You are really better off using your energy to seal against leaks than to minimize the extent of the leak by adding foam. It only takes a small amount of water in the mast to foul up your righting efforts. Think about the amount of water that could be in the places where the foam is not. Eight pounds per gallon. Gaps between foam and mast. Trapped water that may never leave. The possiblity of some of the foam absorbing water. Also, think about the weight of the foam.

Ideally we would have a perfectly sealed mast that was vacuumend or filled with helium.

GARY