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I wonder how water tight these are in practice. Small scale FDM prints aren't airtight, and are questionably watertight. For something like a sailboat that floats in the water it's whole life, even a small leak can be a problem.

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I wonder if they do any post processing on the prints. Some wooden boats are coated with glass fiber & epoxy resin to protect the wood. You could do the same thing with a 3D printed part -- that would make it watertight and would also increase stiffness.

If they do that, it's essentially identical to how you do a cedar strip plank boat. The 3D-print/wood is only used for its compressive strength across the ~1 cm hull thickness, which is really high, and the glass (or carbon) fibre translates bending of the hull into tension of the fibres plus compression of the separating material - loading each material in the way where it's strongest.

The technical word for this is a torsion box, and you find it everywhere, from doors in your house to aircraft wings.

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