Is there a formula relating boat sail area, wind speed, and
horsepower?

The best I can find at the public library are some ranges, ie
hp/sq ft of sail area = 0.015 @ 7-10 kt
0.020 @ 11-16 kt
0.040 @ 17-21 kt
0.070 @ 22-27 kt

I'm trying to get a better idea of how much sail I'd have to put
on a light 15 foot plywood cabin cruiser with appropriate
underwater shape, whose total weight with one occupant and a
week's gear (including accumulated human waste) is 630 lb., to get
it to plane in less than a full gale. TF Jones says 1 hp to plane
each 40-50 pounds.

For comparison, the above table applied to an Albacore racing
dingy (250 lb) with two people aboard (300 lb) and 125 sq ft of
sail says the wind has to be off the above scale for the boat to
plane.

I also found general wind power formulae in books at the library for
generating electicity but there are constants in the formulae, eg
efficiency, which I don't know for boat sails.

I've looked at numbers for several racing boats of about the same
displacement in library books to compare sail sizes but I'd like
to do the calculation for this boat if I can manage it.


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Well ... there's wind ... and there's apparent wind ... and there's a broad
reach (eeehaaaaa on my windsurfer) ... and there's cats that don't tip and
don't lose wind ... and there's displacement hulls that tip and lose wind.
I'm not even a sailor ... but it's obvious to me ... there's a lot more more
to this than windspeed.

"William R. Watt" wrote in message
...

Is there a formula relating boat sail area, wind speed, and
horsepower?

The best I can find at the public library are some ranges, ie
hp/sq ft of sail area = 0.015 @ 7-10 kt
0.020 @ 11-16 kt
0.040 @ 17-21 kt
0.070 @ 22-27 kt

I'm trying to get a better idea of how much sail I'd have to put
on a light 15 foot plywood cabin cruiser with appropriate
underwater shape, whose total weight with one occupant and a
week's gear (including accumulated human waste) is 630 lb., to get
it to plane in less than a full gale. TF Jones says 1 hp to plane
each 40-50 pounds.

For comparison, the above table applied to an Albacore racing
dingy (250 lb) with two people aboard (300 lb) and 125 sq ft of
sail says the wind has to be off the above scale for the boat to
plane.

I also found general wind power formulae in books at the library for
generating electicity but there are constants in the formulae, eg
efficiency, which I don't know for boat sails.

I've looked at numbers for several racing boats of about the same
displacement in library books to compare sail sizes but I'd like
to do the calculation for this boat if I can manage it.


--
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----
William R Watt National Capital FreeNet Ottawa's free community
network
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On 30 Oct 2003 19:13:45 GMT, (William R.
Watt) wrote:


Is there a formula relating boat sail area, wind speed, and
horsepower?

The best I can find at the public library are some ranges, ie
hp/sq ft of sail area = 0.015 @ 7-10 kt
0.020 @ 11-16 kt
0.040 @ 17-21 kt
0.070 @ 22-27 kt

I'm trying to get a better idea of how much sail I'd have to put
on a light 15 foot plywood cabin cruiser with appropriate
underwater shape, whose total weight with one occupant and a
week's gear (including accumulated human waste) is 630 lb., to get
it to plane in less than a full gale. TF Jones says 1 hp to plane
each 40-50 pounds.

For comparison, the above table applied to an Albacore racing
dingy (250 lb) with two people aboard (300 lb) and 125 sq ft of
sail says the wind has to be off the above scale for the boat to
plane.

A 505 dinghy weighs 286 lbs (min) plus two crew and planes in smooth
water in less than 10 kts of breeze. Its main is 117 ft **2, jib 55,
IIRC. The old spinnaker was 210 but they changed to a larger one
recently.

A Thistle weighs closer to what you are proposing (500 lbs, I think).
If you can find a local fleet you can probably observe when they start
to plane.

These boats are sailed very actively. Just sitting in the cockpit
makes planing more difficult. I don't think equivalent hp is likely to
be a useful concept for your purpose.


I also found general wind power formulae in books at the library for
generating electicity but there are constants in the formulae, eg
efficiency, which I don't know for boat sails.

