sail twist

Am I correct in thinking that having the top batten parallel to centerline will allow the top third of the sail to be twisted slightly into the apparent wind paralleling the twist in the apparent wind along the height of the sailplan? I am a beginner and feedback would help me picture this. Frank

Comments

Frank, I am with you, I have never quite understood why the top

Frank, I am with you, I have never quite understood why the top batten should align with the center line of the boat, or with the boom. It has always been my impression that the apparent wind at the head of the sail is more abeam than at the boom, and therefore the sail should be twisted "open" in the head to reduce drag. I have seen many Scot sailors refer to the top batten this way and would love to hear someone explain why this works. In my days of racing planing windsurfers, the head always twisted away, and this was very fast when well powered-up. There was a very large difference in apparent wind speed and direction from head to foot. Any comments would be welcomed. Phil Scheetz FS 4086

Phil Scheetz

FS 4086

Fleet 163, Nockamixon Sail Club

Phil: someone emailed me and who has technical background and

Phil: someone emailed me and who has technical background and said that the issue with the batten parallel to centerline really has nothing to do with the wind speed being greater abeam but with the upwash from the foot of the sail to its head, and that the upper batten position really lowers drag. It is drag in relation to upwash not in relation to the twist in the apparent wind he said. at this point, it beats me what's happening. it seems to me that someone would have done some simple experiments to determine if the sail needs to be twisted at the top because of shifts in windspeed or not. simple measuring devices on the mast would do it. so I do not know at this point. perhaps we could get greg fisher and harry carpenter in on the discussion. what do you think? frank

Phil: someone emailed me and who has technical background and

Phil: someone emailed me and who has technical background and said that the issue with the batten parallel to centerline really has nothing to do with the wind speed being greater abeam but with the upwash from the foot of the sail to its head, and that the upper batten position really lowers drag. It is drag in relation to upwash not in relation to the twist in the apparent wind he said. at this point, it beats me what's happening. it seems to me that someone would have done some simple experiments to determine if the sail needs to be twisted at the top because of shifts in windspeed or not. simple measuring devices on the mast would do it. so I do not know at this point. perhaps we could get greg fisher and harry carpenter in on the discussion. what do you think? frank

The apparent wind will be more forward, not abeam of a boat in m

The apparent wind will be more forward, not abeam of a boat in motion. That is why your masthead wind indicator points so close to the direction you are sailing when on a beat in say; 10 knots of wind. Another way to visualize the effect it is to motor around on a windless day, the apparent wind is always in front of you, no matter what direction you go. On a light air day you will want the batten out more, partly because you are going slower so the apparent wind is less and because you won't want to try to point as high. Sometimes I think we get too technical, the sailmakers that service this class have pretty detailed trimming guides. The best approach for most is to follow their guides, that can speed the learning process qiute a bit.

Dave: I think you missed what Phil and I were saying.

Dave: I think you missed what Phil and I were saying. We are not talking about the wind due to the motion of the boat going forward. We are talking about the increase in wind speed as you move away from the lake surface to higher altitudes. the lake surface causes friction and slows the wind. so some authors say that the wind can be as much as 40% faster 30-40 feet up the mast toward the headsail. If that is so, then the wind will be more abeam at the top. thus the question of twisting the sail. some of us do like to get technical dave! different strokes for different folks!! take care frank

I did miss the point about the increase in wind at the masthead.

I did miss the point about the increase in wind at the masthead. I see Phils reference to that on a windsurfer but having never windsurfed I guess I did not realize that there would be large differences in wind on a sail plan that short. I was thinking that in a puff the boat excellerates (hopefully!)and you trim in slightly to maintain proper trim, except when overpowered.

