--- In harryproa@..., "rob denney" <proa@t...> wrote:
> G'day,
>
>
>
> Todd
> With numbers and design you can make the hull stable when
> flying the ama. Maybe not as stable as if both ama and vaka were in
> the water but stable enough to fly the ama long enough to make a
> difference. To my limited knowledge drag is everthing less the
> better. Check out Sail november 10 issue 2003 page 68. aw yeah lean
> an mean.
>
>
> Rob
> Haven't seen November Sail, but would like a copy of the article if
> possible, or just a description.

Todd
Its showing Fench trimaran sailor Olivier de Kersausons 110 foot tri
Geronimo.


Rob
> Flying the windward hull is certainly less drag, but not a whole
lot less.
> In a Tornado, it is apparently 10% between two hulls and one which
is
> appreciable when you are racing.
>
> Harry has very similar wetted surface with both hulls in the water,
or the
> windward one flying. However 2 hulls will make more waves, so is
draggier.
>
> The only way I know of to design a hull to be stable is to give it
a wider
> footprint. This can be
> done by increasing the waterline beam of the lee hull, adding a
float to
> leeward (trimaran) or a pod (Russ Brown). All add cost, weight,
windage and
> drag. Flying a hull for long periods of time is not an easy task.
Racing
> catamaran crews do it, but are constantly altering sail trim, crew
position
> and heading. To do it on a cruising proa would be impossible, and
unsafe,
> as the smallest loss of concentration coupled with an increase in
breeze
> would see you capsize.
>
> All else being equal a traditional type proa has less drag at the
wind speed
> at
> which the lee hull flies. Above this wind speed, it must be
slower, as it
> cannot use the added power. At a certain windspeed, the same will
happen
> with a harry proa. But the harry will have been increasing speed
up to that
> wind strength.
>
> To get an idea of how this all works, try sailing your small proa
sitting on
> the lee hull. In light airs, the ww hull comes up, you go a little
faster,
> but
> after this you are luffing or easing sheets. Now sit to windward.
You
> continue to go faster until the ww hull flies, which increases
speed a
> little more again, but this then becomes the limit.


Todd
Actually I have little more range than that. I am on the leeward
platform in light winds. As the wind increases I come in on to the
lee hull then out to windward platform as the wind strength
increases. All the while maintaining level sailing with the windward
log just skimming the surface. If I have to go beyond the windward
platform it is of no benefit the hull is over powered and need to
reduce sail.


Rob
> Skinny (>12:1) hulls are a little quicker at high speeds, slower at
low
> ones. Harryproa leeward hulls are >20:1 to minimise weight and
windage,
> rather than to optimise hull shape. The ww hulls are about 11:1 to
minimise
> drag when starting off after a shunt. As the boat gets powered up,
they
> lift and the hull gets a higher ratio. I have no numbers on wind
speed or
> drag at which this happens, but there is no noticable increase in
speed, nor
> change in the helm loads when the ww hull leaves the water, beyond
what you
> would expect from the increased breeze when this occurs.
>
> To design an ultimate race proa, you need to decide on the wind
speed you
> will most often be sailing in, and optimise the hulls for that.
I believe
> that if you go for semi circular sections, no rocker and both hulls
finer
> than say 11:1, the payload, windage, air and water foils are of far
more
> concern than tweaking the hull shape.

Todd
I follow you so far. But lets say you have a leeward hull of 18:1 and
windward hull with the righting moment of the HP but with 35:1 ratio.
i Wouldn't this be a significant gain in speed and stablilty?



