ausbinny said..

mathew said..

utcminusfour said..
If there is not a tack strap in use, then any distance between the pulleys will increase twist.

This is an unusual statement. Please explain some of the details.

Generally race / speed oriented sails as below

Tack strap tensions through the foot of the sail (adding more power) loose will reduce power / release tension

but

If both the pulleys are not close together the foot will have less tension (along the leech) opening up the sail (as the tack pulley will be closer to the clew - further from the mast)

But only the trailing edge of the foot - not upper twist. Again if it was an issue we’d see speed sailors reintroducing tack straps as a tuning tool or adding a sleeve to make mast fatter there to increase tension. But we don’t.

mathew said..

utcminusfour said..
If there is not a tack strap in use, then any distance between the pulleys will increase twist.

This is an unusual statement. Please explain some of the details.

Remember, the boom head can swivel up and down slightly—it isn’t fixed at 90 degrees to the mast. Because the sail’s center of effort is above the boom, loading the sail tends to lift the back end of the boom as well as pull against it. Tension in the sail’s foot limits that upward movement.
If the downhaul is two-blocked, or a tack strap holds the tack close to the mast, that movement is very small. A gap between the pulleys will be deflected and the tack moves aft, allowing the boom to rise farther and increasing twist.
Think of the sail’s foot as a sailboat’s boom vang: leaving a gap between the pulleys effectively eases the vang, with a noticeable effect on twist when you’re sailing.
This is one of the reasons for the classic advice to reduce the gap.