As it was my e-mail to Frank that he refers to, I should join in

As it was my e-mail to Frank that he refers to, I should join in here so that any critique or questions around my interpretation is borne by me. I thought Frank asked one of the better (and most difficult) questions around sail trim. I've wondered the same thing my self over the years and have tried to understand it. This has involved a lot of study in the area of fluids and aerodynamics. It is all self-taught, and I am acutely aware of the limitations in my knowledge and the huge scope of the field. When I read the tuning guides and literature (e.g. Bethwaite), I see many points of "truth"---it's data, it's what works, it's how to do it and where to put it. I don't question the validity of any of this (except for the occaisional author who still thinks the "slot effect" is real). The part that is missing is an intuitive sense of what the air is really doing across and around the ENTIRE rig. How should I think about it? How would I explain the flows in a way that form a robust mental model that is still scientifically valid? What's really going on out there? I think that what is really going on with the twist in the upper part of the main is not a change in apparent wind caused by boat speed. My understanding of the aero literature indicates there is a spanwise flow (toward the mast head) and as the air seeks to equalize the pressure on both sides of the sail, a considerable upwash forms. This upwash is what changes the flow over the sail--what others interpret as a change in the apparent as you get higher up the mast. I believe the position and shape of the leech determines the drag of the sail. So what Greg Fisher is telling you, is if you put the "batten parallel to the centerline" then your leech will be in the optimal position to have the minimal drag for that particular angle of attack for the sail. It has nothing to do with the batten. The sail will need more twist as the angle of attack increases. In aerodynamics, airfoils have a Lift/Drag ratio. You don't get lift with out drag, so as a sailor you are trying to maximize your lift under any given condition---but you must also minimize the drag. We get more lift by bearing off (i.e. we increase the angle of attack) but you get more drag, too. We then try to lower that drag with an optimal trim setting (more twist). Here's how I "think" about this: For windward sailing-- The tiller controls your lift The vang and sheet control the drag The reason is that lift is determined by the sail's angle of attack. We control this by steering the rig (attached to the boat) to the angle of attack we want. Increase the angle of attack by bearing off a little and the lift will go up. The drag also increases, so we would allow a bit more twist to relieve it by reducing the leech tension with the vang and sheet. The literature calls this a "power" or "acceleration" setting and the sail has a lot of twist. You might be tempted to think that a twisted sail has a lot of power in its shape. I don't think this is the case. The power comes from the higher angle of attack and the additional twist is to relieve the drag of the rig--hence you go faster. By the same token, as you point or pinch up, the angle of attack you are steering is less. Consequently the lift is less--and the drag is less; so we need less twist. This is why the experienced sailors don't spend a lot of time in this mode---there is very little power. They will sail in power or normal mode (moderate to high angle of attack) build up speed, and point up, then bear off again to regain power. I tell people to sail like a sparrow flies--power up and glide. The sparrow will make a few wing beats to get its speed up, then will fold its wings into its body and glide ballistically in a low drag configuration. It repeats this sequence every couple of seconds. Buddy Melges called it "scalloping" in his book. If there is a conceptual model to follow in this topic, I believe the concept of Energy--Maneuverability Theory that drives much of high-performance combat aircraft design and flying would be the place to start. We acquire energy from the rig and expend it in maneuvering the boat. There is also a decision process that we follow to ballance all these performance and tactical factors. Those of you familiar with Boyd know where I'm going with this. Thoughts?????

gallus: you do have a way to throw out teasers that invite fur

gallus: you do have a way to throw out teasers that invite further discussion! while I have read that what happens in the slot can be quite complex, (it is not a simple application of Bernoulli's principle to be sure, for example, an upwash of air in the slot that then goes around the leeward side of the jib) I am not at all sure what you mean that the slot effect is not real. could you explain? from my vantage point as a novice, it seems that the "intuitive" pathway at the "hands on" skill level is going to have to be "speed and power"--a sense of what trim makes for maximum speed under what wind conditions and what trim makes for maximum power for what purpose. Do you agree? And at a mental level, it seems that there is simply no "intuitive" picture out there. frank

