Ben
The main difference between say a catamaran and a proa is that the former can be set with an initial trim using weight distribution.
Also the transom on a typical cat hull will suck the stern down at speed. This increases bow up trim and the angle of attack for the hull so there is increased dynamic lift compared with a hull that rides flat.
With a clean canoe stern there is not as much sinkage in the stern. This means there is greater tendency to ride level under power. If the drive is high up then there is a large bow down moment that will force the bow down unless the hull can counter it.
So far I have observed four ways to get the hull to provide bow up moment.
A. At displacement speed having full ends (large waterplane at the bows) will increase the static pressure from the bow wave at the leading end and reduce the pressure at the trailing end. This can give bow up moment below planing speed. For high speed displacement hulls the full ends also produce the lowest drag hull.
B. Flaring the sides of the hull increases the waterplane area in the front end and reduces the waterplane area in accordance with the hull waves noted in A to increase the moment created by the hull waves.
C. The longer the hull the greater the moment from the lift and sinkage created by the entry and exit.
D. Having a flat planing surface in the ends with a slight rocker will provide dynamic lift once the hull approaches planing speed. The rocker on the trailing edge will create suction so it will assist with the bow up trim.
It is certainly a key issue for consideration with a large proa where it is not convenient to move ballast around after each shunt although plenty of racing keel boats use their crew as movable ballast. However I do not believe it results in a limit in speed if the hulls are designed to stay bow up under the drive.
Rick
in the lower speed range. t low counter the trim.
On 17/07/2011, at 1:43 AM, bjarthur123 wrote:
rick,
i've been going back and re-reading all the old msgs on the bow down issue. more than anything i think bow down trim enforces a speed limit due to the possibility of pitchpoling.
what i'm curious about is whether this trim attitude occurs at speed on "normal" boats, or whether this is a consequence of fore-aft symmetry. does having a "bow" sticking way back aft push the front bow down? do the typical square transoms of tackers alleviate this issue?
ben
> There is advantage in a slight flare in the sides - say 5 degrees -
> for improving performance in waves. Take the flare all the way to
> the ends so the longitudinal righting moment goes up as the ends
> bury. But keep the deck line at the ends low so there is not a large
> volume to push through waves or a buoyant trailing end to lift that
> end when running off a wave. Increasing waterplane area as the bow
> initially dips will help avoid burying the bow when in waves. You
> also want a deck shape at the ends to lift easily through green water
> rather than resisting the lift from the bottom and sides of the hull.
Rick Willoughby
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