> Regards,
>
> Rob
>
>
>
>
>
>
>
>
> ----- Original Message -----
> From: "Todd" <ktsrfer@m...>
> To: <harryproa@...>
> Sent: Wednesday, December 31, 2003 2:50 AM
> Subject: [harryproa] Re: equal & Harry Proas
>
>
> > Great Info thanks.
> Todd
> With two hulls in the water as opposed to flying
> > the windward hull on the pacific proa there has two be a
difference
> > in speed.
>
> The wind speed at which the windward hull flies is the max righting
moment.
> Below this wind, both boats are similar (very broadly speaking).
At a given
> wind speed the trad proa will fly a hull, and go a little faster.
As the
> breeze increases, it will not be able to use the extra power, so
won't go
> any faster. The harry will continue to speed up until it also
reaches max
> rm, at which time it's speed will increase a little. How much is
a
> little? For a Tornado, it is apparently 10% difference between 2
hulls in
> the water and one. I suspect that all of this gain does not
suddenly happen
> at the moment the windward hull leaves the water.
>
> Is this part of the sacrafice for stability ?
>
> Yes and no. Flying a hull on a cruising boat is unsafe. The
stronger the
> breeze at which this happens, the safer you are. On a race boat,
it depends
> on the breeze strength in which you want to go fast. But it will
always be
> a lot easier (and safer) to move crew to leeward in light air to
lift the
> hull (harry) than to move them to windward in heavy air.
>
> Can you make
> > up the speed by using a larger sail plan and hull shapes 1/16 or
> > greater?
>
> You could increase the sail area, or decrease beam overall to give
trhe same
> max righting moment, with the same limit on speed. Not sure why
you would
> want to, though, unless all your sailing was in light air. Hull
shapes
> do get higher max speeds as their B/L ratio diminishes. Their
light air
> performance suffers. The difference in performance between 12:1
and 20:1 is
> quite small.
>
> If so how much larger sail plan and is it possible to over
> > power the leeward hull?
>
> How much larger depends on the 2 boats and the wind speed at which
you want
> max rm. At a certain sail area, the danger becomes pitch poling
rather than
> capsizing. Pitch poling is usually more dramatic and has less
warning than
> capsizing, so for this reason, longer lee hulls make sense.
>
> I think Terho eql 7 had experienced somthing
> > like this in strong winds and chop or is this just a matter of
> > reefing?
>
> Reefing will certainly depower the boat, but you still have the
drag of the
> upper mast, so is not generally considered a speed generating
tactic.
>
> It should be borne in mind that the speed difference we are
discussing is
> not huge. You will achieve a faster boat with reduced windage (as
the
> breeze picks up), better foil shapes and a smooth bottom, rather
than
> spending all your sailing time trying to balance a narrow hull
against the
> vagaries of wind, water and crew. Developing the skills to fly a
hull
> consistently is probably just as big a performance gain as the
actual flying
> of the hull.
> >
> > Todd
> >
> > --- In harryproa@..., "rob denney" <proa@t...>
wrote:
> > > G'day,
> > >
> > > The weather hull waterline beam is a function of the length. No
> > less than
> > > 1:10. Beam overall is limited by structural and aesthetic
> > concerns, rather
> > > than handling ones. However, with all the weight in the
windward
> > hull, it
> > > will be much narrower than a Pacific proa of similar righting
> > moment.
> > > There must be a steering limit on beam over all and on weight
in the
> > > windward hull, but we have yet to reach it. If we do, it is
easily
> > > corrected by raising the bow rudder during a shunt.
> > >
> > > The underwater plane of the two hulls does not appear to be
> > important. I
> > > have not done any tank tests, but have used a variety of
different
> > hull, and
> > > it has not made any apparent difference.
> > >
> > > More drag on the windward (or leeward) hull does not appear to
be a
> > problem,
> > > any more than it is on a cat with a hull flying, or a tri with
one
> > (or two)
> > > hulls flying.
> > >
> > > Any problems of this nature will show up during a shunt. For
this
> > reason, I
> > > think large rudders and no fixed foils are important. The
rudders
> > can be
> > > aligned to the water flow and start steering, where a fixed foil
> > will stall,
> > > exacerbating any tendency to luff. The lack of rocker also
helps
> > in this
> > > respect.
> > >
> > > Whether 2 equal hulls are a better idea is a moot point as
unequal
> > drags has
> > > not shown up as a problem. While they would increase the
payload,
> > they also
> > > add wracking loads to the beams, windage and possibly low speed
> > drag (during
> > > shunting) which could be problematic. On W (12m cat with rig in
> > one hull,
> > > single rudder in the other) tacking is better on one tack than
the
> > other,
> > > but this is due more to the rudder being on the inside of the
> > turning circle
> > > rather than the outside. There is no noticable drag dirrence.
> > Terho Halme
> > > has experienced similar handling (albeit only in light air so
far)
> > of his 7m
> > > equal length hulls harryproa.
> > >
> > > Hope this answers the question, if not, please try again.
> > >
> > > Regards,
> > >
> > > Rob
> > >
> > >
> >
> >
> >
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> >
> >
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