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  #1  
Old 03-14-2004
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hrae is on a distinguished road
what''''s up with sail twist?

i''m trying to understand the effects of sail twist. i was reading about it and on the same page the book said that sail twist can accelerate you and decelerate you. and you get sail twist by trimming the sheets and by easing the sheets.
ok, i''m confused.
is sail twist an important element to sailing or is it just fine tuning.

all right-brain-friendly explainations welcome.
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  #2  
Old 03-14-2004
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what''''s up with sail twist?

Twist is an important part of sail trim. In light air it is the only way to deal with gradiant wind effect. Gradient wind results from the friction of air passing over water. In light air the friction of the wind passing over the earth''s surface is significant. At the water surface the wind is not moving at all due to friction. At some point in the air the effects of friction is negligible. In between the windspeed fall somewhere between the ambiant wind speed and the zero at the water surface. This means that the head of the sail feels a faster wind than the bottom of the sail and with greater speed the apparent wind appears lifted so without twist the upper part of the sail would be over trimmed and the lower part of the sail would appear undertrimmed. This is especially important coming out of a tack where the apparent wind angle difference would be greater because of the low speed of the boat. To accelerate you need greater twist. As speed comes up, less twist is necessary.

Twist is achieved by bringing the traveler to windward and then easing the sheet resulting in a more horizontal pull on the boom.

In heavy air twist is a major problem as it means that part of the sail needs to be over trimmed in order to obtain drive. Here the traveler is dropped and the sheet tightened to reduce heel and drag while increasing drive.

Jeff
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Old 03-17-2004
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what''''s up with sail twist?

Jeff wrote:
"This is especially important coming out of a tack where the apparent wind angle difference would be greater because of the low speed of the boat. "

I think you have this the wrong way around.

The apparent wind angle difference (to true wind angle) is LESS at lower speeds. It is the magnitude (and direction) of the boatspeed vector that causes the apparrent wind angle to be different from the true wind angle.

In other words, if you are stopped, apparent wind angle equals true wind angle.
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Old 03-17-2004
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what''''s up with sail twist?

I must say that I am a little baffled here. When I first read your post I thought that I could clearly explain this but as I started formulating a response I began to be unsure that I had a clear picture. I thought that the best way to look at this was to draft a quick vectorial analysis which I did using the CAD program at the office. That should have cleared things up but it has left me more baffled than ever.

Perhaps someone can jump in and explain this. I drew a pretty simple model in which I assumed that the true wind was 45 degrees from the course of the boat. I assumed that the true windspeed at the top of the mast was around 4 knots and that the true windspeed at the boom was half of that or 2 knots. I then looked at what happened to the apparent windspeed with the boatspeed at 0, 1, 2, 4, and 6 knots. I am going to list each result as a series of six numbers. The first # is boatspeed, the second is apparent wind angle at the boom, the third is the apparent wind speed at the boom, the fourth is the apparent wind angle at the mast and the fifth is the apparent windspeed at the mast. The last number in the series is the difference in apparent wind angle between the mast and the deck (angles given in degrees and minutes).

1kt,30d22'',2.7k,36d28'',4.7k,6d6''
2kt,22d30'',3.7k,30d22'',5.6k,7d52''
4kt,14d39'',5.7k,22d30'',7.4k,7d51''
6kt,10d48'',7.6k,17d46'',9.3k,6d58''

When I first started coming up with the numbers I noted that what ''marginal'' had suggested seemed correct. In otherwords the difference in the apparent wind angles was zero when the boat was at rest and quickly jumped to 6 degrees and then to nearly 8 degrees. But then I checked 4 knots of boatspeed and was amazed to see the change in angle actually decrease. I decided to look at the difference in angles at 6 knots just to see what happened and noted that the here the difference in wind angle decreased further. That actually matched my observations out on the water where coming out of a tack I typically ease the mainsheet slightly and bring the traveller up to windward a little, as the boat accellerates I slowly drop the traveler and tighten the sheet reducing twist.

