Foam Flotation Foam

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What is the year and length of the boat you plan on modifying? The old regs I am aware of required boats of twenty feet and less to have positive flotation. Above this, it's manufacturer's option. If the foam vendor indicates it supports 60 pounds per cubic foot, one cubic foot of the stuff will weigh only about 2.5 pounds. Hope you aren't looking to gain any structural capability from the foam.