Old Nick ) writes:
On 1 Nov 2003 13:28:49 GMT, (William R.
Watt) wrote something
......and in reply I say!:

Bill....if I may address you as such...

You ask for power formulae for a _sail_...then argue?

clarify. some responses did not address the question.
you can go back and re-read the original question if you like.

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I scaned diagrams of a possible sprit sail and put them on my website. So
far I just use the wetted surface and displacment to determine the sail
area. I still have more calculations to do for it. There's a sail cutting
diagram too. To use as much of the sail material and to keep the centre of
effort low I did not follow the recommended proportions in John Leather's
"Spritsails and Lugsails". I drew a sail more like older less efficient
sprit sails. BTW if you chose the length of the foot and leach then you
can use Leather's proportions to find the head and luff independent of the
mast length, although he relates them all to mast length. Maybe I'll
describe that in the design text later.

the diagrams are at www.ncf.ca/~ag384/Solo15.htm. I won't link it to my
home page until its finished which could be quite a while yet. Its still
very rough. I'm spending as much time trying to figure out how to use the
computer as I am trying to work out the design of the boat.




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On 1 Nov 2003 13:28:49 GMT, (William R.
Watt) wrote:


no, there's got to be a formula relating wind speed, sail area, and
horsepower.
--

Such a formula could be generated, In fact, a vast family of such
formulas could be generated, all equally valid, and all giving
different results.

The assumptions required to generate such a formula swamp any results
that might be derived from it.


Rodney Myrvaagnes NYC J36 Gjo/a

"Religious wisdom is to wisdom as military music is to music."

Rodney Myrvaagnes ) writes:
On 1 Nov 2003 13:28:49 GMT, (William R.
Watt) wrote:


no, there's got to be a formula relating wind speed, sail area, and
horsepower.
--

Such a formula could be generated, In fact, a vast family of such
formulas could be generated, all equally valid, and all giving
different results.

The assumptions required to generate such a formula swamp any results
that might be derived from it.

yes, but that's true of most things in nature and yet applied mathematics
works. for example, formulae are used to design aeroplane wings. the two
approaches used are to make the most appropriate assumptions and verify by
testing. the results only have to be accurate enough to be effective
design tools.






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William R Watt National Capital FreeNet Ottawa's free community network
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William R. Watt wrote:

yes, but that's true of most things in nature and yet applied mathematics
works. for example, formulae are used to design aeroplane wings. the two
approaches used are to make the most appropriate assumptions and verify by
testing. the results only have to be accurate enough to be effective
design tools.

I don't know that that is a valid comparison. Aerodynamic formulas for
aircraft calculate the amount of horsepower required to overcome drag
and provide lift but that is not what you are after. Also aircraft
foils operate in a very limited angle of attack and with predictable
wind speeds.

If a single horsepower factor for a particular sail area/wind speed
existed the polars developed from VPP programs would be circular. You
can't take a single horsepower and compare it to the horsepower
delivered to a prop. The propeller delivers all its power in one
benificial direction. Wind force on sails has a lift and drag component
and the net benificial power varies depending on the direction of the
lift to the relative heading you want to steer. That is constantly
varying. You might determine an equivelent horsepower for one wind
speed and one aparent wind angle but it would be valid for only that
condition.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

On 3 Nov 2003 13:54:29 GMT, (William R.
Watt) wrote:

Rodney Myrvaagnes ) writes:
On 1 Nov 2003 13:28:49 GMT, (William R.
Watt) wrote:


no, there's got to be a formula relating wind speed, sail area, and
horsepower.
--

Such a formula could be generated, In fact, a vast family of such
formulas could be generated, all equally valid, and all giving
different results.

The assumptions required to generate such a formula swamp any results
that might be derived from it.

yes, but that's true of most things in nature and yet applied mathematics
works. for example, formulae are used to design aeroplane wings. the two
approaches used are to make the most appropriate assumptions and verify by
testing. the results only have to be accurate enough to be effective
design tools.

I am tryng to be helpful. The noise in the set of reasonable
assumptions will produce output noise many times the size of the
signal. An airplane wing is used in one medium away from boundaries.

You could empirically derive some formula from the performance of a
rogallo type hang glider well off the ground. It would, however, be
close to useless for predicting a sailboat's behavior in the real
situation of a water/air boundary, with its normally height-dependent
wind-velocity vector.

I hope the next questions don't offend.

How skilled a sailor are you?

Can you enter a local club one-design race and be fairly sure you
won't come in last?

Have you ever felt the difference a new sail makes?



Rodney Myrvaagnes NYC J36 Gjo/a

"Religious wisdom is to wisdom as military music is to music."

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.

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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

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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On 4 Nov 2003 14:05:19 GMT, (William R.
Watt) wrote:


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.

Have it your way. I tried to help.



Rodney Myrvaagnes NYC J36 Gjo/a

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