Frank, Your note about the slot effect and the mis-applicatio

Frank, Your note about the slot effect and the mis-application of Bernoulli's Principle is what I meant. Over 30 years ago, Arvel Gentry did a very thorough job of putting that explanation to rest---It had been accepted as truth by nearly everyone for decades, (still is by some) but it was incorrect. I believe we need to think about this whole area as more than just the aero-dynamics of the sails. We need to look at it in the context of how we actually sail the boat. I am not suggesting a re-write of physics, just a different persective on what is going on. The notion of Energy and Maneuverability trade-offs makes a lot of sense to me: You get energy from the rig. You "expend" energy maneuvering the boat and from the drag of the rig. If you look at it from an "energy" perspective, you deal with all the forces acting on the boat which impact "maneuverabilty" (i.e. where do I want to put the boat). For example, the Coefficient of Moment (CM) is well known in aero theory but I've never seen it referred to in any technical sailing literature. The CM is the pitching moment of your rig--it is what gives you windward helm. It is what makes bearing off to duck a starboard tack boat so hard in stronger winds. It doesn't effect lift or drag, so it is ignored. It DOES however, impact maneuverability, and this is ultimately what we want to achieve (better boat position on the race course). I suggest climbing out of the box, walking around a bit to another viewpoint, and see how it all lines up from there.

I saw a good article that helped me to understand how different

I saw a good article that helped me to understand how different this is from my old windsurfing days. In windsurfing we frequently sailed in windy (choppy) conditions and thus the wind speed at the foot of the sail is much lower and mainly induced wind. The wind at the top is truer apparent wind, with the wind not tripping over the waves. You only have one sail, and since board speed is close to the wind speed, the twist in the head to account for the apparent wind direction change is very important to reduce drag. The article I read speaks of the interaction of main and jib as one foil. This make sense to me that the jib is a good portion of the lift of the combined foil (with a slot or vent) and that the batten position of the main is to create a clean "exhaust" off the main. Phil FS 4086 Phil Scheetz FS 4086

Phil Scheetz

FS 4086

Fleet 163, Nockamixon Sail Club

Here's the site I was looking at: http://wb-sails.

Here's the site I was looking at: http://wb-sails.fi/news/98_11_PerfectShape/Main.htm Phil Scheetz FS 4086

Phil Scheetz

FS 4086

Fleet 163, Nockamixon Sail Club

Phil, The WB Sails site is an excellent reference.

Phil, The WB Sails site is an excellent reference. You are correct in thinking of the jib & main as one foil. What seems to be happening is that the main interacts with the jib to cause it to experience 1) a higher angle of attack (lift), and 2) cause somewhat more air to passaround the jib luff than if the main was alone (more energy). Most of the WB content comes from Arvel Gentry and a series of articles he wrote for Sail and Sailing World back in the 1970's. You can view them at his web site http://www.arvelgentry.com/. Gentry was an aerodynamist for Douglas Aircraft and approached the questions of how sails really worked from a disciplined, engineering background. He used an analog field plotter and computer programs to get the streamline data--not his imagination! His articles on the Slot Effect and Safe Leeward position will verify your statement. I think he has about fifteen papers altogether. Not necessarily easy reading but it's pretty thorough for us amateur enthusiasts. On the twist question: I called a friend of mine at NASA last week to run a few things past him. He said there are two reasons for the twist. First is to reduce the drag at the higher angles of attack. Second is to reduce the heeling forces on the boat. I'm not sure that the apparent wind explains the twist: The apparent wind change would need to have the follwing conditions: the wind speed would need to be appreciably slower as you went up (further from the surface) but every indication is that it increases in velocity. The wind would otherwise need to change its direction about 15-25 degrees in the space of a few vertical feet. That would be a lot of shear on the local airflow. I'm more inclined to think that the differences are more easily explained by the local effects (e.g. upwash and vortex formation) of the rig itself as it moves through the air. I think you are right in that there is a relative airspeed effect going on, but I'm not confident it is the boatspeed + windspeed = apparent wind condition that accounts for the twist. Jim Davis FS 784

Jim: In response to your comments about twist.

