Cutaways reduce Drag ????
Think you`ve mixed trailing/leading up in your reply but I understand all you are saying and agree with most points but in the blurb starboard claim/make the premis that adding cutaways decreases drag at tail of board.
This is contrary to current fluid dynamics theory.Cutaways or any other manipulation of trailing edge by sheer maths must increase that edges "edge length" (#see point later) This may well be advatageous for other reasons.(ie widening board to increase leverage available for fin but keeping tail area down for top end control) But none of that is mentioned.
#There is a current (yet unproved) theory that applying "saw tooth" shape to trailing edge of a wing helps lift /drag ratio but this is to do with generating tip vortices accroos span of wing and not as a direct way to reduce drag.)But tip vortices and their effects have nothing to do with planing surfaces.Boards plane through momentum exchange between water and board.(ie a board is not a wing/hydrofoil)
With a staight leading it is mathematically impossible for trailing edge to be shorter.A boards leading edge is inevitably straight.(Boards` planing surface is virtually flat in almost every instance) The minimum trailing edge would be another staright line parallel to leading. This would give minimum drag but probably be unsailable ????