Let's talk twist ...

[Scott T-Bird]

I have been reading some postings regarding twist that I haven't agreed with (and the posters are a couple of folks who know a lot more about sail trim than I do, I am thinking) so I feel like I need to hear some thoughts to set the issue straight.

Alan says that sail shape is flattened in gusting conditions to limit power. He and RichH both agree that moving the traveler down or 'playing' the traveler is the correct thing to do when reacting to gusts so that the main doesn't twist off and 'over-power' the main. They contend that easing the mainsheet allows the main to twist off and over-power the vessel at the height of a gust.

I understand the concept that easing the mainsheet 'loosens the leach' and allows twist. Where I diverge is the idea that a twisted-off main is overpowering. First of all, I don't consider that the adjustment of twist is a 'flattening' technique or as a 'power' adjustment. I consider that sail flattening is a function of outhaul for draft shape, and halyard tension and cunningham for draft position (and you could add back-stay tensioning - but doesn't that also loosen the leach?). Yes, the outhaul is for draft shape at the bottom of the sail, and I get it that when talking about the head of the sail sailors will refer to twist, but I consider that twist is either set correctly or it's not. If twist is correctly established, the airflow will not over-power the sail at the head. In other words, you would not purposely reduce twist for gusty conditions, particularly because a sail that is stalled at the top feels over-powered and causes the boat to heel excessively (which leads the sailor to want to change the angle of attack and basically block the boat from the best speed). Basically, I also consider the primary 'power' adjustments to be the outhaul, halyard and cunningham to establish draft shape and position.

I consider that twist is essentially a speed control. For optimum speed, twist is set correctly ... under-twisted is slow, sluggish and creates the illusion that the boat is overpowered because the head of the sail is stalled. Over twisted has nothing to do with power, but it is slower because the sail is inefficient at the head.

If anything, isn't a twisted-off main under-powered?

So this takes me to my experiences ... basically, in gusty conditions, on a close haul, I want twist. I find that if I center the traveler, and sheet in tight, I essentially have stalled the mainsail at the head because I have reduced twist. Since my traveler is too difficult to adjust (it is on track right where people sit and the track has stoppers, not line controls), I ease the mainsheet. Sure it increases twist, but it also changes the angle of attack and really slows the boat. But the worst part is that because the main has no twist to begin with, I get that over-powered, rounding-up problem all too frequently.

I have been experimenting and I find that when I move the traveler to windward, maybe up to half way to the end of the track (from mid-point), I can ease the mainsheet to center the boom AND induce twist. I sail faster, I sail higher, I find that gusts have much less impact (except that I usually get a lift that helps me sail higher) and my adjustments to overpowering gusts are far less severe and less frequent. Rather than over-powering the main when easing the mainsheet, I find that I make smaller adjustments to angle of attack which is the most significant boat slow-down maneuver I can think of.

In short, I find no downside to 'playing' the mainsheet in gusty conditions when I have the traveler set in the optimum position. What am I missing? I am wondering if boat size is an aspect that impacts this discussion.

Also, I have (twice) seen RichH refer to the traveler in regard to twist adjustment. Since we all know that the traveler only impacts angle of attack, I must be missing part of the explanation ... is it with regard to establishing twist as I described above?

 

[Joe]

Heh, heh...this ought to be good.

 

[Todd Smith]

Scott,

I agree that twisting off the top of the sail does reduce heeling ( and power ), but loosening the leech effects the draft of the upper sections of the sail. You might set the controls this way to match the average conditions, but not to depower in gusts.

On playing the main sheet in puffs. If you have the sheet set for the correct tension to set the twist and angle for the 'non gust' condition, then playing the traveler is a faster and easier ( less physical work ) way to dump a little sail and then trim it back in. This is especially true if the main sheet is boned on hard for the average conditions. Maybe in a little boat (a dinghy) this would be possible, but even in a Tartan 28 that I race on, it would be too much work to play the main sheet. On the x-412 ( 41 foot) it would be impossible.

Finally, raising the traveler and easing the sheet is a very normal way to trim the main in lighter air to have the boom near centerline, but not have the leech be too tight.

