Although I have stated on a number of occasions that I think a 12ft boat will be fast enough for children and there is no need to go to 3.9m perhaps I should clarify where this thinking comes from.
The Centre of effort (CofE) of the sail is say 2.5m high, The centre of buoyancy (CofB) of the boat is say 1.8m from the stern.
The drag calculations that I have published on earlier posting show that there is around 200N of drag at 15knots. (I think 15knots is a good speed for a childrens boat)
So in order to balance out the forces, the crew could move his weight (60kg) back to the rear beam a total distance of 1.6m from the resting CofB, and wouldn’t be generating any pitching (i.e. still keep the boat sailing flat). At this distance he can exert a moment of 1.6m x 600N = 960Nm, so the force in the sail he can balance out is 960Nm/2.5m= 384N. Twice what is needed to propel the boat at 15knots. I know that there are other factors involved here but it’s a good starting point.
Of course the extra pitching moment of a longer hull is a good safety device, but with such a lightweight low drag, low CofE boat which will tank along at 15 knots without any danger of pitchpolling is it really required?
