Finding Draft Position and Twist

Don Guillette · Jun 22, 2013

Greg: Thank you for buying my products.

Draft position is the hardest of the 4 elements to see for the first time -- the second hardest is twist, which you've mastered at least with your old sail.

Draft position is an imaginary line that runs from top to bottom through the deepest part (belly) of the sail. Bend your index finger to form a belly and the imaginary line would be at your knuckle. I think some more expensive sails have a draft line but I'm not sure -- I could never afford expensive sails. I'm sure RichH would know. Why it's important to determine draft position is because as the wind pipes up it wants to push the draft position past the 50% or middle of the sail making it a more powerful sail and harder to control. It's not actually the middle of the sail but close enough. Your sail trim controls are used to bring the draft postion back to around 50% or a little less, which is more forgiving. The larger the sail, the easier DP is to spot. This is not a scientific explanation.

Of the "4 elements of sail trim" draft depth, draft position, twist and angle of attack, I think draft position is the least important -- most sailors can't recognize it anyway.

Don Guillette · Jun 22, 2013

Greg: I know because I've been there myself but here's the good news -- if you digest all the material in my book and put it into practice a few times in conjunction with the info on the chart you'll know more about sail trim that 60% of sailors worldwide and you won't need the book and chart -- give it to a friend but not one you might race against!!

If you don't believe me just ask your dock neighbor to explain the difference between twist and angle of attack and then ask him to indicate which sail trim controls he'd use to adjust both elements on the main and jib. After that, ask him to explain the settings he'd use for those controls on the main and jib in 10 knots of wind closehauled. If he gets by that, become his GOOD FRIEND!! But to be sure, ask him where he'd position the crew in the above situation.

Soon, you'll know the answers to most sail trim questions (you'll have solid beginner to intermediate sail trim knowledge) and you'll also know if your dock neighbor knows his onions about sail trim.

TLW · Jun 22, 2013

Well guys, I've been sitting here debating whether to say anything. I don't want to be argumentative, but I believe draft in a sail to be EXTREMELY IMPORTANT. I've always had black draft stripes added to my sails so everyone involved in sail trim can readily visualize it. Although Don prefers to visualize draft from head to foot, the sailmaker adds these stripes from luff to leech basically parallel to the boom. You might have 1, 2, or 3. I like three. The draft may not be the same at all heights if you are twisting off the head to de-power. The greatest control of draft is through halyard tension - more tension moving the draft forward. (certainly, outhaul has a big effect in lower third of sail, as well.) Old blown out sails might not respond at all. Newer dacron sails might need more adjustment than laminate sails where the designed shape is more stable.

Draft position is very important to windward performance. We could really go into a discussion of draft in headsails, but the focus of your thread has to do with a new main. And, you are basically cruising, I think, but if you are of a mindset that wants to consider draft position - and it appears you are - I say go for it, get a handle on it. You don't have to be critical when everyone on board just wants a relaxing sail, but good sail shape is good sail shape and being aware of it adds another entertaining ( if somewhat intellectual) level to your sailing.

shemandr · Jun 23, 2013

geehaw said:
To Start, I just bought a new main sail which was a great improvement on the boat handling.

So it is a better sail?
I really recommend drawing the sail cross sections on a piece of paper using a scale of 1in/cm to a foot. Draw shapes that are mid chord draft (50%) and draft forward (45%). Draw still more with 7 to 10 % chord depth. Compare a 7% depth draft forward with a 10% draft aft. Use french curves and remember that after the greatest curvature of the sail the exit should be damn near flat - no curve. You will develop an eye for the shapes you desire and I think will be able to discern draft forward vs. draft aft.
As for the sail bending at the battens - this unfortunately happens. I don't think it means the sail is no good. You are looking for a even bow shape; but, if the exit of the sail shape is flat it should be a lopsided bow. This flatness will reduce the batten crease. I would try increasing the halyard tension and letting off on the sheet a little. Excess sheet tension may be hooking the leach to windward a bit.
I don't think the lack of twist is a design flaw. You may be used to an old sail which is blown out and takes a twisted shape even if it shouldn't. A new sail by virtue of its stiffness will twist less and that's one reason one design racers buy new sails a lot. A twisted shape is good for certain conditions but ideally it's pretty subtle. The standard rule of the top batten being parallel to the boom, is not a very twisted shape. That closed leach can improve your upwind performance. The top telltale should stall about half the time in a pointing mode. If you want more twist try easing the sheet and pulling the traveler to windward. Normally you would do this to gain attached flow in light air or to depower before reefing.

Don Guillette · Jun 23, 2013

TLW: There are Two drafts - draft position, which is the topic discussion, and draft depth (belly). All 4 elements (draft position, draft depth, twist and angle of attack) are important but for a beginner sailor, if they can master the last 3 they are getting close to home. Personally, I think draft position is better viewed as a verticle line but the forum lister can decide.

Halyard tension is one control for DP but not the only one. There are 4 more -- boom vang, mainsheet, mast bend and outhaul. All the sail trim controls for the main and jib have to be in unison. When you adjust the mainsheet you're also messing with angle of attack and twist. When you adjust the boom vang you're also messing with twist and draft depth. When you adjust the the outhaul you're messing with draft depth and draft position.

It's really a balancing act. A sailor has to master all the sail trim controls for the main and jib or sail trim just becomes a guessing game, but first he has to understand the "4 elements of sail trim" or sail trim will never make any sense.

Jared1048 · Jun 24, 2013

my secret I have not wanted any of the people I race against find is an App from UK Hasley sails called AccuMeasure. Take a picture sighted up the sails and plug that into this app you install on your desktop. You then "fit" the lines to the curve of a section of the sail and it gives you the draft numbers. It also has some other stuff it measures such as twist and camber but I don't really understand how to read the measurements yet....

and I just finished reading your book Don. I liked the way you described things. Thanks!

Don Guillette · Jun 24, 2013

Jared1048; Thank you for the kind words about my book. I'm just a common seaman and that's how I wrote the book so I could understand it.

On the AccuMeasure computer program -- I only know how to send emails so I'd be no help - but I'll bet there are guys on this forum like RichH that could help with any questions you might have and they'll explain it to you in plain English.