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Go Back   SailNet Community > Skills and Seamanship > Learning to Sail > what''s up with sail twist?
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Thread: what''s up with sail twist? Reply to Thread
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Topic Review (Newest First)
09-08-2004 04:00 PM
hrae
what''''s up with sail twist?

hey, this is a lot of fascinating information about sail twist which i look forward to studying the next time i''m at work.
04-02-2004 07:22 AM
RichH
what''''s up with sail twist?

Your statement of ''conventional wisdom....." is profound!
Hint: Compare the ''twist'' on such things as turbine blades and propellers to the ''twist'' on sails. You''ll find that for regimes of greater ''tip speed'' you''ll find correspondingly less ''twist''; the ''conventional wisdom'' here avoiding that the greater the velocity over a foil/wing requires LESS angle of attack and less camber, etc. Just an aerodynamic fact. You dont ever want to disagree with the conventional ''experts''. But then again the same arguements will apply to ''slot effect'' vs. ''conventional wisdom'' ;-)
04-02-2004 05:59 AM
ccboston
what''''s up with sail twist?

I dunno -- I''m not much of a sailing expert -- I''ve spent more time in the library than on the racecourse... But I do know something about the development of technical ideas in science and engineering, and I know that often times ideas are really faddish. The "conventional wisdom" becomes accepted simply by repetition... The experts all start quoting each other, and pretty soon everyone starts accepting something (like "more twist in waves") as gospel truth without actually going out and doing the hard work of experimenting.

That''s why I got interested in your contrrarian position, especially given your willingness to experiment for yourself.
04-02-2004 02:46 AM
Jeff_H
what''''s up with sail twist?

Well, even people like myself can learn something from the likes of Isler, Walker and Brun. I have generally followed the approach of setting the sail for optimum drive at an average position and carried minimal twist (but quite powered up) in a chop. It has generally been quite successful and came to the conclusion that it was faster in one-design keel boat racing where I could check it against competition and the sails, but given the sources, I guess I may need to experiement with carrying more twist in a chop.

The only other thing is that most of my current sailing and racing is in petty large boats (J-22''s, Cat 27''s, J-105''s, Farr 11.6 and a 40.7) and perhaps twist works better in smaller boats. I''ll just have to give it a try next time that I am in a chop and need to open up on a competitor. Thanks for the input.

Jeff
04-01-2004 07:13 AM
ccboston
what''''s up with sail twist?

Jeff -- I''m interested in your comment about not wanting more twist in waves, since it runs directly counter to the conventional wisdom (e.g., Dobbs Davis''s article right here on Sailnet, or Stewart Walker, or Peter Isler / Vince Brun to pick on a few.)

But oftentimes people who defy the conventional wisdom, especially people like you who are clearly coming from a position of solid theoretical and practical knowledge, are onto something interesting. Care to elaborate?
04-01-2004 04:53 AM
Jeff_H
what''''s up with sail twist?

I diagree with the point about needing more twist in waves. More twist in wave and wind means more heeling, and less drive and speed. Trimming the whole sail for an average setting setting (even if that means less twist) means that you have the whole sail working optimally more of the time even if the sail suffers small stalling and low angle of attacks at the end of each swing.

Jeff
03-31-2004 08:53 PM
ccboston
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.
03-31-2004 05:24 PM
joub
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!
03-30-2004 08:49 PM
Randolph Bertin
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
03-30-2004 08:00 PM
paulk
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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