Ben,

  Given the long slender hull on the lw side, I'm not convinced that the current trim is an issue, particularly if you design the first X feet to be wave-piercing shapes that shed water.

  But for the sake of discussion, let's say that it's non-optimal or even risky.  Why not?  Things can always get better, and the proa does not have the advantage of a trimaran in being able to put the rig and the cockpit further aft.

  If this is the case, I'd still shy away from shifting weight, and for three reasons:

  a) EXTRA WORK.  Pumping all that water takes time and effort, and is one more thing to do on a shunt.  Perhaps you have an electric pump, but that's still energy that has to come from somewhere.

  b) SLOW RESPONSE.  Let's say you suddenly need to shunt, even though you were planning to hold the previous course for hours or days.  What then?  Having the weight in the wrong end presents a problem.

  c)  FAIL UNSAFE.  If your pump mechanism or plumbing stops working, the system fails in an unsafe manner, leaving weight in the wrong end.  In a big blow or big seas, that could be dangerous.


  But nixing the weight movement technique doesn't address the concern, does it?

  To make the boat more resistant to pitchpoling, I'd consider four things:

  1) WAVE-PIERCING/WATER-SHEDDING SHAPES.  If the ends can slide through the water, in the way that they now do with the big trimarans and Rick's pedal boats, immersing them isn't necessarily a bad thing.

  2) MORE LENGTH.  What's another two to four feet on each end in terms of cost and weight, particularly with slender ends?  At some point the hull will be long enough to make any pitchpoling arguments a non-issue.

  3) DYNAMIC LIFT.  At low speeds, particularly with longer water-shedding bows, pitchpoling is probably not a danger.  At higher speeds, a bottom that generates lift would add a belt to the suspenders.

  4) TALLER BOW PROFILE.  You could also make the bows even higher with a design more like the CLC 31' proa ( http://www.clcboats.com/shop/boats/wooden-sailboat-kits/proa/madness-31-foot-pacific-proa.html ).  If the concern is the bow under the water, additional height would help mitigate the problem.  Though it would still probably be a good idea to make sure the decks shed water instead of catch it.  Note: this is not an endorsement of the CLC proa, just a reference for the profile.


  I'd probably go for just the first three methods, particularly since the taller profile would create more windage.  But it's worth at least noodling about.

        - Mike

 

thanks for the replies.

rick: call me paranoid, but please let's at least entertain the idea of weight shift and do some rough calculations. i agree the boom idea is a bit crazy, but a sled or a two-tank-with-pump system as arto suggested are not.

arto: 400 kg. is that really how much weight would need to be shifted? or is that the weight you estimate for the things i listed (motor, fuel, batteries, water)??

harryproa.com is still down, so i can't look it up, but i recall aroha nominally weighing 800 kg unladen, and having a design payload of another 800 kg.

let's see here: a moorings 403 has a fresh water capacity of ~500L and fuel of ~130L. a similarly equipped harryproa would then have several hundred kilograms available to shift, but that weight would diminish on a long voyage. would be nice to be able to move the motor and batteries as well.

but still, how much weight must be shifted to compensate for the pitch moment of the sail lift? more or less than the combined weight of the motor, fuel, water, and batteries if placed on the aft beam? it'll vary with wind strength of course, but let's just say 20 knots. this must be a relatively simple calculation.

ben