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I’m surprised this hasn’t been discussed before, even if it would only to be just laughed out of the shop.
Off the top of my head,
a hard bladder could run down the stringer.
On the paddle out open a plug on close to the nose 1/4 and let water in as ballast. Close the plug again. Nose heavy, easier to duck dive, harder to paddle.
When out back, open the plug and push the board vertical in the water column. Because the plug is in the middle the water drains free. Close the plug and we have higher buoyancy.
Why it probably won’t work - Hard to clear the water and still have balanced ballast along the board
One way valve expells the fluid for us
have the system so good that it can deflate for the duck dive only, and still be top buoyancy for paddling; compressed air or something crazy like that
Uli and inflatables. Find a way to inflate these boards in the water. Hollow carbon boards also might allow easier development
Obviously this is a crackpot idea, but at least consider this post prior art to allow all to consider; nothing worse than patents.