Heavy Weights vs Light Weights

Hmmm, I think you made a mistake somewhere.

The draught of th box section with 160kg crew weight is 39mm (in fresh water), increasing to 51mm for the heavier crew.  These equate to WSA of 8.468m^2 and 8.612m^2 respectively, which is an increase 0.144m^2, or 1.7%.

I did a triangle rather than a circle (cos it was easier!) and the draught is 78mm increasing to 102.5mm.  WSA is 8.332m^2 increasing to 8.452m^2 - an increase of 0.12m^2, or 1.4%.  So the trianglular section is 'better' (from a WSA weight sensitivity perspective) than the box section, which is as you would expect (as the waterplane area increases with draught).  However increasing waterplane area (and waterline beam) introduces its own problems...

And, of course, you have to consider the juxtoposition of Jupiter with Venus.

Originally posted by Barty The author assumes and expects that 'armchair' dinghy designers will criticise, neh, mock these calulations

Your diagrams demonstrate that a boat which has vertical topsides above the static waterine will gain WSA much more slowly than one which is flared. This is of course correct.

Where you are going wrong is in not thinking about the actual immersed shape of whole of your boats. Whilst in light air a typical wedge shaped skiff may have the whole of the mid section immersed up to the chine, the aft sections will be much less immersed with bow down trim, thus greatly flared above the waterplane, and its the aft areas where the WSA will come from.

Originally posted by JimC Where you are going wrong is............

Edited by Barty