Rodney Myrvaagnes ) writes:

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.

that's why I'm looking for a formula relating sail area and wind speed to sail
horespower. its wind speed one uses to make any particular boat plane.


Perhaps you would get closer to real questions
if you started with loaded weights and sail areas of common planing
boats.


yes, that's the old way. but as you point out different boats have
different resistance at different wind speeds. if I had a formula for sail
horsepower at different wind speeds I woudl knwo what wind speed and sail
area is needed to pane a hull with a known resistance (pounds) at a given
wind speed. as we know from the lenght to speed formual resistance
increases with boat speed.

however TF Jones says it takes 1 hp for every 40-50 lns displacement to
plan a "good" planing hull and I would use that instead of trying to
determine the resistance of a particular hull under desing. I'll assume it
is sufficietn for my purpose.

its actually possible to predict if a boat will plane given sufficient
sail power. my problem is finding the horsepower of the sail. that's what
I'm lookign for. I have teh table GA notes is in Gerr's book. I forget
where I saw it. I've taken notes from quite a few books.

I do have the formula for pressure per square foot of sail at any wind speed
(pressure = 0.004 times square of wind speed) over some reasonable range
of wind speeds for sailing. I don't know how the 0.004 constant was
derived. I think maybe I can work something out with that.


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.

oh yes it will. wind power is what gets the boat to plane.

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