Jim: In response to your comments about twist. When I read by some author that the wind spped could be 40% greater 30-40 feet up the mast I did find it hard to believe. But as I noted in a previous comment, it seems like this would have been an easy thing to measure. Do you know of any source that has indeed measured the real wind speed 30-40 feet up the mast? Additionally, why do you sya the wind would have to be slower going up the luff? If the true wind speed were to increase, which is logical given surface friction, then the apparent wind would be more abeam, and thus twist at the upper third of the sail would re-position its angle of attack appropriately--keeping the angle of attack up the sail more or less constant. So what did you mean? frank

The author of "High Performance Sailing", Frank Bethwaite and hi

The author of "High Performance Sailing", Frank Bethwaite and his son Julian have done much practical research on this subject. It is explained well in the book. http://www.amazon.com/gp/product/0713667044/qid=1134947531/sr=1-1/ref=sr...

Please note that what follows are responses to our discussion he

Please note that what follows are responses to our discussion here from Paul Bogataj in emails to me and copied here with his permission. Also note Paul's fine discussion at the North Sails website on this very topic. So far, his article at the North site is the briefest,clearest yet technically satisfying explanation I've run across. It has answered many of my questions in a succinct way, and correcting notions out there and/or filling in blanks about the aero-dynamics of trim that I had troubles with. "Thanks for the link. I think I understand the issues now. Actually "Gallus 102" made a decent explanation on Dec. 13, when he correlates the heading angle (through tiller control) with angle of attack on the rig, hence load, and then inputting the appropriate twist for that load using the mainsheet (or vang) to control drag. The objective is to maximize the forward component of force within the constraint of not producing too much heeling moment. Another item that I think needs better definition is that the top batten indicates the direction of the back of the upper portion of the sail, which is not the same as the angle of attack of the top. The angle of attack is the angle is referenced to a line connecting the luff with the leech. For the sail to be cambered, the aft portion must be less twisted than this line, so the batten may appear less twisted, while the sail actually is twisted (because there is more camber in the top, hence the batten angle is more closed even though the sail section is more open). Connect dots on the leading edge with dots on the trailing edge at various heights and the sections should appear twisted. The position of the leech determines the angle of attack of each section. The necessary twist is an affect of the twisted apparent wind, as you understand. I hope this helps. I got the impression you may have been misinterpreting the batten angle as the twist angle. Feel free to include this in the forum. Another resource that I think you would enjoy is a collection of Arvel Gentry's work at arvelgentry.com. It could be useful to include the link to that and to the "How do sails work" article I wrote at: http://northsailsod.com/articles/article6.html. The higher flow angles toward the top are due to a combination of both apparent wind direction due to wind shear, and induced upwash to due the highly tapered planform. The amount that the apparent wind speed increases moving upward depends on where the lower windspeed measurement is taken in the gradient. Theoretically, the wind velocity right at the surface of the water is zero due to friction. A common model is that it increases as a seventh power function in the upward direction, so it quickly increases near the water surface, then becomes quite linear above that with smaller increases for similar increments of height. Having modeled this, the affect of increasing wind velocity with height (Gallus slipped on this, but has most things right) produces only a few degrees of apparent direction change over the height of the rig. While seemingly small, that does influence the amount of load by a significant amount, particularly when you consider that the actual angle of attack of a specific section is really only in the 5 to 10 degree range. The other factor that influences the need for more twist toward the top is induced upwash (as Gallus understands). I think I covered this in the article on North Sails' website. The more tapered a wing is (which sails inherently are highly tapered), the more upwash on the outboard sections. The bottom line is that the flow is what it is (for various reasons) and we need to present the best possible shape to it in order to maximize performance. I am glad this is helping. Paul Bogataj" from Frank with permission

Frank and Hotwheels, Bethwaite is an excellent reference.

Frank and Hotwheels, Bethwaite is an excellent reference. I bought my copy seven years ago and expect to be re-reading it for the rest of my life! If you look in Chapter 3, Bethwaite shows some tests with free-stream ribbons done at different heights. In very light air, (below 5kts) the boundary layer of the air is laminar across the water's surface. There is a difference of nearly 90 degrees in the ribbons' alignment as they get higher and into the moving air. This is the only condition I am aware of where the apparent wind moves aft as the velocity increases with height above the water. At around six kts, the viscosity of the air is overcome and the ribbons all stream in essentially the same direction. The boundary layer has become turbulent and takes on the characteristics of a breeze. The effective difference in sail power from 5 to 6 kts is a three-fold jump---far more than would be expected from the increase in airspeed alone over the sails. Refer to Chapter 5, Figure 5.1 for a quantitative breakdown of the sail forces between these two conditions. Jim Davis FS 784

I read Paul's article last night.

