| Home |
| Search |
| Today's Posts |
|
|
|
#1
|
|||
|
|||
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 |
|
#2
|
|||
|
|||
|
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 × V˛ 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 |
|
#3
|
|||
|
|||
|
sail horsepower?
Glenn Ashmore ) writes:
THe formula for wind pressure perpindicular to a flat surface is P = 1/2p × V˛ 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 |
|
#4
|
|||
|
|||
|
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 |
|
#6
|
|||
|
|||
|
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 |
|
#7
|
|||
|
|||
|
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 |
|
#9
|
|||
|
|||
|
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 |
| Reply |
|
| Thread Tools | Search this Thread |
| Display Modes | |
|
|
Similar Threads
|
||||
| Thread | Forum | |||
| Sail THE TRANSAT from your computer | General | |||
| Sail THE TRANSAT, the one of Chichester | General | |||
| Horsepower Old vs New Outboards ? | General | |||
| Exceeding rated horsepower? | General | |||
| Weekender query | Boat Building | |||
Hybrid Mode