BUT this is really a mystery to me and so I am hoping that someone with a bit more math background can jump in and explain this because It has me baffled.

Respectfully,
Jeff



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Old 03-18-2004
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what''''s up with sail twist?

You''re seeing the the rates change differently, and the area between the two curves builds then diminshes. Think of the extremes of say boatspeeds of zero, 3 kts and 100 knots. At zero, there should be almost no difference in angle from masthead to water, at around 3 kts the difference is maximized, and at 100 kts the difference is present but again made very small. I think you''ve mentioned reading Bethwaite, so if you have his picture of the mast with the 180degree opposed streamers in your mind, remember that the picture is only possible for boatspeeds in the range of the truewind speed less the gradient effect. (For the none-Bethwaite initiated, he illustrated the gradient wind effect that occurs below 6kts true by erecting a mast with streamer ribbons along its length in an outboard skiff and motoring downwind at around three knots in a five knot wind. The streamers at the top streamed forward, since the true wind speed exceeded the boat speed, but the streamers at the bottom streamed almost straight aft, because the air at the water''s surface was essentially still. This has very powerful implications for sail trim on light air days.)

I suspect the sail trimming techniques you mention are firstly of more use in light-air and maybe related to the tendency to foot off initially after a tack to build speed before resuming a close hauled course. A reaching course will have a larger apparent wind angle difference along the length of the mast.

I have a tough enough time getting the sails trimmed right for each leg, let alone for the transient effects of tacks and waves and the like. But I understand (usually) what needs to be improved, and each improvement leads to another order of magnitude of improvements that need to be incorporated. Good thing we''re having fun, right?

Best,
-Chad
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Old 03-24-2004
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what''''s up with sail twist?

this is all very fascinating, or will be when all those new brain cells i just bought on ebay arrive.
but, and i ask this most humbly, can anyone explain it to a newbie?
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Old 03-30-2004
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what''''s up with sail twist?

Essentially, the wind at the top of the sail is blowing faster than at the bottom of the sail. This makes the apparent wind direction change a little bit as you go higher up the mast. You can see this by looking at a pictures of a square-rigged ship sailing on a reach. The sails up high will be pulled in less than the ones lower down. This is because down low the wind seems to come from further aft, (going upwind or reaching) because the slower windspeed is more affected by the boat''s forward motion than wind at the top of the mast. In any case, twisting the sail helps keep it adjusted properly to match this.
The other reason for twist in the sail is so that when it starts to blow too hard, the top of the sail, which has more leverage to heel the boat, releases the pressure and makes it easier to keep the boat flattter.
Its all a matter of balance.
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Old 03-30-2004
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what''''s up with sail twist?


I will try to answer your last question first, specifically:
"can anyone explain this to a newbie?"

Imagine you have a tall pole with a bunch of long streamers or pieces of yarn attached at regular intervals from top to bottom. Imagine that you are standing with the pole in front of you. If there is no wind, the streamers just hang down. If the wind picks up then the streamers will begin to fly/float/flap along the direction of the wind. Imagine that the wind is coming directly from the side (lets say your left side), then the streamers will be blowing out to the right. Now we add a little more complexity to the picture, because when the wind blows, it is blowing slower closer to the bottom (whether water or ground) because of friction, and faster as you go up. So, in a light breeze, the low streamers might barely move to the right, while the streamers up high are flying out quite well (By the way, at very high wind speeds, there is still a difference in wind strength as you move up away from the surface, but the difference is small relative to the overall wind speed, so that in a gale, all the streamers are blowing out straight).

Now suppose the wind dies, and you start moving your pole forward. The streamers begin floating back. If you move the pole quickly (forward) through the still air, then all the streamers will equally fly straight back (since the pole is moving forward at the same speed along its length).

Still following?

