<<But, of course, a plane does not see anywhere near the forces a boat does. >>

  Definitely.  That airframe is highly unlikely to be stressed by hitting a lot, container, whale, or sand bar, particularly in a location the designer didn't plan for.  Assuming the incident didn't sink the boat, we'd want it to be either undamaged, or have the damage localized, but have the rest of the structure continue to work.

  So the design criteria aren't just bending and shear given sailing forces, but also the ability to remain laminated after an impact that deforms the sandwich.  Some core materials remain stuck to their skins, others are more brittle and can experience large areas of separation.

  I'd probably go with a linear PVC, like Airex R63, known for its impact resistance, even if that meant a tad more weight.   Or Corecell's A series, which is supposed to be more resistant to higher temperatures.

    https://netcomposites.com/guide-tools/guide/core-materials/other-foam-cores/
    https://www.compositesworld.com/articles/getting-to-the-core-of-composite-laminates

  (I saw some great destructive testing photos of delamination of different core materials a few years ago, but I can't find them right now).

  For a 40' proa, in terms of percentage of the project cost, what do you think the cost savings would be for going with a cheaper core, including the labor and machine?

  I'm perhaps too conservative in a design sense, and while a proa causes no problems for me engineering-wise (the numbers speak for themselves), I'd be hesitant to make the leap with a core material that hasn't been beat upon in the marine world for a few years.  There would have to be a very significant savings.

  That said, no one proves the value of a new core material without using it.

  It would be great to see what happens to a few test panels of different core materials, including some controlled impact testing, before building a boat.

    https://www.practical-sailor.com/newspics/BOAT_BUILDING_1007.pdf

        - Mike

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