I appreciate ALL of the responses to my questions. There are a lot of good points to consider. I'll have a better idea of which way (or no way) to go after I see the arch of the deck, to see/feel what I might get away with.

: I'm getting ready to start my first stripped deck, on CLC Shearwater 16
: hybrid. Looking ahead, I've always wanted an underdeck storage area for a
: pump and water bottles; would be nice to keep topside uncluttered. I'm
: considering possibly a half-cylinder or cone shape (open at both ends)
: made of strip, for whatever weight penalty may be involved. Not really
: interested in the under-deck day hatches I've seen.

: Are there any established designs for knee tubes that I should look at?
: Problems to avoid? Reasons to reconsider the entire idea? Thanks