|
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?
|
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?
|
sail horsepower?
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. -- ------------------------------------------------------------------------------ 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?
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. :) -- ------------------------------------------------------------------------------ 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?
|
sail horsepower?
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. -- ------------------------------------------------------------------------------ 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: 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 |
sail horsepower?
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. -- ------------------------------------------------------------------------------ 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?
|
sail horsepower?
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. :) -- ------------------------------------------------------------------------------ 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?
|
sail horsepower?
On 2 Nov 2003 16:09:57 GMT, (William R.
Watt) wrote something .......and in reply I say!: 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. A sail cannot be quantified. Too many variables. some responses did not address the question. hmmm... they may not have _answered_ it.... you can go back and re-read the original question if you like. No need. AFAICS, the reason there is such a spread is that conditions apply. ************************************************** **************************************** Until I do the other one,this one means nothing Nick White --- HEAD:Hertz Music remove ns from my header address to reply via email !! ") _/ ) ( ) _//- \__/ |
sail horsepower?
thank you all for the informative discussion.
Henry Miller ("Sailing Yacht Design" (1965) also disagrees with me, writing "As the driving medium, the size of the sail plan obviously should bear some relationship to the resistance of the hull. Unfortunately there is no direct means of evaluating the driving force that can be produced by a given size sail plan under specific wind conditions." That seems odd to me. Surely a somewhat idealized model can be described mathematically and boundary values computed. I'm having a samilar problem comparing heeling force of the sail to righting moment of the hull and crew to see how much sail to carry. The books discuss the Dellenbaugh the angle I was asking about in an earlier thread and another method both of which depend on measurements taken from the dynamics of the completed hull or model. I must declare I'm disappointed. I think I'll keep working on these analytically to see if I can get any useful numbers. I appreciate the need to compare with data from exsisting boats to check any calculations. I realize its the practice among yacht designers to use comparisons where they can't calculate numbers analytically. I've got lots of data on specific day sailers and light crusiers to use, and some scatterplot summaries of data on daysailers and light cruisers, all from books at the public library. The design I'm playing with appears to be reasonable by comparison. As I work on the design I have to answer a lot of questions which refine my ideas about how the boat will be used and under what conditions. Its interesting to try and come up with a roomy ultralight cruiser with sufficient power and response to work the shifty winds on inland lakes and rivers like the waters we have on the Rideau system, hopefully without resorting to auxilliary power. -- ------------------------------------------------------------------------------ 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 |
| All times are GMT +1. The time now is 10:55 PM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
Copyright ©2004 - 2014 BoatBanter.com