Twist - part 2

[Don Guillette]

Fine tuning twist is the most important sail trim adjustment you can make. Please pay attention to this because it appears to be confusing. MORE leeward sag produces LESS power. LESS leward sag produces MORE power. If your confused, don't worry as everyone is at first. The bottom line is that too little and too much twist reduces sail power.You have to tune the twist to match the sailing condition (wind speed and point of sail) for the amount of performance you want. The biggest sail trimming problem you will encounter is acheiving the most efficient total power and the correct mix of sail shape and twist. Additionally, the shape and twist of the mainsail and jib must match or mirror each other. Also, less twist is better for pointing but dialing in a little bit of twist is better for speed and acceleration.The twist controls for the main are the mainsheet and boom vang. Increasing tension reduces twist and decreasing tension increases twist. The twist control for the jib is the fairlead. Move it forward to decrease twist and move it back to increase twist. I can sit here all day and say this stuff but you just have to go out on the water and mess with your twist controls for the main and jib and watch the top of the sails open and close. Once you see it for the first time you won't forget it and you'll see most of the boats sailing on any given day with a bunch of twist anyway, mainly because the don't know any better.In racing situations, I always look at the sail trim of the boats ahead of me. If they have poor sail trim with twist in the main and jib I think that it is just a matter of time before we overtake them. On the other hand, if the sails look great I know it is going to be a long day.What are your best twist indicators for the main and jib? They are your telltales. In part twist #3, I'll tell you how to set twist for the main and jib and how to read the telltales for twist.Any questions so far?

 

[Osprey]

Don't forget the back stay?

When you crank on the back stay it will tend to open the leech of the main...right?

 

[Don Guillette]

Osprey; Not really. It will start to change the draft depth and draft position but won't do anything for the leech.

 

[mike C16582]

to piont or to power up??

I sail on a tidal estuary, so on any give race day, I can be sailing with a wind against the current (up to 2.5 kts.) or wind with the current.Because I am usually reaching across the river at some point, should I have more twist in the sails to keep power in the boat (but I'll be going across the river and have the current set me back) OR have a flatter sail with less twist to point higher into the current and lose less ground that way??Great week Don!!Mike C.

 

[Alan]

mike C

That kind of question is very condition and boat specific. Your best way to find that answer is to check the VMG on your GPS. You are looking for the highest VMG. I would start by sailing as high as possible and note the number, then crack off in 2-3 degree increments, and watch for the change in VMG numbers. When they start to go down, you've found the best VMG.Osprey, adding backstay will move the leading edge of the main forward which flattens the main (reduces draft), but has no affect on the leech. The amount of curve in the leech (twist)is controled by the mainsheet and vang.

 

[Osprey]

Back Stay , mast head rig, stiff mast

Thanks for the reply guy's.I understand what you guys are telling me.Let me try to explain. I race a Newport 28.It's a masthead rig with a huge stiff mast, so any bend is unperceivable.What I seem to notice when I add backstay tension is the sag coming out of the forestay and the top of the mast coming aft. This relives the leech tension and tends to offer a little twist.. Am I off base here?Maybe I am triming in the wrong order?Thanks & Fair Wnds

 

[Alan]

Osprey

I understand where you are coming from. The fact that you're masthead rigged and have a very stiff mast section would certainly not allow you to bend the mast with a backstay tensioner. By the same reasoning, pulling the masthead back will tension the headstay, but will do nothing for the leech curve(twist). The leech of the main will open or close with the height of the boom. This is controled with mainsheet and vang.

 

[Ersin]

Twist in light air

Don,I think I understand the twist very well except one more confusing thing. As you say'Less leward sag produces more power'.Means:Almost close the leech when you need more power.But then, why we should open the leech for a faster sail shape and more speed in light air?As saying in books and in other sources.Don't we need more power in light wind too? Means, almost close leech(less twist)?Thanks..

 

[RichH]

Tell-tales !!!!!!!

IF (a BIG if) you are carefully watching the tell-tales - especially the leech tell tales- twist and setting for 'power' is 'automatic' (as is just about ALL other adjustments) . Especially in light winds the lee telltales will instantly show when/what to adjust because of too much draft/camber ... or twist, etc. that you start to develop a flow *separation* (not a stall !) along the leeward aft portion of the sail. In slow moving airstreams the 'attachment' of the airstream across the leeside is very 'vulnerable' to separate entirely from the sail. Too 'powerful' a shape in light air is just as bad as too flat a shape .... and besides - then you have to 'shift' gears to a flatter shape once the boat gets 'moving'. Same with twist.Obtaining the best shape is like shifting gears with an automobile with a manual transmission. No sense in shaping for power during a tack or when you need to accelerate .... and then leaving the boat "in first gear". But once up to 'speed' you generally need a flatter (speed) shape (high gear).. Watch the tell-tales !!!!!!!! Do anything and everything (*experiment* - with the various controls!!!!) to keep the tell-tales flat and not 'agitated' ... and you will be quite close to a PERFECT set/shape. ....and then watch your knotmeter. For me, perfect shape and set are not only important in racing: I consider good set and shape to valuable as a 'safety' issue... consider: If a storm or squall is on the way, good shape/set will get you back to the dock faster, will allow the boat to perform better and safer if do you get 'caught', etc. I presently also sail a very heavy weight large boat. Without that knowledge of what those tell-tales are telling me, this boat would be a 'terrible' boat to sail because of the slow response and lack of feedback .... without those telltales, I'd be using the engine a LOT more. There is NO way that one can SEE the wind and what its doing to the sails .... you can SEE what the tell-tales are doing! Just keep the leeside tales FLAT and do everyihing possible by the various adjustment controls to keep them FLAT and streaming aft - simple !!!! Experiment. Just remember that what you tension along one edge of a sail releases tension on the opposite edge, etc. (hint: there are always TWO or more adjustments). Think of the sail as being made of stretchy rubber bands ... then all the 'adjustments' will become very easy. Watch those Tell-tales !!!!!!!Hope this helps.

 

[Don Guillette]

Ersin: This is very hard to explain because I am not a scientific guy. It is similiar to the lift/drag curve or prop walk - I don't know about them scientifically but I know how to make them work for me. Your question has to do with the apparent wind angle plus the fact that the wind blows 60% greater at the top of the mast than at the bottom. In very, very light air you want the sail as flat as possable. As the wind velocity start to increase through the range of lite air (2 to 5 knots) the boat starts to move and you go from true wind to apparent wind. This is where the height of the mast comes in because sails are designed to twist off at the top to account for the vertical shift in apparent wind. If you could measure the wind speed at various point up the sail you would see that the velocity increases with height. As the velocity increases so does the apparent wind angle. The wind angle changes or twists and on a 40' mast that twist or change can be as much as 5 degrees or more.So you say so what and is that a big deal? Yes it is because the top of the sail now has 5 degrees more lift than the bottom. What that means is the top is on a close reach and the bottom is beating. To compensate, you have to ease the top of the sail slightly and trim in the bottom harder because the sail must be trimmed to match the vertical shift in the wind.Part of the problem is what is "more" twist and what is "less" twist. Say more twist was about 15 degrees of twist. To power up the sail you would reduce the twist 10 degrees. What you are doing is is asking the wind to turn more, which increases power. When you twist the sail off your asking the wind to turn less, which decreases power.I'm really sorry I had to go through all this because I hate trying to explain things scientifically because I confuse myself. What I would have liked to have said was "in lite wind conditions just twist the sail off a little bit and your boat will sail faster".Next time your out sailing, mess around with the twist adjustment and see what happens to your speed.