Most of the Ureathane-foams can lose significant amounts of strength when they heat up...however, as far as I recall, the strength loss is not permanent—however the deformation may be. I would highly recommend that you paint any foam-cored boat a light color, as most of the foam lose strength at temperatures far lower than end grain balsa, which is still IMHO a superior core material.
Foam as a core material has some strong disadvantages over end-grain balsa. Water tends to migrate along foam cores relatively easily, plywood cores have the same problem. Balsa cores tend to resist water migration. Foam can also have bonding issues with the laminates on either side, as the foam does not wick the resin and create a strong bond, as does end-grain balsa. Also, most foam core materials have a much lower shear resistance than end-grain balsa. Lastly, most of the foam core materials have a much lower deformation temperature than end-grain balsa.
I have been on boats with foam-cored decks, where there were foot prints left in the glass when the glass was very hot from exposure to the sun and then walked on by a very large crew member. This boat was a fairly dark navy blue however. I don't know if the same thing would happen to a white boat under the same circumstances.
Plywood is an inferior core material as it does not have the advantages of either balsa or foam. It wicks water along the core, can have the bond formation problems of foam, and is significantly heavier than either. However, it is an excellent core material for areas under heavy loads, as it resists compression loads and shearing loads far better than either the foams or balsa.
However, the foam cored boats have a few significant advantages over balsa-cored boats. One, they're usually lighter. Two, foam cores resist rot. Three, in some cases, the foam core materials can be cheaper than balsa.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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