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  #1  
Old 07-10-2013
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Twist

Hello, all. See if this sounds about right.

First, I'm a fairly new club racer, nothing serious, several years of weekly (seasonal) skippering and many more of crewing behind me.

Secondly, I believe that much confusion on this topic stems from twist vs. flatness. To the beginner, they sound contradictory, like a twisted sail must be a full sail. The books say no, that one can have twist without fullness. The hows and whys are for another thread or later in this one.

Thirdly, I hear and read that one doesn't usually want twist in the sails "except when" ...

My personal take on all of the exceptions is that one usually wants twist except in one specific scenario.

If we make a grid with wind strength on one axis, and wave state on another, each on a 3-point scale of "mild, medium, severe", that would be nine boxes. I would like to look at only some of them, to illustrate my understanding and ask for comment.

1. Let's take light winds in flat seas (not such light winds that it's virtually calm - that's a different story). The difference between winds aloft and alow is such that twist is desirable to keep all parts of the sail optimally trimmed.

2. Let's take heavy winds. One wants twist because the struggle is to keep power out of the sails (reduce heeling, weather helm, etc.).

3. Medium winds and lumpy seas - the boat's not going to track exactly straight no matter how skilled the helmsman, or in more extreme lumps there will be relative calm in the troughs, so having twist always keeps some part of the sail under power.

4. Medium winds and calm seas. This is the only situation I can think of when less twist is desired, the fabled "upper batten in line with the boom" scenario.

Of course, I'm talking as if twist is either on or off, and it's more nuanced than that.

I'm also not talking (yet) about "how" - a bit of a struggle on my boat currently.

Making any sense so far?

Thank you.

Charles

P.S. See previous threads by others on this topic:
Depowering the sails and twist

what''s up with sail twist?
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Old 07-11-2013
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Re: Twist

It looks like you have a pretty good handle on the various reasons you might want to add twist to your sail. Just be careful not to over think it. Remember they are not hard and fast rules, they are guidelines. Whether to use twist and how much really depends on YOUR boat, YOUR sails and the conditions YOU are sailing in.

The "HOW" is something you should be addressing before you read any more about twist! All that theory doesn't do you a bit of good if you can't apply it and see how it works on your boat.

Why is setting twist "a bit of a struggle" on your boat?
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Old 07-11-2013
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Re: Twist

Thanks, ShockT.

I have a 1981 US Yachts 33 (i.e. feet), built on a Buccaneer plan (long story, they tell me), and it's a lovely, fast boat in many ways, but prone to weather helm and needs delicate attention to trimming when up to weather. So when we're racing, getting the crew to coordinate and play everything well together in concert is a challenge, and we are working on that and getting there. They're all such busy people that most of them absolutely cannot get together on evenings or weekends for practices -most frustrating.

So, that's my "struggle": both technical and human factors.

We have busy races in a 1-mile-wide river, so short legs with lots of tacks, jibes and roundings, so the skipper cannot realistically verbalize every control adjustment on both sails (at least 7 for the main and 4 for the foresail), especially on nights when the winds are gustier and/or shiftier. The crew has to understand what's going on and learn to be semi-independent, and we're working on that too, and making gains.

We have a specific challenge with the main back-winding. A bubble in the forward third is acceptable, but sometimes to reduce a heavy weather helm, we end up flogging the entire sail (but the streamers on the leech are happy!), so we're working on that too. That can't be good, although I know two experienced racers who say "just live with it". I can usually get it all to settle down when I'm sailing alone ... fiddle with things for a few minutes, mainly bring the traveller way up, ease the main-sheet but also ease the foresail. "If in doubt, let it out." "Keep 'er full and footin'!" The boat seems to like to have the main reefed early, say in the 12-15 knots true-wind range, and is quite happy at 20 degrees of heel even in (only) 8 or 9 knots of true wind - it's unclear to me whether that's good, and is specific to this boat, or whether I should be cranking on unbelievable amounts of backstay tension, making other adjustments, etc.

So, the struggle I referred to isn't "twist" alone, but it plays into all of those deliberations. Twist is the thing I hear talked about most, and it became apparent the other day that my crew was confused about the distinction between twist and depth, which has befuddled me in the not-so-distant past as well. For example, raising the traveller and easing the main, leaving the boom just below mid-line on a close haul, is said to add twist but also flatten the sail. That's hard to visualize why that works (but it does work), and I believe that many casual racers never get to that degree of sail-trim knowledge.

Thanks again.

Charles
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Old 07-11-2013
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Re: Twist

Charles, sounds like you are well on your way... US Yacht 33 eh? Interesting...

But they are masthead rigs with low-aspect narrow mains. The reason why your main bubble isn't a huge factor is, the boat drives almost entirely off the genoa.

I am betting when you are racing with that bubble, and pointing, you are sheeted in hard with the genoa. Getting more drive from the genoa is MORE important than using what's left of the main honestly. But if you can get point, and good genoa trim, AND get your main working for you as well, you'll be golden.

