Glad thats sorted, hope I didn't appear too brash. I frequently
appear arrogant in my statements but need feedback to refine my ideas
and I have been known to talk crap and sometimes have niggling doubts
that I may be. This feedback is an important part of this forum for
me,
Thanks
robert
--- In harryproa@yahoogroups.com.au, Mike Crawford <jmichael@g...>
wrote:
>
> << 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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