Don... Can you help me understand "twist" some more?

[weinie]

Don,

In reading your posts, (which I can't get enough of, by the way), I am confused about certain aspects of twist that perhaps you could clear up.

If I read your posts correctly, you posit that the reason sails are capable of twist is because the magnitude of the wind velocity is much greater at the top of the mast, relative to the deck. Yet, I would think that since the DIRECTION of the wind is the same at either heights, the top and bottom of the sail must be trimmed at the same angle of attack for optimal lift.

The only conclusion I can draw, therefore, is that twist allows one to spill some wind from the sail in order to keep the heel of the boat in check. But if that is the case, why not just use the traveler to reduce the angle of attack of the entire leach of the sail? In other words, why just spill wind at the head of the sail as opposed to changing the angle of attack of the whole sail?

 

[Joe]

.................Yet, I would think that since the DIRECTION of the wind is the same at either heights, the top and bottom of the sail must be trimmed at the same angle of attack for optimal lift......................

I'm not Don, but...

NO.... It is the apparent wind that acts on the sail, not the true wind. Take a minute to refresh your understanding of the difference between TRUE wind and APPARENT wind. i.e. an increase in TRUE wind speed will move the APPARENT wind direction AFT. Twisting the sail is an attempt to mimic this apparent wind direction change.

We use the vang and/or mainsheet to adjust mainsail twist(leech tension), the traveller allows us to change the direction of attack without altering the mainsail twist. The leech telltales will give us a visual indicator of how well the air is flowing over the sail at various heights.

 

[Bill Roosa]

As wind speed increses (with constant boat speed) the apparent wind "comes aft". Winds up high are generally faster so the apparent wind up high is more aft than the apparent wind down low. we twist our sails to account for this.

 

[FourPoints]

Take a spare piece of line, and tie 12" long pieces of yarn every 3-4' and hoist it up with a spare halyard sometime while at anchor (dock could work too, but usually the wind is more turbulent because of buildings and other boats). You will see that there can be a significant amount of wind sheer (change in direction as you change altitude) in the 30-50 feet your mast rises above the water. This is caused by drag as the water or ground slows the wind. IIRC there is roughly a 30 deg sheer between sealevel and 100' altitude when the wind is 10-15kts (wind speed affects the amount of sheer present)

 

[Don Guillette]

The only conclusion I can draw, therefore, is that twist allows one to spill some wind from the sail in order to keep the heel of the boat in check. But if that is the case, why not just use the traveler to reduce the angle of attack of the entire leach of the sail? In other words, why just spill wind at the head of the sail as opposed to changing the angle of attack of the whole sail?

Using TWIST and ANGLE OF ATTACK to control heel is sort of like "walking and chewing gum at the same time" - a mate can use both or one or the other. Additionally, there are two sails to consider - the main and jib. Please bear with me as it's going to take a bit to explain as your comments are simple but the answer is a bit complicated.

Angle of attack is your ACCELERATOR. The more you TRIM IN the mainsail or the jib the more you force the wind to turn around the sail and this creates MORE force to move the boat forward. The LESS you trim in the sails or EASE them out the LESS you force the air to turn around the sail and the less power, which translate to speed, you develop.

So the question you might have is what's your point Don? Let's take two examples that induce heeling. Suppose your hit by a gust and the boat heels over. What control do most sailors go to first to reduce heel and bring the boat back on its feet - it's the mainsheet. I don't. I use the traveler and here's why. When you use the traveler, which is the only mainsail trim control used to adjust angle of attack, you do not change the shape of the sail. So, after the gust passes, I merely crank the traveler back to its original position and sail merrily on my way. The mainsheet is used, along with other sail trim controls, to adjust other elements (twist and draft position), which I may not have wanted to mess with in this situation. I don't. I just want to get the boat back on its feet.

In the next situation, let's say the wind is piping up and the boat is starting to heel over but your making good speed and you still feel like your in control of the boat - somewhat anyway. You could spill air and drop the traveler (accelerator) and kill your speed thus bringing the boat back on its feet or what I prefer to do in this situation is induce some TWISTin the mainsail and jib, which spills power from the top 1/3 of the sails but keeps the other 2/3 of the sails driving.

