### Flotation for a large boat?

Posted:

**Tue Mar 17, 2015 8:11 am**It doesn't take much foam to provide positive flotation for a small unballasted wooden sailboat. And it's a USCG requirement for boats up to 22 FT, IIRC. With larger, ballasted sailboats this can be a challenge it seems. So much so that positive flotation is usually not used, and is not required by the USCG.

However, I ran a calculation to find out how much foam would be needed to float 2000 pounds of lead, with the Amigo in particular in mind. This is a sailboat that uses space very efficiently, with little room to spare. One might think there is no room for flotation foam in such a boat.

Here's my calculation:

Cube root of 2000/60, where 60 is the number of pounds that one cubic foot of foam will float.

Result is 3.218. This is the size cube of foam that will float 2000 pounds of lead ballast, that is, each side of the cube measures 3.218 feet. Smaller than I first thought.

Assuming that an unballasted bare wooden hull with fiberglass/epoxy encapsulation will float on its own when swamped, one only has to figure floatation for the ballast, rigging, engine, and weight of other equipment and stores that are heavier than water. Suppose in our Amigo example that that would result in a cube of foam 4x4x4 feet.

Now, how would one fit that inside a space-constrained sailboat hull like the Amigo? I propose that once the hull and bulkheads are complete, that it be sprayed in, lining the hull in an even layer. If the inside of your hull is encapsulated anyway, you wouldn't need to worry about ventilating the wood surface. But how thick of a layer of foam would you need?

If I remember correctly, the Amigo has about 300 square feet of hull area. If you're under 6 feet tall you could add foam to the cabin ceiling also. Let's say this gives you 100 additional feet of surface area and you limit ceiling foam thickness to 1 inch.

Then you'd need to figure out how many cubic inches are in a cube measuring 4x4x4 feet. One cubic foot has 1728 cubic inches (12x12x12=1728). Multiply this times 4 cubed, = 110592 cubic inches. Divide this number by the square inches and you get the thickness of foam needed to line your hull. But first, subtract the cabin roof square inches, since that's limited to 1 inch thickness.

100x144=14400 square inches, same as cubic inches for ceiling foam.

110592-14400=96192 cubic inches of foam for hull lining.

300x144=43200 square inches for hull

96192/43200=2.267

So, if you line your Amigo's cabin ceiling with one inch of foam, and your hull with 2.267 inches of foam that provides 60 pounds of floatation per cubic foot, that will be the same as a four foot cube that provides 3840 pounds of flotation, rendering your Amigo unsinkable.

Side benefit: Your Amigo will be well-insulated too.

These calculations could be done for any boat. You just need to know the square footage of the hull and cabin ceiling. This technique might work best in hull forms that use framing timbers, because you can fasten to it attractive interior planking to hide the foam.

However, I ran a calculation to find out how much foam would be needed to float 2000 pounds of lead, with the Amigo in particular in mind. This is a sailboat that uses space very efficiently, with little room to spare. One might think there is no room for flotation foam in such a boat.

Here's my calculation:

Cube root of 2000/60, where 60 is the number of pounds that one cubic foot of foam will float.

Result is 3.218. This is the size cube of foam that will float 2000 pounds of lead ballast, that is, each side of the cube measures 3.218 feet. Smaller than I first thought.

Assuming that an unballasted bare wooden hull with fiberglass/epoxy encapsulation will float on its own when swamped, one only has to figure floatation for the ballast, rigging, engine, and weight of other equipment and stores that are heavier than water. Suppose in our Amigo example that that would result in a cube of foam 4x4x4 feet.

Now, how would one fit that inside a space-constrained sailboat hull like the Amigo? I propose that once the hull and bulkheads are complete, that it be sprayed in, lining the hull in an even layer. If the inside of your hull is encapsulated anyway, you wouldn't need to worry about ventilating the wood surface. But how thick of a layer of foam would you need?

If I remember correctly, the Amigo has about 300 square feet of hull area. If you're under 6 feet tall you could add foam to the cabin ceiling also. Let's say this gives you 100 additional feet of surface area and you limit ceiling foam thickness to 1 inch.

Then you'd need to figure out how many cubic inches are in a cube measuring 4x4x4 feet. One cubic foot has 1728 cubic inches (12x12x12=1728). Multiply this times 4 cubed, = 110592 cubic inches. Divide this number by the square inches and you get the thickness of foam needed to line your hull. But first, subtract the cabin roof square inches, since that's limited to 1 inch thickness.

100x144=14400 square inches, same as cubic inches for ceiling foam.

110592-14400=96192 cubic inches of foam for hull lining.

300x144=43200 square inches for hull

96192/43200=2.267

So, if you line your Amigo's cabin ceiling with one inch of foam, and your hull with 2.267 inches of foam that provides 60 pounds of floatation per cubic foot, that will be the same as a four foot cube that provides 3840 pounds of flotation, rendering your Amigo unsinkable.

Side benefit: Your Amigo will be well-insulated too.

These calculations could be done for any boat. You just need to know the square footage of the hull and cabin ceiling. This technique might work best in hull forms that use framing timbers, because you can fasten to it attractive interior planking to hide the foam.