Twist ..... or why allowing the boom to rise to depower a sail is not the 'best'
Twist is the constant change of needed angle of attack in a sail due to the differing wind speeds (gradient wind) immediately above the water. The gradient wind is caused by the action of friction as the airflow gets nearer to the water, the wind will be ‘faster’ further away from the water. But hold on – its not going to be ‘all that faster’ at 50-60' feet off the water. The higher the windspeed the less the ‘gradient wind’. Only when the windspeeds are ‘light’ will twist need to be exaggerated and ‘enhanced’ by the trimmer; in the higher windspeeds twist is not that important to keep a boat moving because there is LESS gradient wind effect. Typical sail design, and for ‘non-precise’ cruising sails will have a ‘inbuilt’ twist in the neighborhood of 5-10 degrees of twist 'already' designed into the sail; usually in a well designed sail to get the twist correct usually requires ‘little’ adjustment (traveler position, etc.) if the sail is otherwise properly raised and otherwise 'shaped' by the sailor.
The chief ‘control’ of twist is ‘how hard’ one pulls **DOWN** with the MAINSHEET. This puts strain along the leech and ‘twist’ is reduced, the overall sail camber increases with increasing MAINSHEET strain and the resultant is that the sail ‘powers-up’; conversely, easing the mainsheet tension ‘opens’ the leech, allows more twist aloft, can allow flow separation (flow no longer attached) at the head panels which depowers the sail especially in higher windranges which causes less heel, etc. Looking at ‘overtwisting’ in another way: the top of a sail that is ‘overtwisted’ is akin to putting in drastic ‘overdraft’ or so much draft aloft that the flow separates at the area of ‘overdraft’ .... a separation stall under high speed wind flow because the shape is ‘too round’ and the draft ‘way too much’ !!!!
The LIMIT of how much the sail can be twisted is set by the VANG and the Traveler ..... because the traveler can set to be almost directly UNDER the sail which can ‘substitute’ for the amount of mainSHEET tension. Using the traveler is faster and vastly more precise to control the amount of twist.
Setting up the CORRECT amount of twist is to get precise MAXIMUM speed/power out of a sail, is NOT a long term substitute for reefing nor flattening nor for ‘blading-out’ a sail to correct/trim for too much ‘heeling’ moment, etc.
In ‘blading-out’, an adjustment done with solely the traveler – sometimes called “‘playing’ the traveler”, the already set angle of attack ‘all the way up’ through the panels stay essentially the same or unchanged, and the sail begins to ‘stall’ under a uniform manner and at a consistent (and usually predictable) manner along the WHOLE sail.
In contrast, by allowing the boom to rise, when ‘dumping the mainsheet’ to allow more twist at the top, the leech does ‘open’ but also at the same time the lower and MIDDLE panels of the sail become MORE DEEPLY drafted (powered-up). Yes, the top panels are now ‘essentially flogging’ (reducing the heeling moment); but, the center panels are now set up to be in SECOND GEAR deeper draft and thus ‘progressively vulnerable’ to drive the boat further over onto her beam ends, if the helmsman isnt that quick or experienced ..... all set up for a ‘dumping’ if the wind is more powerful than the boat/helmsman can handle ... OR if there is a significant windshift .... because the twist (overdraft) is now down into the middle and lower portions of the panels and they are now at MORE draft than when the 'twist' was set correctly.
“Blading-out” with the traveler doesnt allow the draft to change to a more ‘powered up’ shape, the sail stays flat and is restrained from changing to a ‘powered-up’ shape .... because the boom can’t rise (even ‘rigid vangs’ will have some amount of ‘stretch’).