Draft Position

[John Scharer]

I have read Don's book and have his sail charts which I think are great. I would like to clarify draft position on the sail. In the book he talks about the point along the chord line of the sail where the maximum depth falls. If you take a straight line down from that point to the chord line determine draft position. If your chord line is 10 feet then a 50% draft position would be 5 feet. My question is this, isn't the overall length of the chord line determined by the position of the outhaul? If the outhaul is tight versus being eased the 50% draft postion would vary or do I have this wrong. If I am correct then what is a easy way to record these settings based on the conditions?

 

[Alan]

Draft vs Chord

The draft position is the point along the chord where the depth is the greatest. The length of the chord is, for this discussion, irrelavent. The depth vs the % of chord is the main issue. For any given chord length there is in theory an infinate number of possible draft positions. This position is adjusted with halyard tension, mast bend, cunningham, outhaul, vang, sheet, etc. Each of these control draft at varying heights of the sail hoist. As a result draft can be controled at the top, middle and bottom of the sail. So to answer your question, a 10ft chord could have its max draft at 5ft and therefore have a 50% draft, but by changing the control line tensions the draft,or max depth can be moved forward to say 40% or more to 35% as it is needed.

 

[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.