Hi Mark!
With a genoa, jib, or cruising spinnaker, let's assume you have the right lead spot for a beam reach, for example. How do we know it is the right spot?
Because the luff is "hitting" the wind at a constant angle from top to bottom-meaning the entire height of the sail is working for you as it should. How can we tell if the luff angle of attack is constant from top to bottom? If you have luff telltales at 2-3 different height on the luff, if you slowly head up from you properly trimmed beam reach-without touching the sheets, you will see the top and bottom inside telltales "break" or begin to lift at the same time.
If the upper telltale goes up and the lower stays flowing, it means the top of the sail has a SMALLER angle of attack than the bottom, and is luffing eariler than the bottom (in effect, it is sailing closer to the wind than the bottom).
If the lower breaks and the top is cool, then the lower part of the sail is "sailing closer to the wind than the top. The by product of this, if the sail is designed and built right, is the you have proper flow from the leech.
Now, lets say you bear away to a very deep angle and let the sheet out accordingly, but with out moving the lead forward. As you ease the sheet, the top part of the sail always "goes out" more than the bottom, since the vertical loads on the sail are the ones which get reduced more than the horizontal loads as you ease-therefore you are easing the top of the sail more than the bottom- This causes the leech to rise up. and if you imagine the whole sail following that movement, the top part of the luff also goes "up" and in effect gets closer to the wind (the angle of attack up high gets smaller relative to the bottom part), and you now have that same situation as if the lead were too far aft-the top of the sail is not working for you.
I have a cold and am kinda fuzzy today-let me just boil this down: On your genny, when you are reaching along (beam-broad reach), have you ever noticed the top of the sail is luffing and the bottom is not? And you don't see this when close hauled..This is because you eased the sheet while leaving the lead alone. If you pull the lead forward until the telltales break together, you will see the whole sail is working without the top luffing.
OK, back to the kite. If you ease the sheet, the top of the sail will become soft and not work as hard-there is less load on the leach than there was when you had it trimmed in, right?
So, as you ease and see the top of the sail getting soft, just move the lead ahead until this is no longer happening. A good way to tell is with luff telltales, or, to get in the ballpark, another way is to imagine the sheet going from the block, through the clew and extending an imaginary line to the luff.
This imaginary line should hit the headstay about 55-65 % of the way up the headstay. If you see it is hitting at 50% or less, move the lead forward. Higher than about 70%, move it aft. The ideal % for a genoa is 55%-this is how we design the sails when figuring where it will sheet on the boat. For kites it is 5-10% higher.
In the case of that photo I commented on, the lead is forward, pulling too much on the leech, causing helm and heel and less thrust, and I would bet if you extended the sheet towards the headastay, you would see it intersecting the HS at about 80% or higher. I looked, and it does!
Sorry to be long-winded-I am under the weather, so to speak. let me know if this was not clear. Having the lead right can easily make a difference of up to a knot of boatspeed in many conditions...
Safe sailing!
S