Draft Position

[Don Guillette]

John Scharer: I’m going to keep this very simple so I don’t confuse myself. The chord line is the horizontal luff to leach measurement of your sail. In other words, the foot of the sail. That measurement never changes no matter what sail trim adjustments you make. If it did, you could not make the proper adjustment for a given point of sail and wind condition. Assume the wind conditions called for 50% draft position and 20% draft depth (belly). Without knowing the chord you could not make that adjustment. Draft depth (belly) and draft position work together and to get both positions where you want them you have to use all the sail trim controls for both those items. Please refer to the Quick Reference that came with your Sail Trim Chart. Note that the sail trim controls for draft position are the boom vang, Cunningham, mainsheet, mast bend and outhaul. The sail trim controls for draft depth are mast bend and outhaul. So, you can see in your question, that when you adjust the outhaul you are primarily adjusting the draft depth (belly) but you are also adjusting the draft position.An easy way to record the optimum setting for each point of sail and wind condition on your boat is to do just that – mess with the settings and record the best ones.Draft depth, draft position, twist and angle of attack (The 4 Elements Of sail Trim) are hard to understand at first. That is why I say in my book, do not proceed past the chapter that explains those elements until you fully understand them as nothing that follows will make any sense. Once you understand those concepts, the flood lights come on and you know more about sail trim than 75% of the sailors worldwide!! I hope I’ve answered your question. If not, ask again and I’ll give it another go.

 

[Paul]

Chord line? Nah.

The last time I worked in an aeronautical engineering shop, the chord line was the distance between the leading edge and the trailing edge of an airfoil.That is not the same as the foot of the sail, which is generally forced into a flat line by the boltrope. Sails are never cut completely flat, they always have a curvature.On a loose-footed main, the chord line is the distance between the tack and the outhaul car--at least at the foot of the sail. If I move the outhaul, then I've changed the chord. The most general definition of chord is the *straight line* distance from the leading edge to the trailing edge.I have attached a link to a site with definitions of chord and camber.

 

[Don Guillette]

If I asked you to set the the draft position at 45% and the draft depth at 25%, what would that mean to you? How would you figure out where the draft position would fall on the boom? How much belly would your sail have?

 

[Paul]

It means nothing at all in context

You're perfectly right, they are nonsense terms.When you write about setting the camber of the sail to "20%" that doesn't mean anything to me. I doubt if it does to anyone else, either.But all I'm really doing is telling you what the definition of chord is, because it ain't what you said.

 

[Don Guillette]

I know it doesn’t mean anything to you but don’t assume just because you don’t know that all sailors don’t know either because draft depth and draft position are always expressed as a percentage. The percentage is obtained by comparing the location or distance of the draft from the leading edge to the over all chord of the mainsail or jib. On your mainsail and jib, a draft position of 45% means the imaginary line falls at the 4’5” mark on a 10” chord. A draft depth of 20% means you have a 2’ belly in the sail on a 10’ chord. I don’t know how you could discuss draft depth or draft position on the main or jib without using percentages.To any beginner to intermediate sailors that are following this discussion here’s what I suggest you do. Get a good sailing book (not some airplane book) like Dennis Conner’s “Sail Like A Champion” and look up the definition of chord, which is “the measurement of a sail front to back horizontally”. That means to measure the foot of the mainsail or jib from the tack to the clew. Pull the outhaul car all the way back.before you measure the main. When you get the measurement, divide it in half and mark the 50% location on your boom and on the foot of the jib. That is your point of reference. Forget about the movement of the outhaul car. It only moves about 6” to 10” anyway, so that amount isn’t much of a calculation factor anyway. All your trying to do is get in the ballpark so you have a general and visual point of reference to compare your sail shape to. If you want to do it the way this guy suggests, then factor in the 6” to 8” movement of the outhaul car. The percentage change will be insignificant as it applies to what we are trying to do. If you do it his way, you have more time on your hands than I do.