So, when you are using TWIST to spill some wind from your sails, that's exactly what you're doing - spilling SOME wind. When you use ANGLE OF ATTACK you're not spilling SOME wind, you're spilling ALL the wind along the entire shape of the sail. As always, this is what I do and the reasons I do it but there might be other ways to get the same result. I do and expound on what works for me.

I could have said the above paragraph in the first sentence but I felt I needed to explain my position. Did I answer your question? It's like writing a mystery book - I knew what the ending was going to be but I had to set it up!!

PS: for beginners to intermediates reading this thread. The first chapter of my book deals with draft depth, draft position, twist and angle of attack. At the end of the chapter I state that if you don't understand those "4 elements of sail trim" do not proceed further until you do because sail trim will never make any sense to you.

 

[RichH]

Weinie
-
To understand twist more correctly one has consider not only what the sail is 'doing' but also what the air is doing as it moves around (gets out of the way of) the sail ... and the 'anomalies' that are caused TO the wind BY the sail ... just because the sail is 'there' and the wind has to move 'aside' to get around the sail.

It is quite true that the closer the air is measured to the 'ground' (or any other surface) the slower the air will be that surface - all caused by friction. However, 'triangular' (pointy top sail) shapes have a 'special problem' .... at the very top of the sail (head) there will be an invisible 'rotor' following and just behind at the very top of the sail .... if you could see it it would look like a small 'horizontal tornado' following behind the sail at the very top !!!!!! The 'pointier' the sail the larger the 'following rotor'. If you notice the modern FLAT-top sails, such as being raced in the current americas cup or other match racing events you will notice that these 'flat top' sails have very little 'twist' (and if you could see it that 'rotor' following the top of the sail would be greatly reduced for a 'flat top' ... will we be seeing 'winglets' at the tops of sails soon?).
Also, The faster the boat is going and the faster the wind is blowing, less 'twist' is needed.

Secondly, the larger the 'cord length' (distance between luff or leading edge and leech / trailing edged) the more 'upwash' in front of the sail - the 'upwash' is the wind 'that is getting out of the way of the sail just because the sail is "there"'. The longer the cord length of the sail the more the 'upwash' ... so the bottom of the sail produces more upwash (sail can operate at a higher angle of attack) than the top of the sail --- then on top of all this, add the 'slow down' friction as you get near the bottom of the sail and you need to have the sail 'twisted', especially at the slower boat speeds and slower wind speeds, .... less so at the higher boat/wind speeds. All this is in respect to getting the 'most efficiency' out of the sail.
Twisting-off to reduce sail efficiency, to obtain reduced heel is an entirely different concept ... safety - so that the boat doesnt 'go over'.

For simplicity, when attempting to get the 'most' out of a sail, you can easily visualize the tell tales at the luff, midcord, and especially the leech.
For setting the correct amount of twist - Once the luff tell tales on both sides are flying 'perfect', then notice the leech tell tales and if the leech tales are not streaming 'straight back' then the 'twist' is incorrect for the present wind speed and boat speed.
If the leech tales are going 'forward' onto/on the leeward side of the leech then that section of the sail has its angle-of-attack us too great and should be 'opened'/lessened by releasing slightly the mainsheet and easing the traveller towards leeward, etc. ... until the leech tell tale (in that section of the sail only) goes straight back. You will most probably have to make several adjustments of mainsheet tension and traveller position until the sail is correctly twisted and ALL the leech tell tales are flying straight back.
Note: in sail design especially in the 12-15 kt. range and if the sail was *correctly raised* with correct luff/halyard tension you shouldnt have to make such twist adjustment as the sailmaker probably already designed-in the correct amount of expected twist into the sail for the traveller ON the centerline and with the proper amount of mainsheet tension. (http://forums.sbo.sailboatowners.com/showthread.php?t=120970)
In contrast, If the windward leech tales are 'unstable' AND the leeward side are 'unstable' (and the very top of the sail may be 'luffing/shaking'), then the amount of 'twist' is too great ... and the mainsheet/vang should be addtionaly tensioned and the traveller is set too close to the centerline or above the centerline and should be 'eased' a bit and/or the mainsheet tightened up a bit.