And it sounds like all your assessments are correct... Couple observations, and I am still sorting all this out myself sailing with a smaller masthead rig myself... Take what I say with my own novice observations.

Reef the main if you find that you are sailing a lot with a bubble ALL the time (regardless of these tips below).

To prevent the bubble, (or reduce it), you can crank on the outhaul and flatten. You can also (and want to as you sheet in hard, or vang hard) apply cunningham. If you don't have one, and your main is not brandy new, you'll want to add one (they are easy to build, just a 3:1 becket and blocks with a cleat... with a hook on top (to go through the cringle above the tack), and a hook on the bottom to attach to the bottom of the mast. Anyway the cunny pulls the draft forward as the vang/mainsheet flatten and pull the draft back. You may have a lot of bubble because your draft is deepest at the exit angle of the genoa... getting that draft forward, puts the deeper part of the main towards the deeper part of the genoa (and gets the shape better for lift)... General rule is draft should be in the first 40%, I am betting your draft in yoru main has gone to 60% or more when you are sailing like this.

Dump the traveler in the puffs to depower some (as soon as you feel that puff drop in power, get that traveler back up!), don't release sheet unless you have to (if your vang is on holding the boom from lift, releasing the sheet is less a problem), or/and head up, take advantage of the temporary lift. If your point is good and you are still overpowered, you can add twist to the top of the genoa by moving the cars back a bit on the tracks, to depower that as well.

MOST of your trim focus should be on that genoa (keep low, medium, and high luff teltales streaming, know when they should stall and when they shouldn't). Keep up as much genoa as you can racing, pick headsails for your non-puff winds, as large as you can go where you aren't overpowered (keep heel under 20 degrees use all the rail meat you got to determine that)... REEF the main before you lower headsail size. When you downsize headsails, go in small increments. 155-140-130-110.. if you can (if you have them).

One of the things I find MOST useful to climb upwind, is to feather.... head up for a couple seconds, adjust trim, until you feel the boat starting to slow, then foot off, retrim, pickup speed, then repeat... don't ROW while doing this, make the steering adjustments slow, otherwise for sure you'll slow down. Practice this before you race, it takes a bit to know what is too much adjustment, or to fast.

On another sailing site (SA), there was a huge racer's discussion on "sailing by the luff." That I've just started to understand myself. I found it interesting and yet another confirmation of genoa trim (it's harder to see with a tuff-luff or furler, but it's still usable).

PS: it hadn't dawned on me that adding twist in the light stuff (although I had been doing it, I hadn't realized that's why it worked) because of the differential between low altitude and high altitude winds. I've observed it by looking at differences between shroud tale and mast head though.

Oh one final note... with a masthead rig, you can usually get away with a larger spinnaker (symmetric) than these fractionals are allowed to run (think square footage). That's where your advantage is biggest. I can walk away from most of the fractionals downwind, but they beat me up, coming upwind.
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Last edited by SHNOOL; 07-11-2013 at 07:08 AM.
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  #5  
Old 07-11-2013
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Re: Twist

Quote:
We have a specific challenge with the main back-winding. A bubble in the forward third is acceptable, but sometimes to reduce a heavy weather helm, we end up flogging the entire sail (but the streamers on the leech are happy!), so we're working on that too. That can't be good, although I know two experienced racers who say "just live with it". I can usually get it all to settle down when I'm sailing alone ... fiddle with things for a few minutes, mainly bring the traveller way up, ease the main-sheet but also ease the foresail. "If in doubt, let it out." "Keep 'er full and footin'!" The boat seems to like to have the main reefed early, say in the 12-15 knots true-wind range, and is quite happy at 20 degrees of heel even in (only) 8 or 9 knots of true wind - it's unclear to me whether that's good, and is specific to this boat, or whether I should be cranking on unbelievable amounts of backstay tension, making other adjustments, etc.
It is true that on the '70s and '80s era high aspect masthead rigs it is normal to carry a bit of a bubble in the luff of the main when sailing upwind in breeze, but you still want to try to minimize it. The genoa is where most of your power is coming from, and the main is little more than a "trim tab" for the genoa. First, make sure your genoa is set properly. On my boat in upwind trim the foot of the genoa is almost touching the shrouds, and it is a couple of inches off the spreader. Make sure your genoa car is set correctly so that your upper, middle and lower telltales break evenly. (in other words, the right twist!) Also, if the car is too far forward you will have too much leech tension which could cause a "hook" in the leech. The hook can also be caused by a over tensioned leech line if you have one. That hook will exaggerate the backwinding of the main.

