Sail Twist

Sails should twist to work efficiently.

Now before y'all start citing this class and that class, all boats do
something else, they heel and therefore spill wind off the head.

Whereas in that more sail efficient world that I have always existed and I
note will come to pass in the future with foiling moths, the rig is canted
over to weather, so the sail must work automatically and efficiently to
ensure continued power and no stalling action so twisting off at the head
or what became known as 'floppy leech' worked out to be very very much
faster than sails that didn't twist and remained tight leeched and more
stable.

The reason is exactly the same as having a tuned exhaust on a race car,
get the fuel in extract the power and get it out, so it is with air, use it and
exhaust it before it turns into something else.

I've also been told down the years by everybody that knew anything about
dinghies, that 'twist' and the way twist occurs is very important, so if you
can adjust your rig so the lower half remains powerful with a tightish
leech, then the further up the leech the more it 'twists' off in the puffs,
the better it is.

But equally I've also found that the way the jib reacts and the gap that
brings a venturi effect of speeding the airflow over the main, without
backing the leading edge of the main is also of paramount importance to
an efficient and fast rig, that bit I found out for myself, the hard way and
the jib itself is very inefficient in that it has a propensity for stalling the
flow rather than twisting off at the head, which in turn adds to the
turbulence over the main, another reason not to have it sheeted too hard
in a breeze.

Edited by G.R.F.

Originally posted by ellistine At Rutland at the weekend (very windy), while beating the wind was hitting the back of the sail so hard that it was popping the battens the wrong way. Presumbaly in this case the sail was no where near flat enough?

It probably wasnt flat enough. but i know in some classes if you over tighten the kicker the batterns will invert. so perhaps it needed to be eased a few inches.

also when dropping the mainsheet the jib should be going down as well to stop the front of the main backing. it keeps the slot open as well. Its far faster in those conditions in all venues (well all the ones ive visited) to keep the bow down ease jib and main together and power on through the gusts and watch the guys that pinch and rag their sails go backwards. only problen is that it is seriously physically demanding.

btw this is just something that ive found in the RS classes, it does vary as Jim has said between boats, and sailors

Thanks all.

At Rutland at the weekend (very windy), while beating the wind was hitting the back of the sail so hard that it was popping the battens the wrong way.

Presumbaly in this case the sail was no where near flat enough?

> (it is not really possible to have a flat twisted
> sail, 'cos the kicker must be eased to get twist, but
> easing the kicker straightens the mast

Note for other folk, this is not really the case with modern full batten/large roach stable mylar rigs, where the downhaul is the key control for bending the mast. Different rigs really do march to quite different drummers.

In general you want an untwisted leach so that the air flows across the sail in a uniform way at all heights up the mast. In light winds some twist can be good, as the wind gradient means that the angle of the wind is different at different heights (due to friction with the rotating earth). Therefore twisting the sail to suit the wind direction maintains the uniform flow. You are right, twisting the sail in strong winds is one way of spilling excess power, but it is not generally the best way.  It is usually faster to flatten the sail as much as possible (lots of kicker, downhaul and outhaul), lift the daggerboard, hike like hell and ease the sails if needed.

In my experience it is better to have a flat, flogging sail than a full twisted sail (it is not really possible to have a flat twisted sail, 'cos the kicker must be eased to get twist, but easing the kicker straightens the mast and therefore powers up the sail). 

I've not sailed a Vago, but you have an untapered (and therefore stiff) mast I think.  This probably makes it hard to get enough mast bend to flatten the sail fully by pulling on the controls.