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sail horsepower?
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. -- ------------------------------------------------------------------------------ William R Watt National Capital FreeNet Ottawa's free community network homepage: www.ncf.ca/~ag384/top.htm warning: non-freenet email must have "notspam" in subject or it's returned |
sail horsepower?
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. -- -------------------------------------------------------------------------- ---- William R Watt National Capital FreeNet Ottawa's free community network homepage: www.ncf.ca/~ag384/top.htm warning: non-freenet email must have "notspam" in subject or it's returned |
sail horsepower?
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sail horsepower?
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. -- ------------------------------------------------------------------------------ William R Watt National Capital FreeNet Ottawa's free community network homepage: www.ncf.ca/~ag384/top.htm warning: non-freenet email must have "notspam" in subject or it's returned |
sail horsepower?
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. -- ------------------------------------------------------------------------------ William R Watt National Capital FreeNet Ottawa's free community network homepage: www.ncf.ca/~ag384/top.htm warning: non-freenet email must have "notspam" in subject or it's returned -- 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 solicitations. Reproduction or conveyance for any unauthorised purpose is THEFT and PLAGIARISM. Abuse is Invasion of privacy and harassment. Abusers may be prosecuted. -This notice footer released to public domain. Spamspoof salad by spamchock - SofDevCo |
sail horsepower?
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. -- ------------------------------------------------------------------------------ William R Watt National Capital FreeNet Ottawa's free community network homepage: www.ncf.ca/~ag384/top.htm warning: non-freenet email must have "notspam" in subject or it's returned |
sail horsepower?
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 |
sail horsepower?
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. -- -------------------------------------------------------------------------- ---- William R Watt National Capital FreeNet Ottawa's free community network |
sail horsepower?
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. -- ------------------------------------------------------------------------------ William R Watt National Capital FreeNet Ottawa's free community network homepage: www.ncf.ca/~ag384/top.htm warning: non-freenet email must have "notspam" in subject or it's returned |
sail horsepower?
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