Thanks, Jeff. I've got a few dilemmas in there. Let's say "fairly flat seas but with a bit more wind than 'moderate' for the boat". You're suggesting twist in the foresail but not the main. Did I understand that correctly? The books say that usually both sails will have similar twist.
I'm not sure what "blading out" is. But strangely - and I've come across this in several places and confirmed it with some high-level racing sailors - easing the mainsheet, letting the boom rise and freeing the leech (opening, even though the tack and head are on a shorter line to each other) adds twist but flattens the main. This of course means raising the traveller to restore angle of attack with the boom near centre, whereas you say lower it. I can't quite visualize why that flattens, although I can see how it adds twist. But so be it. It also means having a very tight backstay.
Can you clarify any of the foregoing?
The two explanations above are both essentially correct. To more specifically try to clarify my points, there are two schools of thought on heavy air mainsail trim. Done skillfully, either can work well. The flat and twisted approach is the 'old school' approach. In this approach, huge amounts of halyard, and outhaul are applied. This 'pulls fabric out of the middle of the sail'. The risk is that it moves draft aft, and so some cunningham is applied to move the draft forward. (I personally try to avoid ever having to use the cunningham, and have actually no longer have it rigged on my boat. But as sails get older or are stretchier, the cunningham becomes your friend.)
This is a very old technique and so old, in fact that it used to be called a 'fisherman's reef' from the days when folks fished under sail.
The good news about the flat and twisted approach is that the upper part of the sail is twisted off reducing heeling, when coupled with a twisted jib, the slot remains nearly parrallel, and its pretty quick to play in and out. In smallish boats, when coupled with the vang, a good mainsail trimmer can play the sheet with each wave allowing the boat to be steered with the mainsail and not with the helm reducing drag and building up Popeye arms on the mainsail trimmer.
The downside of this approach is that the geometry of most mainsails mean that as the boom lifts adding twist, it also adds fabric in the middle of the sail, powering the sail up. The result of that means that beyond a very small range of adjustment, this quickly becomes a twisted and not
so flat approach. For that reason, the flat and twisted technique works best on boats with lots of mast bend control and the ability to play the backstay quickly and easily, so that applying the backstay will quickly open and flatten the mainsail without changing the boom geometry.
(Here is a picture of my boat from astern illustrating the flat and twisted approach. This was right after a mark rounding, a little over powered since it was just as the wind had come up to roughly 20 knots apparent. I am single-handing and so was not settled down in a position for subtlies yet. The backstay is on hard, the sails flattened, at that exact moment I am adjusting the traveler to flatten the boat. I am using flat and twisted since that approach tends to be a little more forgiving and so made sense here while I dialed in.)
In my lifetime, the approach to main sail trim in heavy air has changed over to blading out the sail. The idea here is that the sail is very flat and has minimal twist and is rotated using the traveler to increase and decrease the angle of attack. Conceptually, there is comparatively little gradiant wind angle difference during heavy air. Consequently, the sail does not need twist to have the same angle of attack along the span of entire sail. The net result is that the flat sail maximizes drive relative to side force, and then the angle of attack is precisely rotated with each gust and lull.
To do this, all control lines are set up very tightly (that means halyard, outhaul, flattening reef if you have one, vang, backstay, and mainsheet) so that the sail is flat and with very minimal twist. The boom will be below the centerline pretty much all the time. The mainsheet is pretty much cleated off, and the traveler is played between full on to totally flagging, but mostly with a small bubble near the luff.
In practice, on a bigger boat with really good sails, a good mainsail trimmer will cycle through a mix of both techniques, slightly freeing the sheets and lifting the traveler in lulls and blading and dropping the traveler in the gusts.
This cycling can happen with great frequency in waves and the mainsail trimmer really can make a huge difference loading and unloading the helm. In gusty conditions I would cycle through this 3-4 times a minute on the 40.7 I used to race on. Timing is critical on this. As the gust hits the boat, the mainsail trimmer needs to delay a moment so the boat starts to take a bite to windward, and feathers up slightly, but then the load is released just before the helmsman needs to move the rudder to stay on course. As soon as the gust wanes, the process reverses. When trimming mainsail on a boat with a wheel I make sure the center spoke is marked and carry tape in my seabag so I can mark the center spoke if its not. The mainsail trimmer needs to be aware of the angle of helm, and use his sail trim to minimize drag. That is usually easier when the sail trim starts out being bladed out and only uses small amounts of added twist.
The other part of this is communication between the helmsman and the trimmer. The helmsman needs to be able to convey input into feel and simple stratetic decisions such as, "Need point", "Need speed", "lots of helm", "feels slow", "need more steering" and so on so that two can work as a tight knot team. Of course, after a while, a bit of clairvoyance sets in so that team learns to read the other person's mind.
I hope that this somewhat clarifies what I was trying to say.