On the mainsail, you want to try to flatten the sail out as much as you can. If you are running old dacron sails that will most likely mean cranking the cunningham (to pull the draft forward) and cranking the outhaul on hard (to flatten the sail). As the wind picks up you will also want to start cranking on backstay tension. If you have an adjustable backstay you should be in the habit of pulling it on for upwind, and letting it right off for downwind. Don't be afraid to crank it on hard. Even unbelievably hard! (It is not uncommon to see 3000psi on my hydraulic backstay) That will do 2 things; it will reduce headstay sag, flattening the genoa, and it will bend the mast flattening the mainsail. Both are desireable for upwind work. I believe the US33 has a baby stay as well? If it is adjustable crank that on too. That will help to induce more bend in the mast.

Once you have done all that set up the main so that the boom is on centerline and adjust the sheet tension so that the top leech telltale streams about 50% of the time. Once the sheet is set for the wind conditions, leave it alone and work the traveler to control heel angle/weather helm. That maintains your desired sail shape, and just changes the angle of attack of the sail. Every boat is different, but typically 20 degrees is too much heel, and that could be why you have a weather helm issue. The more the boat heels, the more rudder input is required to keep it going straight, and more rudder angle slows you down. I try to keep my boat at 10-15 deg. If you have a trimmer on the traveler you have to communicate constantly and let him know when you need it eased. I prefer to control it myself for quicker response. When sailing through gusts I may ease it to the point where the whole sail is backwinded except the last couple of feet of leech. You can also control gusts by "pinching" into them. (what Schnool refers to as feathering).
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Old 07-12-2013
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Re: Twist

Schnool and ShockT

I wrote a long reply and the forum software lost it.

The short version is that I have learned some new things, confirmed some old things, and will be working on the new ones.

Thank you both very much!

Charles
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Re: Twist

Shock I am quite glad you followed on, confirmed what I was saying and added to it... TONS more expertise than me Charles, his word is golden.. Also glad I wasn't dead wrong. I've always know to crank on the backstay to depower, had no idea you could crank on so much... The adjustable on my Capri 25 is slow to add bend, I'll be looking to add more backstay quicker here on out.

Amazing the quality of sailors here though!
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Re: Twist

Quote:
Originally Posted by SHNOOL View Post
Shock I am quite glad you followed on, confirmed what I was saying and added to it... TONS more expertise than me Charles, his word is golden.. Also glad I wasn't dead wrong. I've always know to crank on the backstay to depower, had no idea you could crank on so much... The adjustable on my Capri 25 is slow to add bend, I'll be looking to add more backstay quicker here on out.

Amazing the quality of sailors here though!
I appreciate the kind words, but really I am just passing on what i have learned from much better sailors than myself! (and a fair number of years racing helps I guess!) That's what makes this community great!

One word of warning on backstay tension. Some boats have tree trunk stiff masts that really don't bend very much, in which case you are only tightening the forestay, but even that is worth while. Don't forget, if you do crank the backstay to bend the mast, you will have to pull on more sheet to compensate, and of course when you let the backstay off, your sheet will probably need to be eased a click or two.
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Re: Twist

Shock, yes and the Capri 25 fits into that category... mast-bend is very little... but the headstay sag is controlled by backstay... I've found already that cranking on the backstay tightens the luff of the genoa, reducing sag, and providing more point as the winds pipe up, by moving draft forward on the genoa (bringing it back to where it should be, cause it creeps as the wind picks up). I've considered adding a downhaul to the genoa as well but I don't know how much MORE that'd provide.

Another thread on another board commented on "modern sail cut," about how shapes are pre-determined on load-path sails... and moving the draft is a "minimal" movement (I've observed this especially with cunningham)... but on dacron it's can be a huge adjustment.
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Re: Twist

Quote:
Originally Posted by SHNOOL View Post
Shock, yes and the Capri 25 fits into that category... mast-bend is very little... but the headstay sag is controlled by backstay... I've found already that cranking on the backstay tightens the luff of the genoa, reducing sag, and providing more point as the winds pipe up, by moving draft forward on the genoa (bringing it back to where it should be, cause it creeps as the wind picks up). I've considered adding a downhaul to the genoa as well but I don't know how much MORE that'd provide.

Another thread on another board commented on "modern sail cut," about how shapes are pre-determined on load-path sails... and moving the draft is a "minimal" movement (I've observed this especially with cunningham)... but on dacron it's can be a huge adjustment.
I thought the Capri had a fairly "sporty" mast. It doesn't look like an overly heavy section to me. You might be surprised how much it will bend. Next time you are at the dock, attach your main halyard to the gooseneck and pull it tight. Observing the gap between the halyard and the mast half way up. Now crank the backstay on full and snug up the halyard again. That will show you how much bend you can get.

From your avatar I see you have plastic racing sails. They don't typically need nearly as much halyard tension as old dacron. Often times you don't even need a winch, just a good hard yank is enough. I think a jib cunningham is redundant on your boat. you can achieve the same thing with halyard tension. Typically you find jib cunninghams on small boats and dinghies that have locking halyards that can't be adjusted on the fly.
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