Robert wrote:
> I am not quite sure if I am on the same planet as those who persist
> in a lw pod for a sailing proa. Am I missing somethingin my
> understanding of hydrodynamics and hydrostatics. My calculatons for
> the hydrostaics show the boat has a greater tendancy to keep going
> over if slightly >90degrees as the ww hull is more likely to go paast
> top dead centre.
I think some of the pacific proa designs can be amazingly efficient
when given just the right conditions, which is probably why some favor
them.
With more weight in the large leeward hull, you can theoretically cut
down on hydrodynamic drag. Check out the comparison chart halfway down
the page on
http://proafile.com/view/weblog/comments/cheap_capable_cruisers_iii/
If you can get the weight in the long hull, and then either lighten
the windward hull, fly the windward hull, or use it as a vector fin, you
can probably get more speed for less sail area. I've not taken fluid
mechanics, but I'll buy the argument. This would be why Jzerro can do
20 knots without a whole lot of righting moment. It's kind of an
evolution of the proa mantra: less weight, less drag, less sail area,
and yet more speed.
This really comes true in the vector fin proa that is shown at
http://www.proadesign.com/ . I'm sure that thing is wicked fast given
the right conditions. The designer even claims it will hang on the edge
of a vertical wave face, which might even be true if you're moving the
right way. This might be true, but since he won't let anyone sail with
him, or race against him, it's tough to tell.
However, we don't always sail in just the right conditions. As Rob
has pointed out, if that windward vector fin were to come out of the
water, or get fouled with a plastic bag or seaweed, you could be in a
world of trouble. Likewise, if you were to get a solid gust when fying
a hull on Jzerro, there wouldn't be much to stop you from going over.
The leeward pod overhang would help, but I'm not sure I'd want to try it.
The Harry proa might have more drag than a traditional pacific proa,
but there's more to a fast boat than drag. A greater righting moment
means more sail area, and more lift, and that can mean a lot. Then
Imagine what you can do when skimming or flying the ww hull of a harry!
It's also nice to have a greater righting moment if you're heading
windward in big weather, or trying to ride out a storm. With rudders up
and sails or bare poles to leeward, the harryproa will tend to itself
quite nicely.
The "dutch proa" at http://www.bijlard.demon.nl/ is pretty
interesting. That additional leeward pod/ama would certainly help, and
would be a great point of balance where you could fly a hull, and then
adjust sheet loads to keep both amas out of the water without worrying
about imminent capsize. As some have noted, though, it's really a
stunted trimaran, and the extra weight would be better used for righting
moment.
In the end, the harryproa seems to be the best approach to a series of
conflicting design goals (including speed vs. safety), and as side
benefits, it also manages to keep masts and daggerboards out of the hull
with the accommodations, and it can even be tacked or backwinded without
major issues.
However, i can see why the purists out there like the less-is-more
solution of the pod in the leeward hull.
- Mike
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