rick,

i understand about bow waves creating lift. but your analysis was done for flat water, no? what about storm waves?

it would be prudent in rough conditions to have a way to lift the bow even higher than occurs with the bow wave alone. so that when you nose dive into the back of a wave you don't pitchpole.

this is normally done by putting weight aft. from the crew, or water ballast. i don't see a way to do it in a "normal" proa. hence my inquiry about a pantograph.

ben

> The bow wave creates buoyant lift and the water leaving the stern
> creates a trough causing the stern to sink or squat. Once the hull
> is moving there is a tendency for the bow to "float" higher and the
> aft to squat. These are purely displacement phenomena with no planing
> forces involved until the speed increases.
>
> The wide transom on hulls designed for planing cause the stern to
> sink at displacement speed and give the pronounced bow up attitude so
> these sort of hulls will leap onto the plane if there is enough
> power. Long slender hulls with canoe sterns do not trim bow up as
> markedly but the do trim bow up.
>
> I have analysed this for what Dennis Cox was playing around with
> earlier and it is not difficult to produce bow up trim with typical
> sail plans and proa style lw hull optimised for high speed. The
> fullness of the ends have a lot to do with this trimming. Very fine
> ends tends to be easily pressed deeper - small bow wave. Full ends
> want to lift at the bow and squat at the stern. Once they trim up
> dynamic lift comes into play at higher speed.
>
> There is drag from the waves produced by the ends but there is also a
> reduction in wetted surface with the high resulting Cb that reduces
> total drag at higher displacement speed. So longer hulls are not
> necessarily best for higher speed even in displacement mode.
>
> The Flotilla software I use has the ability to determine the trim for
> any hull form in displacement mode. So it is an iterative approach
> to see if there is enough bow-up trim to counter the bow-down
> tendency from the sail drive. The height of the CoE is a key factor
> as well as the shape of the bows.