Hello, all. See if this sounds about right.
First, I'm a fairly new club racer, nothing serious, several years of weekly (seasonal) skippering and many more of crewing behind me.
Secondly, I believe that much confusion on this topic stems from twist vs. flatness. To the beginner, they sound contradictory, like a twisted sail must be a full sail. The books say no, that one can have twist without fullness. The hows and whys are for another thread or later in this one.
Thirdly, I hear and read that one doesn't usually want twist in the sails "except when" ...
My personal take on all of the exceptions is that one usually
wants twist except in one specific scenario.
If we make a grid with wind strength on one axis, and wave state on another, each on a 3-point scale of "mild, medium, severe", that would be nine boxes. I would like to look at only some of them, to illustrate my understanding and ask for comment.
1. Let's take light winds in flat seas (not such light winds that it's virtually calm - that's a different story). The difference between winds aloft and alow is such that twist is desirable to keep all parts of the sail optimally trimmed.
2. Let's take heavy winds. One wants twist because the struggle is to keep power out of the sails (reduce heeling, weather helm, etc.).
3. Medium winds and lumpy seas - the boat's not going to track exactly straight no matter how skilled the helmsman, or in more extreme lumps there will be relative calm in the troughs, so having twist always keeps some part of the sail under power.
4. Medium winds and calm seas. This is the only situation I can think of when less twist is desired, the fabled "upper batten in line with the boom" scenario.
Of course, I'm talking as if twist is either on or off, and it's more nuanced than that.
I'm also not talking (yet) about "how" - a bit of a struggle on my boat currently.
Making any sense so far?
P.S. See previous threads by others on this topic:
Depowering the sails and twist
what''s up with sail twist?