Ben
You have interpreted correctly. The trim results were based on an approximated height of CoE for the full wing or sail plan. If the speed can be achieved with a reefed sail then that reduces the bow down moment. Benefits can be achieved with end plating the bottom of wing or sail and/or "twisting" the sail or wing to lower the CoE. The twisting could be achieved through more subtle means than actual twist.
With regard to the pantograph - years ago I took out a provisional patent on a self-righting catamaran where the hulls folded together. It did work in small scale. When I thought about a full scale version I felt there was a lot that could go wrong with nip points and high stress pivot areas. This adds a level of complexity that does not appeal to me. In my view it is better to keep simple hulls without pivoting beams that require adjustment while sailing.
With a cruising proa you need to be aware of the potential for pushing the bow under and hence taking time to do the trim analysis or full scale testing to be confident that it will not be a problem.
The intuitive solution of adding reserve buoyancy is probably not the best choice for reducing the tendency to pitchpole. A lot of reserve volume inevitably results in rapidly increasing drag once the bow is submerged causing the boat to decelerate. Then the similarly buoyant trailing end (bow) gets lifted on the following sea while the momentum of the boat wants to keep going - all combining for a dramatic swim.
I am pretty happy with my V15 style hull for wave piercing. There have been a few developments since I last posted on this.
A friend in Seattle has now tried the design and makes comment on the bow in item 4 at the bottom of the linked page:
This comment was only after his first outing while testing in boat wakes but he is experienced enough to know how more conventional bows work. This is a video he took some years ago in another pedal boat that has wider beam, wide transom and more conventional foredeck:
I have had both my sister's boat and my new boat in similar conditions. This is the 5m version in waves:
Notice how the beamier hull with more reserve buoyancy in the ends in the first video tends to pitch much more - what goes up has to come down. The V15 hull just goes through waves. This also verifies the lower tendency to slam with the V15 hull despite having a flat bottom. The bang you hear toward the end of the second video is a wave clipping the stabiliser brace that was a bit too loose and allowed movement. It all looks a lot worse when the ride is up and down compared with riding level without constant pitching.
I have taken the wave piercing concept a bit too for with my latest variant:
I am finding the side windage over the foredeck makes it more difficult to turn through the wind. I think the 45 degree inverted "V" (90 degree peak) as used on the first two is about the best.
Rick
On 17/07/2011, at 10:38 AM, bjarthur123 wrote:
thanks.
so the short answer is yes, the bow down problem is specific to proas due to their fore-aft symmetry.
do i recall correctly that your graphs of trim vs. speed were made by adjusting only the coefficient of lift of the sail at speed or maybe its AOA, and specifically not the height of the lift? in other words, maybe the trim wouldn't be so bad if the sail was reefed?? not possible with your snazzy wing i know, but applicable to most proas.
your four ideas on hull shapes all make sense to me and are very good in that they require no moving parts. i still can't help but wonder though whether the best solution is a pantographing ww hull. it would put the weight right where you want it. such freedom of movement could even possibly be useful as a folding mechanism when trailering.
ben
(RC'ing for J/24's tomorrow,
next weekend crewing on a corsair 28cc,
i miss my weta already!)
> 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 Willoughby
03 9796 2415
0419 104 821
