When you make a boat lighter you can reduce the bouyance of the hulls. This has a drag lowering effect by reducing the wave-making drag. Also some reductio in wetted surface area drag is achieved but the amount by which this is relatively small.

Think of the hulls as a long rectangular box. Now small reductions in the width of the box lead to large changed in overall bouyancy, but only is small reductions in surface area. However, small changes in length result in small changes in bouyancy while having relatively large effects on surface area.

If you make large reductions in overall weight, then you can make large reductions in bouyancy. If you effect this ONLY by reductions on width then the wave-making drag drops considerable but by that time it was already a rather small part of the whole. The wetted area is still only lightly reduced while it is actually a larger part of the whole drag. At that time only reducing hull length will result in significantly reductions of overall drag.

Pretty much this means that a long water line length is not always a good thing. When large reductions in bouyancy are achieved then a shortening of the hulls must be considered also otherwise the reductions of bouyance are not really effective beyond a certain level.

Therefor "ratio of wetted surface area to bouyance (=overall boat weight) does play a factor"

I still think that this is one of the pitfalls that was not carefully analysed in the M20 design. I still say that it is probably best to have 5.75 mtr long hulls like the Taipan 5.7 and then reduce overall weight of that boat to 120 kg (and no more) and have it feature a sloop rig and 2.8 mtr width. I suspect the quickest design setup to be found around those parameters while still having a good stable feel to it.

I've written many times on this forum, but people seem to forget about it, that the 5 mtr long hull on the F16 is actually a speed enhancing feature. The boat will actually have more drag at 107 kg and say 17 foot hull length. Making a F16 5.50 mtr long will also be a bad move, although it will make the boat feel more stable. But from a drag perspect it will be inferiour.

Too often I see people just "feel" that longer hulls are just faster. This I refered to by "extrapolating to extreme points". Normally a longer hull is better, but not when you also reduce the overall weight significantly at the same time.

Wouter