Headstay sag is determined mostly by your headstay length, but is also affected by backstay and mainsheet tension. Headstay sag is nice for 2 reasons in light air:
-Additional power is generated because the extra sag makes the jib deeper, more depth means more differential between high pressure (windward side of jib) and low pressure (leeward side of jib) and more differential means more pull!
-Better pointing can be achieved by having the luff of the jib set off to leeward, which means that the angle of attack (the angle of the jib to the wind) is wider, which lets you sail higher.
How much sag is right? The Shields is damn slow in light air, so anything to make power and helm is good, but how do you know how much is right?
-By the numbers, I think about 12" of sag in the light and flat stuff seems right
-By the feel of the boat, the sag should be enough that the jib looks round up front, but not enough so that the rig shakes around in waves (or worse, the rig shakes when not in waves!) If you rig is bouncing around a lot, you may be overdoing it.
-By jib trim, if you sag too much, you may find that the jib leech telltales will stall unless you're sheeted way out. The jib leech telltale is a critical thing to watch here, and if you have to ease that sail out beyond the spreader tip to get the telltale flying, it means that the jib luff has sagged so far to leeward that bringing the sheet in closes the leech off well before you're trimmed properly. This is bad.
On our boat, in the super light air, we were sheeting the jib so that the leech of the jib was between the spreader tip and about 4" in. It's important to ease out when coming out of tacks, or when hitting chop, or pretty much anytime the boat felt slow. In race 2 we were locked up with 67 pretty much the whole race. We didn't feel straight line fast, but we were consistently ableto make gains because we were constantly changing sheet trims when conditions would change. It helps to have a spare body looking ahead, and if they see a puff, or waves, or a light spot to call out "ease sheets, in 3, 2, 1, ease" and the commensurately calling for the sheets to be trimmed back to normal mode once the boat is back up to full speed.
Headstay sag is great for the jib but it also affects the main. The first few years we had the boat we struggled in light air, and it took a long time to find out why. We had a good jib shape, but always tended to keep the backstay totally slack, after all, you don't need backstay in light air right? It took quite a lot of experimentation before we realized you need to make the main shape right, and in light air the mainsheet alone won't do it.
The light air main shape is wrong when you have appropriate sag in the jib, but the mainsail top leech telltales just won't fly. The solution to this appears to be ease the sheet until the top telltale is streaming about half the time. Of course, when you do this the top of the main opens way up, and you lose quite a lot of power and pointing ability. Additionally, easing the mainsheet can make the headstay sag too much.
The solution is more backstay, which sounds wrong for light air, but is the only way to get the main right sometimes. Add a little backstay, until you can get the main top telltale flying, with the boom within a few inches of centerline, and the top batten angle not too open. Once the main looks good, double check headstay sag. If the sag is too little, you have have to loosen your headstay a bit, but often times the crazy sag we sail with can handle a little backstay tension without making the jib luff too straight. The shape of the main will change as the mast bends, generally making it flatter. We all know we want flat main for big breeze, but a flat main for very light air can be fast as well, as it's a low drag shape that will get you your best upwind speed.
The above is good for straightline speed, but what about tacks? When we tack the boat in light air, we do a few things to help get it up to speed fast.
-We coordinate our weight so that when starting the tack, we're all in the middle, and when coming out of a tack everyone is to leeward, sometimes hiking hard, to help the boat heel to leeward and get airflow back over the sails.
-We have both jib sheet and mainsheet eased after the tack starts, so that when we come out on the other board, both sails are eased out to help build speed. After 10-20 seconds we'll trim slowly back in to a true upwind course. This is usually about 2" eased on the jib, and about 4" eased on the main.
-We ease the backstay as well, to make the main deeper, which helps build power faster. Once we're up to speed and starting to trim the sheets back in, we'll trim the backstay as well to make the main flatter and help the top telltale fly.
-The helm comes out of tacks a few degrees low of course, to build speed faster
When chatting after racing last week, both 196 and 130 were interested in better main shape to match a saggy jib. The thing to try out next light air weekend is adding a little backstay to the main to get the shape flatter and make it easier to trim. Bonus points for matching that to the headstay sag! Additionally in light air being vigilant with trim is even more important, and every time the wind lulls or puffs, or the boat is about to hit waves, trim should be adjsuted.