Todd

 

[Don Guillette]

Scott: I'm not sure where to start. Let's start with how attractive your wife is!!

Now that I've got that out of the way, I'm going to deal with the first 3 paragraphs and will get to the rest in the next message.

"flatten the main in gusting conditions" - a flat sail is a less powerful sail and a full sail is a more powerful sail.

"move the traveler down (or wherever you move it)" - you're changing the angle of attack only and there is no effect on the shape of the sail.

"twisting off the main is overpowering" - just the opposite. Power is being spilled off the top 1/3 and you're reducing power.

"sail flattening is a function of the outhaul" - right and if you have a bendy mast you could also use the backstay adjuster. The backstay adjustment takes the fullness out of the sail.

One last point for this session. Twist is built into every sail. The reason is that the wind blows 60% harder at the top of a 45' mast than at deck level. If you didn't have the ability to control twist you could not control the sail.

Now I have to go to the ice box and get a "tinney" (Aussie for beer). Be back shortly.

 

[Alan]

Maybe I can shed a little light here. A sail can be shaped all the way from nearly flat to blowing like a flag (no controls) and all the possibilities in between. Of course, the least powerful shape is the flag which effectively has no drive force. The next least powerful shape is a fully flattened sail. From this point on every adjustment that adds shape to the sail increases power until it reaches the point of the flag.
In gusting conditions, sailing with a relatively flat sail in the lulls and then dumping the sheet to the flag condition will certainly reduce all power. However, to change the mainsheet setting from full in to all out requires going through the full power up stages of the sail shapes. This change leads to sudden heeling, weather helm and loss of drive. As Todd rightly points out on larger boats dumping a highly tensioned mainsheet can be near impossible and a ton of effort.
The alternative would be to start with the same flat main and as the gust hits reduce power (and heeling) by simply lowering the traveler. This is highly effective and easily done without the need for huge and powerful adjustment or exposing the boat to unwanted addition power. More importantly the traveler can be 'fine tuned' to any increment to help maintain some driving force while holding heel and weather helm to a minimum.

 

[Paul J. Rusyniak]

Scott...
If what you're doing works for you, why worry about theory or anyone else's opinion?
It's obvious some respondents didn't read your comments about the difficulty you'd have playing the traveler on your boat.
Paul

 

[Alan]

Paul, let's be serious about this. I read Scott's post and understand his issue with his traveler. Not making adjustments because someone is in the way or his gear doesn't work well is not an answer. Building in additional issues for his particular boat doesn't change how things are designed to work. Sure he could stick with what he is doing but he raised the question in the first place. He wants to understand how it's supposed to work and why we do it a different way than he thinks.

This is exactly the type of thing that this forum was intended to address. To his credit Scott states what he thinks should work and wants to know why others disagree. Doing it your way because you like it is not the answer, learning the right way is.

 

[Don Guillette]

Scott/Paul: I hadn't gotten that far in your message but Paul brings up a good point. Here's the solution - replace the traveler. The traveler is a very important mainsail sail trim control. You can use the mainsheet to adjust angle of attack but it is a distant second. My 1981 C30 had the curved track traveler, which looked nice as it matched the curve of the cabin top, but what a piece of junk. It was loaded with friction and difficult to use. To move it I had to put my foot on the cabin!! At C30 National Regattas, where they gave us the crappy boat they could find (even with that we never finished wworse than 2nd), I cringed when I saw the curved track. My job on the national team was traveler/mainsheet. I loved it when the boat had the Garhauer straight track traveler.

Sailors will do what is easy and not what is hard and that also applies to pin type fairlead adjusters. Most that I see are frozen in place because they are too hard to mess with, especially under load. Check out the Garhauer adjustable fairlead system.

So, Scott take a look at what Garhauer has to offer in the way of travelers. For a few bucks you'll be happy with the results.

I agree with Paul - do what works for you. The advantage you have is you have a good idea of what you're trying to accomplish.