Now, suppose that while you are moving forward, the wind begins blowing from your left. Now the streamers that were floating backward because of the motion of the pole forward through the air, are now also being blown toward the right. And the ones near the top of the pole are being blown by a stronger wind than those at the bottom (because of the friction at the surface, the wind speed is slower). Suppose at the top of the pole, the wind speed is equal to the speed at which the pole is being moved through the air. The top streamers would be floating out halfway between ''right'' and ''back'' (45 degrees). The streamer at the bottom would be pointing more toward the back, because the ''wind'' caused by moving the pole forward would be stronger than the wind blowing naturally from the left (which is not as strong at the surface than it is up high, where we said the two winds were equal). So, if you look up your pole (or mast) you would see a set of streamers (or you can imagine them as wind vanes) that are pointing in a gradually shifting direction as you go upwards from closer to straight backward near the bottom to 45 degrees at the top. This should give you a picture of the what the wind appears to be from the perspective of the leading edge of your sail (we are ignoring the problem of the pole/mast itself interfering with the wind).

As you might know, you want to trim your sail relative to the wind direction. As you have seen, though, the wind direction at the top of the sail is a little different from the direction at the bottom. Having sail twist allows the sail to be trimmed appropriately from top to bottom (the top of the sail would be at a greater angle to the centerline than the bottom of the sail, which would be closer to the centerline).

Of course, you don''t always need or want sail twist (or the same amount of twist) for optimum performance in different conditions. In very high winds, the true wind is blowing much harder than the "wind" caused by the boat moving through the water and the difference in speed and direction between the top and bottom of the mast is much smaller as compared with the overall wind speed (meaning the difference is negligible) and you might be mostly concerned with flattening the sail as much as possible to minimize heeling and avoid being overpowered etc. This is admittedly a simplification, as all sailing theory and discussion is necessarily a simplification of the continuously varying conditions actually involved in sailing a real boat in the real world.

Now, if you are still with me, I will try to answer your original question:
"is sail twist an important element to sailing or is it just fine tuning"

This all depends on what your sailing abilities are and what you want to get out of your sailing experience. For many, part of the joy of sailing derives from understanding and controlling the boat as precisely as possible to optimize sailing performance. That is, the joy of sailing comes from the joy of sailing well. I should add that in some cases, having a thorough understanding of the many and various factors involved in handling a sailboat in a variety of conditions can spell the difference between life and death (though sometimes that too can be a matter of simple chance or fate). BUT, for many, the joy of sailing comes from being out and moving modestly through the water from one place to no particular other place and being with friends etc in not too hazardous conditions. And if that is part of the experience you cherish, then I would say that you can hoist your sails and trim them modestly well and relax and have a good time and not be particularly concerned about whether you have just the right amount of twist in your sails. You can still move through the water at a reasonably fast pace by getting the basics right. Later on, if you want to do more, there is always room to improve.

I hope this answered either or both of your questions
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Old 03-31-2004
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what''''s up with sail twist?

Very nice explaination Randolph! You covered a very complex subject in a very understandable manner. You should be a teacher!
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Old 03-31-2004
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what''''s up with sail twist?

There''s another aspect of twist: More twist makes the sail more forgiving of minor changes in apparent wind angle or speed (such as those caused by waves acting on the boat, or by a steerer who is merely human.

Picture a sail with a fair bit of twist. Somewhere around halfway up, it''s perfectly trimmed for the conditions of the instant. Below that point it''s too far in; above that point it''s too far out.

But as the boat speeds up and slows down (waves) or wanders above and below the desired course (waves, steerer) there is always *some* part of the sail correctly trimmed for the conditions of the moment.

Contrast this with a really rigid sail, trimmed perfectly all the way from top to bottom for the conditions of the instant. Now, if you head up only a tiny bit the whole sail is luffed; if you bear off only a tiny bith the whole sail is stalled.

The implication? everything else being equal (which of course it''s not): Bigger waves or a less accurate steerer: more twist. Flatter water and a god-like steerer: less twist.
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