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.
Rick
On 13/05/2011, at 5:30 AM, bjarthur123 wrote:
a pantograph is definitely against your philosophy of light, simple, and hence cheap, which i certainly appreciate otherwise.
longer lw hull would definitely reduce pitchpole likelihood, but the bow would still be down i think, just not by as much. i don't see how planing can happen with a bow down trim.
the other thing to consider is that if you do manage to pitch the bow up (by whatever means), then the overall CLR is going to move aft. if you've lifted the forward rudder (to maintain helm balance b/c it's a unarig or schooner w/o jib), then the overall CLR might coincide with the aft rudder, and you'd have no steerage.
difficult problem. will have to chew on it. sure would be nice to take one out for a sail instead of just thinking about it!
ben
> No doubt about needing to counteract the nose down trim off the wind. Just
> whether it is better to do it with a pantographing hull or a longer lee
> hull. Depends on all the secondary requirements, I guess. I am happy to
> draw a pantographer if you want to build one to test the theory.
Rick Willoughby
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