I've looked at numbers for several racing boats of about the same
displacement in library books to compare sail sizes but I'd like
to do the calculation for this boat if I can manage it.


Rodney Myrvaagnes NYC J36 Gjo/a

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

thanks for the comments.

for planing purposes angle of heel isn't important. there isn't any. since
the boat can be assumed to be on a beam reach the full horsepower of the sail
can be used in the calculation.

I've planed Albacores. The planing wind speed I got using the numbers
available does seem high, but then I never had a wind speed (apparent)
indicator in the boat.

When someone designs a planing boat, and there are light displacement
planing cruisers although much larger displacment that this tiny one I'm
playing around with, he or she has to use a formula for predicting sail
area for the boat to plane. That's what I'm looking for.

I'm having a good time working out all the numbers with the Plue Peter and
Carlson hull programs (and my own skiff program) and comparing them to
numbers in boat design books from the library. I want to put a description
of the design project on my website. I see lots of descriptions of amateur
building projects on websites, but none of an amateur design project.

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As large as a sail is, if it is not a rigid foil, parts of it
will not be working while a section in the middle will.

Sail twist is a method used to automatically reduce power in
gusty windy conditions: The top part luffs, the middle is about
right, the bottom section is stalled. Very common in cruising
sails, very easy to see who has a tight leech and flat sails, who
is just plonking along, who is racing hard, who has reefed early,
who needs to reef now, etc.

Just look at the tell tales which should be everywhere, about a
foot apart.

A boom vang will tighten the leech, flatten the sail (as opposed
to let it twist, not wrt the camber, or depth of the airfoil
shape, but the conformance to a continuous angle of attack from
the top to the bottom of the sail.) With a tight vang, the entire
sail performance more closely approaches consistant efficiency
from top to bottom, but it requires constant attention to keep
the boat going to the max. Boats with large sails and tight
leeches react instantly to shifts and gusts, and need instant
attention to main sheets, or they get depowered or dumped,
broached, turned upwind, spun around and even tacked
uncontrollably.

Power boaters do not understand this, that a moment's
inattention, like when a flying fish lands in your eye, can
result in a 180 degree course change and the sailor hanging by
the neck in the mainsheet, clobbered by the boom, and still
trying to get his contact lens back. It's all perfectly routine
for an experienced sailor.

Sails with enough twist sag a little more in a gust, luffing more
at the top, and reducing the amount of sail in the middle
actually working at top efficiency, reducing the power increase
otherwise generated by a gust on a 'flat', or 'stiff' sail,
resulting in less weather helm than might otherwise occur, and
giving a more sea kindly, 'forgiving' ride.

It's all in the sail trim, lad.

Further, a sailboat in motion in a 10 kt true wind may see an
apparant wind on board of anywhere from say 5 kt when going
downwind, to perhaps 20 kt if really boiling along upwind.

The hp / sq meter figures do not take this apparant wind effect
into account, but only calculate one absolute value.

Once the boat has gathered way, the doubled apparant wind yields
4 times the power, or is that 8 times as much, dependant on
twist? Some aerodynamacist mathematician should know if power
increases as the square or the cube of the airspeed, which is the
speed at the sail, or apparant wind. Sailors more taste it than
cook by recipie. Finding 'the groove' is done by buttock feel.
Er, sorry;-)

The planing hull shape might be more important than the sail
area. The most efficient planing hull might be one that traps a
big bubble of air under it at speed. Ask any hovercraft driver.

I would be interested to see a sailboat with a bunch of tall
pipes connected through the hull to a point well above the
waterline, with some sort of sensor in each pipe to measure
airflow, pressure and suction against the hull at speed,
investigating the way a hull's shape can affect hydrostatic
pressure, and co-incidentally, friction against the hull. Could
pumping air under a hull reduce drag? Ask any air hockey puck.

P.C. will expound that Viking boat hull shapes have air tunnels
on either side of the keel, and benefit when air is trapped under
the hull at speed, regardless of how it was constructed.

Terry K

"William R. Watt" wrote:

Is there a formula relating boat sail area, wind speed, and
horsepower?

