<< In general we don't have a great deal of disagreement >>
Agreed.
I just misunderstood what you meant by the leeward pod in the initial
posting. I'm so used to people on other forums getting religious about
Pacific versus Atlantic that I ended to going overboard in the
explanation. Oh well. Better than than flaming you...
I do think the leeward pod on a design like Jzerro
could be an asset if you like to keep the boat on the edge, and are
quick enough to dump the sheets once that pod does start to dig in. It
could buy you a few extra seconds if an unexpected gust comes by.
However, this only goes for when you're out having fun daysailing.
As you point out, the pod could be a liability once you get into real
weather.
I think your comment about the leeward pod diminishing the effect of
mast buoyancy in a capsize is particularly salient given the current
self-righting discussion.
- Mike
Robert wrote:
--- In harryproa@yahoogroups.com.au, Mike Crawford
<jmichael@g...>
wrote:
>
>
> 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.
In general we don't have a great deal of disagreement
I agree about the pacific proa. For example Elementarry. Though
Elementarry may be even faster though not as easy to sail in Atlantic
mode. keeping the lee hull just out of the water in low winds and
more righting moment with the ww hull just skimmming the water in
higher winds. Would not be easy but theoretically faster. I have
sketches for a monocoque proa with a fold out cockpit based on an
elementarry that I reckon would be a little bit better as an Atlantic
schooner
>
> 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.
I have actually taken fluid dynamics. I am suspending judgement on
the vector fin and generally agree that if you keep the weight on one
hull then you lessen drag. You can lessen drag at low speeds even
more by going to a fat mono. But you also need righting moment to
generate power. Probably this can be done with a vector fin but it
seems much more reliable to use weight not needed in the lw hull. If
the vector fin works as claimed , then it is contributing to extra
downward force overall on the boat,ie equivalent to extra overall
weight similar to water ballast. Jzerro used water ballast to get the
righting moment and needed more sail area than equivalent length harry
This doesn't tally with less weight less drag less sail area.
I spent a few years in PNG and had experience with some of their 40'
racing canoes. They had plenty of crew constantly racing backwards
and forwards keeping the canoe balanced but once a bit of wind came
up they were all, but the steersman, on the outrigger. This is in
steady trade wind conditions.
If you are going to rely on moving human ballast around constantly
then a Pacific proa works very nicely and using semistayed masts is
probably the lightest engineering. With the use of unstayed msats I
calculate things change to an Atlantic rig. For a long term offshore
boat you don't want to be living on the edge constantly so for safety
considerations you put all the necessary weight that you can in the
ww hull as permanent righting moment- within reason- you need some
weight in lw hull. The Atlantic rig has the problem of interfering
with accommodation and sheeting arrangements that make it difficult
for weather cocking. The amount of weight in the rig seems a good
compromise for the lw hull and bouyant masts can help prevent
complete overturning if someone is mug enough to leave the sheets
cleated with ridiculous amounts of sail in lumpy seas.
My objection wasn't with the Pacific proa. I have great fondness for
it after my time in PNG. I believe it has its place but not for long
haul cruising. My objection is the use of the lee pod as I believe
that it is an unseaworthy attachment. I can almost see some value in
it for flat seas as training wheels and as a lee platform for
drifting conditions but not once there is any swell. I can't see how
it can help prevent combination wind/wave capsize. On the side of a
wave the boat would have to be about 90 degrees before the leepod
contributed to righting moment. In breaking seas, if hit from the
side by a breaking wave it could dig in and flip the boat. It also
makes the boat less likely to be stabilised by mast bouyancy from
going over further.
Rob has iterated most of these point many times. This is based on
his undertanding of the engineering and many years experience in all
sorts of sailing craft in all sorts of conditions. My experience of
the sea as a professional fisherman working in Bass Strait and many
years of surfing various craft, coupled with a degree in mathematics
fully support his reasoning.
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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