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 ZachW. 01-23-2002 02:37 PM

Ayrodynamics of Sailing

Ok, How do you know were to position the sail to get trust? I know the sail acts like a airplane wing in the sence that when air flows over the inside it slows and increases pressure. the opposite on the outer side. Vernullies Principle right? But how do you possition the sail according to where you want to go and direction of wind?

 Jeff_H 01-23-2002 03:49 PM

Ayrodynamics of Sailing

That is one question that really cannot be answered on a bulletin board. You are really asking how do you adjust sails and that unfortunately is a question that would take a book to provide the technical answer. If that is what you are seeking I suggest that you try to find a copy of C.A. Marchaj''s ''Sailing Theory and Principle''. Other good sources are Bethewaites ''Performance Sailing''. If you are only looking for the answer in a simple enough form that you can get out on the water, then my current favorite ''learn to sail book'' is Steve Colgate''s wife''s book called something like ''Sailing for Women''.

Jeff

 HHJ 01-26-2002 06:33 AM

Ayrodynamics of Sailing

Jeff H is right that this is a very technical quiestion, but there''s also an easy answer - trimm the sail all the way in for sailing close to the wind (30 -45 degrees off your bow), half way out when the wind is across your beam and all the way out when the wind is behind you. Now make fine adjustments for everything in between.

 BigRed56 01-27-2002 03:47 AM

Ayrodynamics of Sailing

Ahoy,Zach W. th answer is as simple as that you cannot always get the boat to go where you want by just adjusting sail. You may have to tack and change course constantly to make progress over the ground(bottom) to the place you want to go . I agree with the other posters there are many fine books available on the subject nut just doing it in a small boat can teach the basics book or not. Be carefull about the thinking anology of the airplane wing though similar there is little resemblence between flight and sailing. The physics and the dynamics of movement are totally different. Sail trim is only the fine tunning of machine and windf and wave and geography . Many variables ,and there are even more than I mentioned like nude 1st mates , rum, slippery decks eyc. Good Luck Big Red the Pirate of Pine Island

 dhartdallas 02-10-2002 06:26 AM

Ayrodynamics of Sailing

Ahoy, Zach.I will answer you query in two parts. First part: The effect you are talking about is called the Bernoulli effect after Jakob Bernoulli, a Swiss mathmetician and physicist who illuminated it. It says, in simple terms, that the faster a fluid flows past a sidewise surface, the less pressure it exerts. Pressure decreases as velocity increases. The air doesn''t slow as it flows over the inside as you said. The air speeds up as it flows over the outside. Sail makers take advantage of the Bernoulli effect by a simple mechanical means. Imagine an airplane wing. The bottom surface is a straight line from leading edge to trailing edge. The top surface is curved making the line from leading edge to trailing edge longer. Now, the wind must travel farther over the top than over the bottom. In order for air split at the leading edge to arrive at the trailing edge at the same time and equalize as it must, the air on top must speed up. The faster air exerts less sidewise pressure. There is no vacuum. There is pressure on both surfaces, but the upper surface has slightly less pressure. A fluid flows from greater to lesser pressure, so the air underneath tries to flow toward the air on the upper surface to equalize the pressure. But, the wing is in the way so it gets pushed in the direction of the upper surface. That gives the wing lift. Turn that wing vertical and you have a sail. Sails and wings work on pressure differential. End part 1.