The best I can find at the public library are some ranges, ie
hp/sq ft of sail area = 0.015 @ 7-10 kt
0.020 @ 11-16 kt
0.040 @ 17-21 kt
0.070 @ 22-27 kt

I'm trying to get a better idea of how much sail I'd have to put
on a light 15 foot plywood cabin cruiser with appropriate
underwater shape, whose total weight with one occupant and a
week's gear (including accumulated human waste) is 630 lb., to get
it to plane in less than a full gale. TF Jones says 1 hp to plane
each 40-50 pounds.

For comparison, the above table applied to an Albacore racing
dingy (250 lb) with two people aboard (300 lb) and 125 sq ft of
sail says the wind has to be off the above scale for the boat to
plane.

I also found general wind power formulae in books at the library for
generating electicity but there are constants in the formulae, eg
efficiency, which I don't know for boat sails.

I've looked at numbers for several racing boats of about the same
displacement in library books to compare sail sizes but I'd like
to do the calculation for this boat if I can manage it.

--
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Terry K - My email address is MY PROPERTY, and is protected by
copyright legislation. Permission to reproduce it is
specifically denied for mass mailing and unrequested
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purpose is THEFT and PLAGIARISM. Abuse is Invasion of privacy
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SofDevCo

Terry Spragg ) writes:

It's all in the sail trim, lad.

there's none of that stuff on a beam reach


Further, a sailboat in motion in a 10 kt true wind may see an
apparant wind on board of anywhere from say 5 kt when going
downwind, to perhaps 20 kt if really boiling along upwind.

wind speed is wind speed. its a simple matter to calcuate apparent wind
and add it to ambient wind speed and use that in teh formual, in fact it
can be included in the formual, whatever it is.

I would be interested to see a sailboat with a bunch of tall
pipes connected through the hull to a point well above the
waterline, with some sort of sensor in each pipe to measure
airflow, pressure and suction against the hull at speed,
investigating the way a hull's shape can affect hydrostatic
pressure, and co-incidentally, friction against the hull. Could
pumping air under a hull reduce drag? Ask any air hockey puck.

P.C. will expound that Viking boat hull shapes have air tunnels
on either side of the keel, and benefit when air is trapped under
the hull at speed, regardless of how it was constructed.

thanks but I'm not designing a cathedral hull.
in teh 1920's MIt did some tests on sails with primative equipment that
you oudl dupicate if you want. one neat thing they did was to put a smoake
bomb on the end of a pole and stick it out from various parts of the boat.
they made a film of it. you coudl do it with a camcorder. you'd need
someone in a powerboat with a walkie-talkie to record views from outside
the boat.

no, there's got to be a formula relating wind speed, sail area, and
horsepower.
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William R. Watt wrote:

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

THe formula for wind pressure perpindicular to a flat surface is
P = 1/2p × V² the p is the specific mass of the air which varies due to
temperature and humidity but somewhere around .0022 but that is just the
beginning. There are other rather complicated formulas for calculating
lift and drag that are way over my head.


In Dave Gerr's book there is a sort of rule of thumb wind speed/HP
table: At 9-10 Kn a sail can produce .015 HP/sq. ft. At 13-15 Kn it is
..020 HP and at 19-21 Kn .040 HP.

--
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

Frank Bethwaite's book "High Performance Sailing"
has some info about planing potential, sail area and
weight. I don't have my copy any more but it may
be at your library.
Matt


no, there's got to be a formula relating wind speed, sail area, and
horsepower.
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William R Watt National Capital FreeNet Ottawa's free community
network

Glenn Ashmore ) writes:

THe formula for wind pressure perpindicular to a flat surface is
P = 1/2p × V² the p is the specific mass of the air which varies due to
temperature and humidity but somewhere around .0022 but that is just the
beginning. There are other rather complicated formulas for calculating
lift and drag that are way over my head.

I'm sure its all been worked out by aeronautical engineers, but for higher
relative wind speeds.

In Dave Gerr's book there is a sort of rule of thumb wind speed/HP
table: At 9-10 Kn a sail can produce .015 HP/sq. ft. At 13-15 Kn it is
.020 HP and at 19-21 Kn .040 HP.

thanks. I have that. Its the best info for sails I have been able to find
to work with so far, but a bit course.



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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?

Terry Spragg ) writes:

It's all in the sail trim, lad.

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