will displace 0.03611 pounds of water.

Best of luck with your project. Keep us posted about the outcome.