 dhartdallas 02-10-2002 07:36 AM

Ayrodynamics of Sailing

Ahoy, Zach. (Part 2: How do you position the sail according to where you want to go and direction of wind.) I refer you to my discussion under Learning To Sail, Sail Trim For Beginners, and to my discussions under Learning To Sail, Heeling Paranoia. Also, my discussion under General, to Bob Ohler, regarding Mast Rake. These illuminate in a simple way the fundamental forces acting on a sail boat and their implications.
What we are striving for is to maintain the sail as a wing so that we get the lift we were talking about in part 1 at all times. In order for that to happen, there must be smooth air flow over both surfaces so that Bernoulli effect can operated. The wind must strike the leading edge, the luff, and part, running down both sides with as little turbulence as possible. We put tale-tales on the sail because we can''t see wind, but can see its effects on the tale-tale. When they are streaming back, laying along the sail, we can see that we have smooth air flow. When they flutter, dance, run up the sail vertically, or hang straight down, we know that the air is disturbed. To have smooth flow, the air must stay in contact with the surfaces of the sail. If we tilt the wing too much, the wind will not be able to do this. It will break away from the surface, become turbulent (we want laminar flow), and not flow along the surface in the desired manner. We say that the wing has stalled.
Now, if I face directly into the wind, I will find, try as I may, that I can not adjust the sail to create a wing. I will also find that if the wind is behind me, I can not. Down wind we are just pushed along by the wind striking the sails from behind and are not lifted as we would be if the sail were acting as a wing with lift. Into the wind we will stop and make no progress at all.
Now, if you begin easing the bow from dead into the wind to off the wind, you will find that at about 45 degrees (it is different for different boats), you can adjust the sails so they take up the desired shape and act as a lifting force again. As I continue to turn the bow of the boat away from the wind, I find I will have to continually ease the sail out to maintain that proper angle to the wind (called angle of attack). At 90 degrees to the wind, my hull is at right angles to the wind, and my sail is out over the lee side of the boat. Beyond that point, I can no longer adjust the sail to the proper angle to the wind because I can not move it any farther forward. At this point I enter the push mode and not the lift mode and that holds all the way through dead down wind.
I go into the wind with a series of runs across it, called tacks. I sail at 45 degrees to the wind on one leg then tack roughly 90 degrees and sail back across it the other way. That allows me to keep the sail angled correctly to the wind and it lifts me up to wind. I is slow, hard, sloppy going to wind relative to other points of sail, but I can get there.
As I turn away from the wind, I am not beating into waves and running so directly into the wind. My hull doesn''t heel so much , the ride is easier and I sail faster. Beyond 90 degrees I am being pushed.
Generally speaking, you will have your main sheet cleated on a beat and maintain the angle of your sails to the wind by using your tiller to turn the bow into and away from the wind as your tale-tales indicate. Up in a puff, and down in a lull.
On a reach, because you want to stay on your course, your rhumb line, you will adjust your angle of attack with the sheets, easing in and easing out as you need to to keep the tale-tales streaming. Always trim, whether with sheet or tiller, toward the stalling tale-tale.
A good way to trim sails is to ease them just until the wind starts getting behind the luff. It will break and bend inwards. Then harden your sheet just until the cupping effect, the break in your luff disappears. You will have the right angle of attack and smooth flow over the sail.
Finally, you will normally have at least two sails, the foresail and the main. Always trim the foresail first, then the main. You will have a bi-wing effect. Air is compressed and forced through the slot between the two sails, speeds up even more over the main, increasing lift. This is called slot effect.
The trailing edge of both sails should be as parallel as possible all the way up to ensure as much slot as possible, and your leeches should allow smooth reconnection of the upper and lower wind stream at the trailing edge. Too loose they flap and too tight they trap the wind stream breaking the smooth flow. IT IS SLOT SHAPE AND LEECH TENSION THAT DETERMINE THE DRIVE EFFICIENCY OF YOUR SAILS.
Summary: Ease the foresail just till the luff breaks and then harden slightly till the break disappears. Do the same with the main. Keep this angle of attack on a beat into the wind by steering the boat, and on a reach by easing and hardening the sheets.
Hope this helps. Good luck. Go sailing. There is no wind in books.

 Bobola 02-14-2002 12:49 PM

Ayrodynamics of Sailing

ZachW,
There''s a good book all about sails called &quot;sail power&quot;, written by Wallace Ross(?)
Best thing is to get out on the water with a small boat and just ''do it''...

 Bluesmoods 05-10-2002 03:38 PM

Ayrodynamics of Sailing

Thanks VERY MUCH dhartdallas !!!

 MaryBeth 05-21-2002 04:29 PM

Ayrodynamics of Sailing

Oh, Guys,

I never even thought before how hard it would really be to try to explain how to sail without actually being on the boat! I now realize how much I have to be thankful for. Whether delivery or cruising or racing (which I found to be hectic and exasperating, but fun), I have been so lucky, I guess, to be able to get the boat into a groove and go. Though I have always had a great deal of patience while teaching crewmembers, from reading these boards, I can see that it really doesn''t come easily to a lot of people.

Best wishes,
MaryBeth

 EscapeArtist 05-30-2002 10:41 AM

Ayrodynamics of Sailing

It''s a snap.

1. Point the boat where you want to go.

2. Let out the sail(s) until they flap a little.

3. Pull in until they just stop flapping. Don''t pull in any more.

4. When you change direction, repeat 1-3.

You won''t win any regattas like that, but it''s good enough for now. Sailing is a simple thing.

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