 

[Stu Jackson]

Paul nailed it, as I read it. The answer, Scott, is that you ARE inducing twist in the entire sail, not just the top, when, you release the mainsheet with the traveler in a set position. The boom has to go somewhere, and where it goes is UP, which adds twist to the main. It's kinda as simple as that.

You mentioned this twice, IIRC: I consider that sail flattening is a function of outhaul for draft shape, and halyard tension and cunningham for draft position...

But it's more than just those things, and the mainsheet affects them, too, and in most cases much more. Think belly plus simple draft.

Let's take your example: close hauled, traveler fixed, whether above centerline or midships, then you turn from close haul to close reach, you let out some mainsheet, instead of dropping the traveler. You have, effectively and realistically changed the shape of the sail. You are going faster (close reach vs close haul) BUT your sail is now deeper and more powerful, pretty much just when you wanted it just as flat if not flatter than when you started.

I fully understand your concept, but agree that what you're saying is that this works for me on my boat with my limited motion traveler. That's cool, good for you. But it doesn't change the fact that by releasing your main you ARE changing the shape of the sail throughout the entirety of its dimensions, not just the top.

Here's another / two examples: in the late 70s early 80s the Catalina 30 came first with those fixed-pin travelers like yours and then with arched curved traveler tracks. Many discussions, similar to this one, occurred until folks began to understand what Don's been telling us all these years.

Cunninghams, outhauls, vangs and halyard tension all are parts of the equation, but the biggest change in sail shape will be evident by releasing the mainsheet. Period.

Once you get to and past a beam reach, the vang just ain't gonna be able to do it all and the boom will rise, too. The reason is that I play with these combinations of vang and mainsheet all the time going downwind.

We, too, have a "larger" boat, and I gotta tell ya, releasing the mainsheet is the VERY LAST thing ever done on our boat, and we sail in breezy conditions more often than not. Why? Because it is a blasted b*tch getting it back in, when dumping the traveler takes four nanoseconds to release and pull back up on our Garhauer 6:1 traveler system. How do I know this? I've keelhauled crew who've released the mainsheet first, and then I make them pull it back in!!!

We, too, move the traveler above the centerline and use the mainsheet to shape the sail, usually with the boom on the centerline or a tad below. But that's basic sail shaping and positioning 101. And we usually move the boom above the centerline because the mainsheet can't get the boom over enough if the traveler is centered and the main backwinds. There's a word for that, right, Don?

 

[Clark]

I'm going to chime in with a little bit of agreement with Scott and personal experience. I read that too about "twisting the main will overpower" and said "Huh?". I'm pretty comfortable with setting things up on our boat but maybe I'm just not using the proper degrees of tension on the controls.
BTW, I don't play the mainsheet either in gusts; it is too difficult on our boat too. I try to flatten the main when the wind builds but sometimes it seems better to trim so that the top twists off and reduces lift up there and reduce heeling. When the wind pipes up, I try my best to respond with the traveler and it usually works OK until I'm just waaay overpowered - easily done with our fat-roached main and mid-boom traveler/mainsheet.

 

[Winkfish]

This is how it was explained to me a long time ago. As Don said, the wind blows harder at the top of the sail than at the bottom. This also means that apparent wind at the top of the mast will be different from the apparent wind at the bottom. So the best thing to do is have a different angle of attack at the top of the sail from the bottom. Since a sail isn't divided into two pieces, one at the top and one at the bottom, you use "twist" which means adjusting the sheet and the traveler so that the top and the bottom of the sail are each at the correct angle of attack.

A corollary the Don's rule of wind speed is that it takes less power to push a lever at the end than it does at the middle, so the wind at the top is exerting more force on the mast (and therefore contributing to heel) than at the bottom.

So, assuming you need to "dump" wind, you want to do it as little as you can in order to keep the driving force. If you can do it by just dumping the top, with the sheet, you would do that. That would reduce heel but leave the bottom of the sail still driving. If still overpowered, then you dump the bottom as well ,with the traveler.

If you're talking about gusts or puffs, then the increased wind, as Don says, will have a greater effect on the top of the sail as opposed to the bottom, so, in effect, you would want to change the twist of the sail in the increased wind, which would be by easing the mainsheet.

