Subject: [harryproa] Re: Ratio of ww to lw hull lengths
From: "Robert" <cateran1949@yahoo.co.uk>
Date: 2/15/2009, 11:33 PM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

There is the theory that at low speeds the shorter fatter hull will
have less wetted area and thus be a little faster and as the speed
increases the hull unweights form the sailing loads and the hull then
gets a bit skinnier. My calculations show that the boat should get up
to a speed where the extra wave resistance is significant a fair bit
before there is significant unweighting. The advantages I see for a
shorter ww hull are less wracking loads as the ww hull more easily
follows the lw hull and the attack to the waves is closer in time than
if the ww hull was well ahead of the lw hull. It should also make
shunting easier as there isn't the extra bow length to drag around on
the inside of the shunt. It is much more important to have the lee
hull long as this is the one that is preventing pitchpoling. I do not
know what the optimum ratio is between the hull lengths but the 2:3
seems a reasonable compromise for cruiser between reasonable fore/aft
stability and becoming a bit unweildly and the need to have righting
moment without going too fat in the w hull . The trailerable one for
Raps has a smaller ratio, ie a longer lee hull and shorter ww hull
proportionally, and is designed for going faster through waves. It is
to be actively sailed closer to the edge of capsize.A faster boat
means greater wracking forces and therefore extra strength leading to
extra weight if the ww hull is longer.
I don't necessarily agree with the ww hull beam ratio. I don't see
why it can't be skinnier and deeper, depending on the loads to be
carried. My calculations for light weight cruising with a 15m lw hull
gives slightly less wetted area for a skinnier ww hull (10m ww hull 1
tonne payload at 6000mm wide, 150mm radius on the bilges with a
prismatic coefficient of over 0.8, compared to 850mm wide using a
smaller prismatic coefficient and more parabolic shape. If you start
to increase this weight, then this no longer applies. There is also
the point about wanting a fatter hull for accommodation. This can be
accomplished with a flare above the waterline, but this does lead to
extra complexity and a little more weight- I figure about 20kg for the
ww hull and I am not sure about for the lw hull as there are a lot of
pluses and minuses in stress analysis.

--- In harryproa@yahoogroups.com.au, Gardner Pomper <gardner@...> wrote:
>
> Hi,
> I am unclear on why proas, particularly harryproas have a ww hull
that is
> shorter than the lw hull. If it is a "weight to windward" proa, then
50% or
> more of the weight might be on the ww hull, and keeping it short
makes it
> beamier. Doesn't the short/"fat" ww hull cause more resistance when
it is in
> the water?
>
> Note: I am obviously speaking of conditions when you are not flying
a hull,
> or even close to it. Specifically, in terms of cruising boats like Rare
> Bird, or even heavier. Why wouldn't it be better to have a ww hull
at least
> as long and narrow as the lw hull to carry the weight?
>
> I see the advantages to a longer ww hull. What are the disadvantages?
>
> Thanks,
>
> - Gardner Pomper
> York, PA
>

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