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Hi Bob!
When you are beam reaching(or close to it-NOT DDW), you are right that you want to ease the headsail-WITH the lead forward. How much? We have discussed having the sheet be angled such that if you continued the line from the block and "through" the clew, it would hit the headstay about 55-60% of the way up. Another method, is to ease the sheet as you bear off like you normally would, using the LOWER telltales as your trim indicator at first. When you are at the desired sailing angle, and have the sail trimmed well at the lower set of telltales, you will notice that the top of the sail is luffing at this setting, or at the minimum the upper set of telltales indicate a huge undertrimmed condition. Simply move the lead forward until the top set behaves like the bottom set-you will then have the lead properly located for the sailing angle. Check it by easing the sail a bit too much, and the windward telltales should break (lift) together (at the same time), OR it is fine for the top set to lift JUST a hair before the lower set-but not much. If the lower set breaks first, then the lead is TOO far forward. Once this is done-ease the main until it begins to luff, trim just so it stops, and make sure you have flow off the leech (by making sure you don't have too much vang loading). If you find the top of the main is luffing while the bottom looks good-add some vang until the top and bottom behave in the same way-just like with the genoa leads. You will now be so close to perfect trim that trim is no longer your concern.
However, when VERY broad reaching (almost running), the deeper angle you get to, the less effective and reliable the telltales will be-especially if your sail is a bit too heavy in weight for the wind speed and/or wind angle. So, there will come a point where the telltales simply are not worth fussing with-especially if you have the heavy sail situation-they just run out of effectiveness-and think about this-the deeper you get-you will at some point have the air entering the sailplan at the LEECH of the sails instead of the luff-When ddw, this is the case for sure with the genoa, and sometimes the main.
This obviously makes any conventional interpretation of the telltales incorrect.
So, when running wing and wing (truly wing and wing-as opposed to reaching with the genoa poled out) STOP worrying about flow!! If you are really ddw-just tweak the genoa/whisker pole so it "feels" fullest, and push the main ALL the way out. Telltales will not help you one bit in this condition-they may or may not fly, but when ddw, the fact of them flying or not is not indicative of anything. If you reach up 10-20 degrees, keeping the pole wung out, then yes, work the sail to get the telltales at least flowing off the mainsail leech.
This is why Bob has to let this go-the flow over the leech may indeed have become the "leading edge"-fuhgeddabouditt. At least when sailing really deep.
As far as "straight for the mark" goes; sometimes yes, sometimes no. Depends on the boat, the wind speed, waves, etc. Are there conditions when it will work on your boat-espeically in JAM? Yes. Even without "flow" it might be the call in the right conditions.
But, as we have said before, to chase these trim points to this level is extremely difficult without a high resolution knotmeter. The changes we are talking about yield .1, or .2kt. benefits. You can't feel this and you can't see it on a round guage instrument-you need the digital readout with better than .1 resolution if you want to work at this level of precision. A person could drive himself/herself nuts chasing these things without this tool-and we should be having fun, right?
Fair winds!!