To get all this 'straight', just watch the leech tell tales and do whatever it takes by mainsheet tension and traveller position to ensure that ALL the leech tales are flying 'straight back' (but the 'aft end' of the boom NEVER goes above/beyond the centerline). When 'beating' usually leech portion of the second from the top batten will be or should be parallel with the boats centerline at between 12-15kts. .... and all the leech tell tales are flying 'perfect' or 'straight back'.
Ditto with the jib/genoa, but of course on most boats you will not have a 'traveller' on the jib controls so just do the best you can with fairlead fore/aft position and jibsheet tension ... but get those jib leech tell tales flying 'straight back'.

hope this helps
;-)

 

[Stu Jackson]

Some extremely good advice here. One of the better "twist" discussions I've read.

In addition, go out and practice. Sail your boat, have someone at the helm, and sit UNDER the main. Get on a close hauled or close reach point of sail, with the traveler amidships with a loose vang. Looking up at the main, harden the vang and see what happens. Then release the vang and drop the traveler. Once you beging to personally SEE the affects of what Don described, you'll do either of two things: 1) have a WOW, NOW I GOT IT!!! moment, or 2) re-read this thread and buy Don's book! Then go out and do it again.

Then try this again on a beam reach. When it's windy on a beam reach, popping the vang puts a good deal of twist on the top of the main and the boat starts to sail flatter and doesn't round up (like fighting you to go to close hauled).

Try it, you'll like it.

 

[weinie]

I'm not Don, but...

NO.... It is the apparent wind that acts on the sail, not the true wind. Take a minute to refresh your understanding of the difference between TRUE wind and APPARENT wind. i.e. a change in TRUE wind speed will move the APPARENT wind direction AFT. Twisting the sail is an attempt to mimic this apparent wind direction change.

We use the vang and/or mainsheet to adjust mainsail twist(leech tension), the traveller allows us to change the direction of attack without altering the mainsail twist. The leech telltales will give us a visual indicator of how well the air is flowing over the sail at various heights.

If the head of the sail is moving in the same direction as the foot relative to the "ground" (which of course, it must) then assuming the true wind's direction is constant from top to bottom, both the wind's apparent and true direction must be the same from top to bottom.
Of course this doesn't take into account other effects like the vortexes and turbulence described above.

 

[FourPoints]

then assuming the true wind's direction is constant from top to bottom.

Wind speed and direction are almost NEVER constant from the top of the sail to the bottom, and the taller the mast, the more true that statement becomes.

 

[RichH]

If the head of the sail is moving in the same direction as the foot relative to the "ground" (which of course, it must) then assuming the true wind's direction is constant from top to bottom, both the wind's apparent and true direction must be the same from top to bottom.
Of course this doesn't take into account other effects like the vortexes and turbulence described above.

Let me jump in here again .... the wind/air MUST get out of the way of the oncoming sail thus the 'true wind' is no longer from the same original direction therefore no longer 'true' .... as it was a long distance in front of the sail before it was 'influenced' and redirected by the sail .... therefore true wind has NOTHING to do with how a sail is reacting, only as a reference to what the original direction of the wind long before it came 'near' the boat.

True wind is used only as a 'far away' and 'non-influencing' reference ... or if the boat/sail is NOT moving relative to the wind or the wind has completely 'died' and the sail/boat has come to a complete stop.
With wings/sails/fluid dynamic 'foils', forget 'true wind' as such 'never operates' in 'true wind' ... its only a FAR upstream 'reference' in the 'undistorted flow' (unless you want to do some unnecessary severe and complicated math).

 

[weinie]

Wind speed and direction are almost NEVER constant from the top of the sail to the bottom, and the taller the mast, the more true that statement becomes.

