BoatBanter.com - View Single Post

On 3 Nov 2003 20:23:46 GMT, (William R.
Watt) wrote:

first comment: the angle of the sail to the apparent wind is naturally
assumed to be 22.5 deg (I think that's it) for max lift. the angle of the
sail to the course of the planing boat is naturally assumed to be 90 deg
on a beam reach for max power. the angle of heel when planing is naturally
assumed to be zero as the boat will be planed flat. the sails are assumed
to be new. that's how applied math works. the assumptions made are an
idealized standard. they don't have to include variables for skill or
equipment wear. all I'm looikng for is the power of the sail. the
resistance of the boat is a separate calculation, or tank test.

second comment: Oh yes, I've planed a dingy, in the middle of the night in
a 24 hour race by observing the angle of some boats swinging free on their
moorings in the moonlight and going wide on the downwind leg and screaming
past the the gybe mark on a plane thereby gaining several boat lengths on
the hot shots in the other boats. The sails were not new. Our pickup team
did not come last against the hand picked hot shots. We came second last.
I also recall observing the progress of a pleasure sailor and swinging
accross the back of a fleet of racing dingys to overtake them from the
rear and round the gybe mark on the inside screaming "ROOM, ROOM" with
great gusto. What a mess. Sailing speed has more to do with reading winds
and currents than boat speed. That why its so hard to beat the locals.
However that should not prevent us while in the boat design stage from
calculating the sail area needed for planing the boat when we do get
'round to reading the winds and currents.

OK, that is enough that you should have observed that the delivered
'horsepower' fluctuated by at least an order of magnituded over a
fairly short timespan. Perhaps you would get closer to real questions
if you started with loaded weights and sail areas of common planing
boats.

For example, a Snipe, with very small sails and a quite heavy hull,
can still plane under ideal conditions. So can a J 24. But neither
does so often enough to be a common occurrence.

50-year old designs like 505 and FD can plane under ordinary
conditions, and routinely do so. They are much lighter for their sail
areas than the Snipe and J 24.

More modern designs, like the Bethwaite 18-ft skiff and the 49er,
plane almost all the time as far as I can tell watching them go.
Unfortunately for your purpose, these make use of high-tech materials
to make the dead weight small compared to the movable ballast (crew).
They also carry sail area that could only be manageable with a very
sophisticated rig.

You could find out the relevant ratios (sail area/dsiplacement,
displacement/area of planing surface, displacement/length) for all of
these, and try to pack what you want into a weight that fits your
desires.

You will end up looking at carbon fiber composite and a very spartan
interior. But at no time in this process will a horsepower formula
help you.

Rodney Myrvaagnes NYC J36 Gjo/a

"WooWooism lives" Anon grafitto on the base of the Cuttyhunk breakwater light