Like Scott, I want to know if my thinking is wrong, and learn why (ease of using the traveler versus the sheet notwithstanding).

Thanks!!!

 

[Paul J. Rusyniak]

Alan...
If I wasn't being serious, I would have ended with a
Paul

 

[Scott T-Bird]

I'm getting a clearer picture ...

But here is the problem I am having ... in a post about travelers RichH says 'the traveler allows or controls the proper twist in the upper portions of the sail'. He goes on to say 'the traveler allows the sailor to change the angle of attack in gusty conditions without the mainsail powering up when releasing or easing the mainsheet'. These aren't direct quotes but close enough to make the points clear and I didn't write the statements or understand them or agree with them. Alan seems to be agreeing that twisting off the main overpowers in gusty conditions.

But Don and others are agreeing with me that twisting off the main is depowering. Actually, I technically don't think it is depowering but rather speed reducing to be twisting off excessively. However, I still contend that when the leech is tightened to inhibit twist, the sail is definitely stalled and feels 'overpowered' so you don't want a tight leech in gusty conditions.

Now I want to talk about sail 'flattening'. I think of flattening as a shallow draft and a fine entry (draft moved aft). When 'flattening' a sail, I don't want a tight leech to inhibit twist. A Pringles potato chip is flat but it is shaped nicely to fit in your mouth and curve around your tongue . A barn door is also flat but you would never want your sail shaped that way because it would be similar to trying to handle a sheet of plywood in a strong wind. Not that I want my sail shaped like a potato chip, but the image of a shape that manages the wind properly is what comes to mind!

Two points I disagree with ... Stu, there is no twist at the lower section of the sail, the shape is called draft. Todd, there is no draft at the upper section of a sail because the draft disappears into twist. I fully understand your notion that allowing the boom to lift changes the shape of the sail, but you are both missing the point, I believe.

I also agree fully with Alan that an improperly functioning piece of equipment is no excuse for not using it correctly. It then simply becomes a practical matter of fixing the assembly so that it is used properly. But in this discussion, we have heard from Todd, Stu and Alan that playing the mainsheet is a BIG problem on a large boat and that dumping the traveler is far easier and the most expedient means of scrubbing wind. So it seems to me, THAT is exacty the crux of the issue. It is simply easier to dump the traveler for them, not necessarily the best method to adjust for a gust.

First of all, when sailing in a particular direction, changing the angle of attack will have the most dramatic effect on boat speed ... it is like thowing an anchor in the water. So why, then, would I want to throw the traveler down to scrub boat speed just because I am finding a gust of wind? If the wind doesn't change direction but suddenly picks up velocity, the true wind stays in the same direction, but the apparent wind moves aft (probaby even more so at the head of the sail). Ok, so instead of scrubbing boat speed by throwing out the anchor by moving the traveler down, I want the head of my sail to twist off so that it doesn't stall and make the boat over-powered. So I ease the mainsheet. Even Stu says that easing the mainsheet lifts the boom for twist. That's EXACTLY the point. I want twist and LESS change in angle of attack. In fact, with a gust, I want to take advantage of the lift that I get when the apparent wind moves aft. Changing the angle of attack is doubly bad in this case. So I want more twist to reduce the overpowering of a gust (overpowering because the head of the sail is stalling when the leech is too tight) and I want less change in angle of attack which only tends to luff the mainsail just at the moment I want to ease my heading upwind to gain with the aid of a lift.

That is why I say that dumping the traveler is bad in a gust and easing the mainsheet is better. I am beginning to think that you large boat sailors only do it to be expedient and that it has nothing to do with what works best.

 

[Winkfish]

Re: I'm getting a clearer picture ...

Scott, I think we must have gone to different schools together.

 

[Cayennita]

Are we forgetting that a good driver with information from the rail can feather up and increase VMG up wind without any sail changes? We use back stay and traveler 95% of the time in the Virgin Islands where I race. At the same time the crew hikes harder and the boat can climb to windward against the competition without much effort. The Mumm 30 was able to do 22 degrees apparent. Not by dumping the main, but by paying attention. When cruising, drop the traveler and sheet in. control twist also with a powerful vang. Check out a Star boat or for that matter a big tri.