I will grant you that wind SPEED is usually not constant from top to bottom, however, I will argue that DIRECTION (baring the effects of turbulence, which is of course becomes increasingly significant in gaining every extra fraction of a knot in racing) is in general, constant. Of course if we had a hypothetical mast hundreds of feet higher up into the atmosphere, this would not be the case.

EDITED to add:
I think we are going off topic discussing apparent vs. true wind.

I think I am answering my own question as to the reason why sailmakers build twist into the sail. I think it is to account for the GRADIENT of wind speed from the head of the sail to the foot (apparent, true, whatever, not the issue here). The force applied to any point of the sail is directly proportional to the cross sectional surface area. Since we never know how much the magnitude of the wind at the head will differ from the foot, we need to adjust for this rate of change. Since we cannot "re-cut" the sail when it is up, in order to decrease the surface area when the wind picks up at the top, we can reduce this cross-sectional surface area by adding twist; in effect reducing the area of the sail exposed to the wind at the top. As this gradient decreases, the twist would therefore need to be reduced to increase the cross sectional surface area at the top.
If this is indeed the case, one would have to conclude that the GRADIENT of the magnitude of wind is much greater at higher wind speeds than at lower wind speeds. Can anyone confirm if that is true?

 

[weinie]

So, when you are using TWIST to spill some wind from your sails, that's exactly what you're doing - spilling SOME wind. When you use ANGLE OF ATTACK you're not spilling SOME wind, you're spilling ALL the wind along the entire shape of the sail. As always, this is what I do and the reasons I do it but there might be other ways to get the same result. I do and expound on what works for me.

Don, if that is the case, I would argue that we could just spill a "little" bit of wind from ALL of the sail by just letting the traveler drop a lesser amount to the same effect.

I hope I am not coming off as being too argumentative, being a newbie here and all (as well as to sailing in general).

 

[Stu Jackson]

It always kinda amazes me when someone asks a question, is given great answers and then complicates issues without trying the recommendations.

You can discuss gradients all you want, but the real answer is simple. It's like the marketplace: you can develop all sorts of theories, but what you can actually purchase is what is being made and available.

You have a boat, you have a sail (or two) and a vang and a traveler. You have wind, and I agree that apparent wind is all that you can deal with when setting sails. Most people tend to forget that the real reason folks want to know true wind is to define which is the favored tack for either racing or getting uphill faster. In most cases, that can be found by luffing up and checking the compass or reading the waves.

That said, if you REALLY want to get into the dynamics of this new question you've just raised, then go and buy SAIL magazines great two books: Sail Trim and More Sail Trim. That's why they write books.

I read them when I was a newbie and they made my head hurt. I went out and practiced what Don's described in his excellent book, and learned what it all meant. By doing it and then comparing it to the esoteric material in those SAIL books. Not Don's - his is very understandable.

And I'm an engineer!

The combination of theory and practice is what makes anything work.

Rant over, sorry if I stepped on your toes.

BTW, you have a great boat with all the bells & whistles on it to try it out on the water. Heck, take the day off work and try it out today. Tell your boss we told you so. :dance:

 

[RichH]

Weinie -
I would suggest that you segment the amount of twist that results in A. the maximum amount of force produced for sailing in typical 'normal condition' and then (maybe by another independent posting) for when B. 'survival conditions' without reefing, etc.

These are two VERY DIFFERENT and quite dissimilar aspects of 'twist'. Confusion occurs when these two very very different concepts of trim/shape are discussed within the SAME discussion. ;-)

 

[red_dolphin]

Wind speed and direction are almost NEVER constant from the top of the sail to the bottom, and the taller the mast, the more true that statement becomes.

IMO, this is a key point [but certainly not the only point] to understanding twist.

A awhile back, in a book about full-rigged-ships, I read, "The wind is always fairer aloft."
This statement was illustrated by an image similar to the one linked here.

Since square-riggers have multiple sails on a mast, each sail can be trimmed to the apparent wind at the height of the individual sail. The result is 'twist' that is easily seen.

