RE: Heavyweights Waveboard
Agree with your ratio idea but your calculation assumes a constant "rig/board/suit/harness" weight which is why your maths appears to help heavier sailor.
In actual fact heavier sailor would (all things being equal) have
a) heavier board b) bigger sail/heavier rig c) a larger wetsuit holding more water.
Looking forward to seeing new board but suspect at 100litres it will be on small side for me.(Depends on other factors too)
By experience I feel 105 is about my practical (optimum) minimum.(My all up sailing weight will be approaching 130k (including board weight)