My rule of thumb with respect to MAX righting moment is :

boat weight x boat width x 0.5 + number of crew (on traps) x (boatwidth + 1 mtr).

So for a 150 kg crew we find : max rigting moment is equal to :

107*2.5*0.5 + 150*(2.5+1) = 133.75 + 525.00 = rounded 659 kgm

In my earlier posts I used a lighter crew of 140 kg which is the average crew weight for competitive F16 crews.


I feel the formula that you use/presented is in error and I think what the error is.

You write :

1/2 the beam x (weight of boat + crew weight) + number of trapezing crew x COG of average person (0.93cm) x average weight of person worldwide (75kg)


Basically you have excluded "1/2 * beam * crew weight" from the total. Rearranging the items will show this (while replacing the factor 0.93 by 1 mtr for ease of calculations).

1/2*beam*boatweight + 1/2*beam*crewweight + number crew*1 mtr*WeightOfPerson =


rearranging further, assuming number of crew = 2 and 2*1 = 2 again

1/2*beam*boatweight + 1/2*beam*crewweight + 2 * mtr*WeightOfPerson

Note how the first term is valid but the second and third terms are insufficient when combined as the feet of the crew are located at the luff hull which is TWICE as far removed from the fullcrum point then 1/2*boatwidth. I hope this is clear enough.


Anyway back to the topic at hand.

2.30 mtr wide boat 2-up sailing with 150 kg crew ; max righting = 618 = 100.0%
2.50 mtr wide boat 2-up sailing with 150 kg crew ; max righting = 659 = 106.6 %

The difference being 6.6% in favour of the wider boat

Now typically a displacement hull travelling through the water at the surface can be crudely approximated by a drag curve that is 2.5 power of speed.. I say crudely as wave-making drag can place peaks or depressions on different locations depending on hull designs specifics.

This means a speed increase of 102.6 % = 2.6 % due to the later beam is all other things are equal and the max righting moment is the only limiting factor in achieving higher speeds (this implies identical trim, heading and windstrength etc)

Righting moment is not nearly as important on the downwind legs and we assume that it is NOT the speed limiting factor on these courses (diving is !) Also from measurements I have found that going downwind takes less time to cover the same distance then going up wind the ration is about 40-60% or 45%-55%. The first seems to be a very good upper limit. Meaning that only for max 60% of the time the extra leverage is effective (note 0.6*2.6% = 1.5%). This means that the wider boat completes a single upwind/downwind course in 55 secs less then the more narrow boat in a hour of racing (=40 secs in 45 min race). Of course in my other post I covered a range of percentages and that lead to an average conversative performance increase of 25 secs in 45 min race.

In this post I also simplified a few higher order dependencies (to keep things simple) and part of the differences will be the result of that, an example is the change in saildrive due to high boatspeed (higher apparent wind and smaller angle of incidence). However, I think the general method will however be clear.

Wouter