 

[Clark]

Sounds good to me Scott with this exception:
"there is no draft at the upper section of a sail because the draft disappears into twist". The top of the sail can definitely be twisted in relation to the lower sections but it still has draft (except for maybe the topmost 3-4").

 

[Todd Smith]

Scott,

I think that you have an unusual definition of draft and flattenning. Draft is the amount of curve in the local section of the sail. A flat sail has little draft. You definately have draft in the upper sections of the sail. Twist is the angle difference between the chord line of the bottom and top sections of the sail.

As far as sailing to weather, during a puff we don't need all that extra lift, we have plenty, so we dump the traveller. What we don't want to do is increase drag. Added twist and draft increase the drag, which slows the boat down.


PS. I find the way sailors use the term "depower" very awkward. It does not directly corelate with reducing lift or spilling wind, but changing from a rounded "high lift/high drag" shape to a flatter "medium lift/low drag" shape.

 

[ffaubert]

PS. I find the way sailors use the term "depower" very awkward. It does not directly corelate with reducing lift or spilling wind, but changing from a rounded "high lift/high drag" shape to a flatter "medium lift/low drag" shape.

OK..., I've got a lot to learn.

Is this all in relation to a "Close Reach, Close Hauled" kind of thing or does this apply to any point of sail? I would think that past a beam reach, baggy is better.

Frank

 

[Don Guillette]

Going back to basics, since I'm having trouble taking in all the TWISTS!! and turns of info this thread is developing (Joe said "hee, hee.. this ought to be good" and he was right) there are only 2 sail trim controls that are used to adjust TWISTand they are the boom vang and the mainsheet. The easiest way to adjust twist in the mainsail is with the boom vang, if you have one. If you don't have a boom vang your only recourse is the mainsheet but you have to be careful as you'll see in a moment.

Since we are talking about adjusting sail trim by using methods we are comfortable with, here's why I personally prefer the boom vang over the mainsheet for mainsail twist control. Of the "4 elements of sail trim" which are draft position, draft depth, twist and angle of attack the mainsheet is involved with 3 of the 4 - angle of attack, twist and draft position. The boom vang is only involved with 2 - twist and draft position and its effect on draft position is minor. So, while your messing with the mainsheet to adjust twist you're also adjusting other elements you may not want to be adjusting.

As the Mexican peasants said of Pancho Villa - "where is Rich H when we need him".

 

[Scott T-Bird]

Frank, my discussions have essentially been about handling gusty conditions on a close haul. Todd, I don't think my definitions are any different than yours so I will have to mull over your thoughts while driving.

I am thinking that if draft position at the lower section of the sail is mid-chord, then draft would essentially disappear about half-way up the sail when the leech is loosened sufficiently to allow twist. I will have to look at that more carefully, but I seem to think that my upper 2 full length battens are essentially flat when I don't have the mainsheet totally cranked down. I think you have a point that I am going to consider.

We have talked about how easing the mainsheet increases twist, but does easing the mainsheet increase depth of draft? If that is the case, than I can more readily admit that easing the mainsheet increases drag. That is one of the things I'll mull over. But doesn't shifting the angle of attack also have a dramatic impact by increasing drag and losing drive at the same time? That is the primary reason why I object to moving the traveler down.

BTW, I also think that the term depowering is used inaccurately. I think of power and speed as a form of compromise. Needing power for choppy or rough conditions is said to be achieved by deepening the draft and moving the draft position forward as a compromise to speed which likes a finer entry (draft aft) and flatter sail (less draft depth, less drag). I don't think my definition of sail 'flattening' is inconsistent. Controls like the halyard, the cunningham and the outhaul, as well as a backstay tensioner if you have one, are what I consider flattening tools. I consider 'flattening' and 'shape' as distinquishable terms. Shape includes the use of twist which can optimize the airflow over a 'flattened' sail.