I read Paul's article last night. WOW! This article is REALLY good. It reinforced everything I have learned previously and filled in the gaps on what I had not. I suggest we refer to it in all future technical discussions and questions. "How Sails Work" is precisely what I wanted in my original post from 13 December--a concise explanation of how all the flows and forces interact with one another. Paul ties it together very nicely. Paul has another article on the North Sails site, "Wind Angle X Upwash of the Sails". There are two computer generated images shown where you can visualize the flows, pressure gradients and vortices coming off the rig. They appear to be of two modes, pointing (low angle of attack) and footing (higher angle of attack). I'd never seen the information presented like this. You can quickly grasp the concepts that Paul explains in his article. BTW--Although we have been focusing on the upper end of the rig in this forum, take a look at the huge vortex coming off the boom. I knew it was there, but didn't realize it was as significant as the simulation shows it to be. If there are other illustrations like this available, we could show how the rig is working at different wind speeds and modes (i.e. varying Reynolds numbers and angle of attack). You could have a visual "mental image" to complement the theory part and I think this would be very useful. Jim Davis FS 784

Excellent timing gentlemen.

Excellent timing gentlemen. Now I will have some reading for the holidays. Can't sail the Scot on ice, so reading sail trim articles will have to suffice... The Gentry articles look interesting, as well as the article by Paul on the North site. Phil Scheetz FS 4086

Phil Scheetz

FS 4086

Fleet 163, Nockamixon Sail Club

Jim: While I have not read the book you refer to yet, but I w

Jim: While I have not read the book you refer to yet, but I will purchase it, I still have the question about what you said regarding the wind speed as we climb the sail. Paul seems to be saying that the apparent wind speed does increase a small amount, and that the shift aft is small but has substantial impact on the sails---thus one reason for the need for twist (besides upwash). You seem to be saying that from your reading that the wind does not change speed as you go up in height beyond 6 knots. This seems like a contradiction between the two of you, or what you have read I guess. Yet you praised Paul's article, and didn't mention the difference. What am I missing or misunderstanding? I too felt like Paul's article gave me a conceptual model. Frank

Frank, The short answer is that Paul is right and I was in er

Frank, The short answer is that Paul is right and I was in error---I didn't think that the apparent wind gradient changed as much as it does. When you factor that in, it all makes sense. I went back to Bethwaite and looked a little closer at the streaming ribbons and there is that 5-10 degree difference that Paul refers to. This is what Paul meant when he said I "slipped a little"--I hadn't taken that gradient into account. It wasn't lack of awareness--I just didn't think (until now) that it had a significant variation in the short distances on a modest sized rig like the Scot's. If I understand Paul's explanation correctly, most of the flow is accounted for by the upwash with the apparent wind accounting for another couple of degrees of angle of attack. I suspect this is going to vary somewhat under different Reynolds numbers, but we should consider it as one of the parameters. Jim Davis FS 784

quote:[i]Originally posted by Gallus 102[/i] [br] Gentry was an

quote:
[i]Originally posted by Gallus 102[/i] [br] Gentry was an aerodynamist for Douglas Aircraft and approached the questions of how sails really worked from a disciplined, engineering background.
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Cole: what do you mean?

Cole: what do you mean?

There is an article in the latest issue of Sailing World (Jan-Fe

There is an article in the latest issue of Sailing World (Jan-Feb 2006) on roll-tacking. The roll-tack uses twist in the upper mainsail to get some forward thrust as the mast rolls in an arc to windward. The article focuses on the crew choreography of actually doing it. They don't show the upper part of the sail, but they discuss some of the sail trim points. Roll tacks are are kind of neat because they combine the apparent wind, forward momentum, upwash, and dynamic lift in one quick maneuver. Jim Davis FS 784