The shark hull just passed a 48 hours float test. There was no change in weight or other signs it absorbed water. Using the bathtub enabled float testing in clean water – and this hull is huge so the bathtub was the only volume in which it would fit. That said – it turns out explaining why there is a robot floating in the bathtub can lead to unexpectedly awkward conversation.
The new hull uses a blown polyurethane hull filler, which cast under pressure. That creates a closed cell foam but one dense enough for the hulls littoral pressure ratings. Tests show clear for moving on the rest of the design.