When calculating transverse frames what would be ...

This is probably the course I'm going to take ... (since I'm doing a reconstruction of the boat, and this is the part which is "fixed") I will probably take B as unsupported, calculate the frames needed, and (since they won't satisfy the section modulus) just put down ... L50x30x5 and let the gentlemen at the classification society decide whether they will allow it.

The current side frames satisfy the requirements were "A" was the length, but not quite were I was to take "B". So it is an edge case.
But since the boat is build for restricted waters (20NM off the coast), and it will probably never sail in rough weather ...

 

I was going to ask you that.

I will go for "B" as that satisfy the class rules. The ends are are bracketed where it connects to the floor and the deck.

Small yachts are sometimes built with different floor levels and are "discontinous". Purist would say anything in between the floor and the deck is a 'tween deck" (in between decks).

I also agree with Perm Stress, that is, you can skimp on the cross tie beam connection on "A" since the "B" (the longer span) has already been satisfied.