 

[Bill Roosa]

I'm with Stu on this one. A man with an experience is never at the mercy of a man with a theory. Take it on faith that the harder the wind blows the more you need to add twist. Then just set your tale-tails correctly.

For the record, you do not need to understand how something works to use it effectively. There are lots of folks that have no idea how the accelerator or brake pedals work but they are fine drivers.

 

[weinie]

It always kinda amazes me when someone asks a question, is given great answers and then complicates issues without trying the recommendations.

You can discuss gradients all you want, but the real answer is simple. It's like the marketplace: you can develop all sorts of theories, but what you can actually purchase is what is being made and available.

You have a boat, you have a sail (or two) and a vang and a traveler. You have wind, and I agree that apparent wind is all that you can deal with when setting sails. Most people tend to forget that the real reason folks want to know true wind is to define which is the favored tack for either racing or getting uphill faster. In most cases, that can be found by luffing up and checking the compass or reading the waves.

That said, if you REALLY want to get into the dynamics of this new question you've just raised, then go and buy SAIL magazines great two books: Sail Trim and More Sail Trim. That's why they write books.

I read them when I was a newbie and they made my head hurt. I went out and practiced what Don's described in his excellent book, and learned what it all meant. By doing it and then comparing it to the esoteric material in those SAIL books. Not Don's - his is very understandable.

And I'm an engineer!

The combination of theory and practice is what makes anything work.

Rant over, sorry if I stepped on your toes.

BTW, you have a great boat with all the bells & whistles on it to try it out on the water. Heck, take the day off work and try it out today. Tell your boss we told you so. :dance:

If it wasn't for Don, I wouldn't know half of what I know about sail trim! In fact, I think he has made me into a perfectionist in my sail trimming ways!!! I do try to manage my sail twist as dictated by my tell tales as well as all the other controls on my sails as best as possible rather than just working the sheets.

My problem is that although my body is here at work, my head is still on my boat... and maybe still in my old physics classes.

And yes I could've hit the library, but I'm too damn lazy!!! And I don't know about you Mr. Engineer but I gave up multivariable calculus and fluid dynamics after grad schoo.

 

[Don Guillette]

Like Stu, when I decided to get back into sailing, I read every sail trim book and article I could find and purchased a bunch of videos also. Only one problem - I had no idea what they were talking about because I couldn't figure out what they were adjusting in the first place. Finally, after endlessly reviewing the 9 books listed in my books bibliography until they almost fell apart, the sail trim lights went on and I saw and understood how simple sail trim actually is. I really wish I was able to understand and more importantly to explain things to the depth of RichH and Joe from San Diego and a few others but I'm not an engineer nor do I have any knowledge of aerodynamics (I just had to look up the spelling of the word!!).

In the forward of my book I state "This book may not be for everyone. If you wanted a scientific presentation that included intricate detail, together with complicated formlae for hull speed, wave lenghts and displacement/length ratios then this book is not for you". Same goes for the Sail Trim Forum. I give simplistic answers and then RichH, Joe from San Diego and a few others fill in the needed details. We all should be glad these guys are around. I don't care where you look you'll not find a better sail trim info site than this one

The book forward goes on to say that "sail trim is an acquired art and simple to learn and understand when it is presented in an easy to understand fashion". That was my goal with the book (I actually wrote the book for myself so I could understand it) and is my goal on this forum. In other words, don't fight it, just go out and do it. See if it works for you. Then, if you have some time on your hands, figure out scientifically WHY it works. My make up is I don't care why it works as long as it works for me. It's like the lift/drag curve. I don't know what it is but I know when I'm at the top of it because the boat/sails tells me - boats talk to me. My wife says they don't and not to tell people that because they'll think I'm crazy but she's wrong and what does she know. Alan says his boat talks to him also. My wife thinks he's crazy too!!

 

[weinie]

Stu:

Google (and you!) are my new best friends:

http://www.onemetre.net/design/Gradient/Gradient.htm

 

[Stu Jackson]

Stu:

Google (and you!) are my new best friends:

http://www.onemetre.net/design/Gradient/Gradient.htm

Always nice to make new friends.

Mine was thermodynamics!