bow up trim because of the bow wave, yes, i understand how that arises in flat water at moderate speeds. but i don't think that alone is enough to handle 3-4 m waves. this is the situation i worry about, because as you previously said, pitchpoles most often occur when you stuff the bows into a wave when off the wind.

correct me if i'm wrong here, but don't most if not all of the 60-70 foot round-the-world ocean racing classes have stern water ballast? is it not the case that the reason is precisely to induce a static upward pitch of the bow to prevent pitchpoling?

if so, then how is a static bow up trim not a desirable thing in a racing proa as well? not with water ballast of course, added weight is very bad. but with moving the existing weight to the right spot.

that's one of the founding principles of a harry proa, no? put the accommodations to windward to balance the heel. but that only applies when close hauled. off the wind you want them aft!

man, what we really need is a PANTOGRAPH!! it solves everything in one fell swoop. weight is moved aft and inboard at the same time!!! and not just the easily moveable weight but the entire windward hull including the crew.

a cruising proa is a different beast and, no, i would not advocate anything that requires extra work or is fail unsafe there. shorten sail in the lumpy stuff. but i get bored if i'm not racing, even if there's conversable eye candy onboard.

your point is well taken that the mast height to water line length ratio is absurdly low for a harry proa in comparison to normal boats. perhaps this is a big enough of a safety/performance boost that quibbling about the second-order effects of optimal weight placement is not worth it.

in that vein, if i had to buy a proa right now i'd probably be happy with a una-rigged expeditionarry. but since i've got some time, i'm going to wait until the pantographing version comes out :) you all are right, this sled / boom business is for the birds!

moving parts blah blah blah. it's like retractable landing gear folks. the performance advantages hugely outweighs the added complexity. make it work! criss-cross some 50-mm thick 3-strand shock-absorbing eye-spliced nylon that prevents it from racking more than +/- 30 degrees.

the beams can attach to the masts of a schooner just below the booms. just need to figure out how to attach them to the windward hull...

ben

--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
>
> Ben
> The 18m proa has bow-up trim from 12 to 20kts if hull is clean. This
> is due entirely to buoyancy. Its largest bow-down trim under 20kts is
> 50mm at 10kts. Above 20kts it starts to dive. If it had flat
> sections forward then they would provide substantial lift at 20kts.
>
> These numbers do not account for the poor shape of the bow that lifts
> water at substantial flow rate above 14kts.
>
> For pitchpole resistance hull length is the main factor followed
> closely by height of CoE. Other factors are height of CoG, pitch
> inertia (generally lower inertia the better so concentrate weight
> around the middle in a harryproa), flat sections provide greater
> pitch damping than round, bows that dynamically lift rather than
> dive. Any bow with a flat deck and round bottom will dive if pressed
> below the surface at speed.
>
> I have not yet determined how much buoyancy in the ends is enough
> other than for any giving panel area it is better making the hull
> longer rather than higher for resisting pitchpole. For a length
> constrained hull I am yet to work out if more buoyancy is better.
> With a proa the bow eventually becomes the stern and a buoyant stern
> will increase bow-down trim in following seas.
>
> If you look at the progression of the bows of modern racing boats
> like A-class and amas on large trimarans you see lower buoyancy in
> the ends; flatter underwater sections in the bow and peaked or
> rounded foredeck that is narrower than the waterline. These factors
> contribute to the hulls drive through green water without much drop
> in speed while generating uplift through both buoyancy and dynamic
> lift. Apart from length these are the factors I would concentrate on
> to reduce tendency to pitchpole when driven hard.
>
> If you have low buoyancy in the ends then good buoyancy in the mid
> sections of the hull contribute to lifting the hull above the
> troughs. This means the ends are submerged less.
>
> If you are intent on shifting weight then move as much as you can at
> the level of the keel so it stays low, as little distance as you can
> to keep the pitch inertia low.
>
> Rick