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Is Polyurethane foam good to use for sandwich construction of a hull? If not why and is there an alternative at a comparable cost?
Thanks,

The foams that are used are usually PVC. The core is structural. After the laminates are applied to each side, you may be able to stick your fingers into softer foam and tear it apart. If the core fails, the boat is useles and there really is no fix. Unless you can get authoritative assurance that a particular core material is suitable, I would not use it.

Core construction is usually about twice the cost of plywood and is somewhat heavier.