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.

On some classes (RS400 for example) the boat is EXTREMELY sensitive to jib sheet tension.  Always let the jib out if you let the main out. 

BTW GRF, the jib slows down the air flow over the main, not speed it up.

The tendency of the jib is very similar to the lee bow effect you get from
another boat ahead and to leeward of you.

Too tight and it will 'head' the main which backs it. Even I know that
much, but still made that mistake early on because at the end of the day
the crew operates it and when it's windy we were more intent on
surviving, we're past all that now.

But it is a fact that the slot between the jib and the main, and this intel
goes back to Scorpion times, actually venturis the airflow as well as
slightly heading the main sail hence why you do all your pointing with the
jib leading edge and it those tel tales you watch.

Balanced properly a two sail dinghy rig is really acting as one unit as far
as propulsion is concerned, but if you get the balance wrong you are
effectively creating an air brake, which done wrong produces all those
parasitic drag things I crack on about.

We most of us these days with these modern boats are over canvassed
more often than not, so getting the depower right is very important and I
wish I could sort the magic rig formula, the controls open to us are so
over complicatedly useless compared with what i"m used to.

Bang the Gnav on, it will tighten the leech, but it also now bends the
mast forward and flattens the sail unless you tighten the lowers which
then prevents the cunningham from bending the mast to twist the top off
because the jib tension will bend the top of the mast ahead of the bottom
unless you then tighten the upper shrouds which then rakes the mast
back and it doesn't point as well... it goes on.

We've got a new main now, a good metre and a half smaller, but with very
little twist at the head, I've not used it in anger yet, but it'll be ages
before I work out what to do with it and when, life's a lot simpler for a
windsurfer with just two controls, downhaul and outhaul.

Sail twist (wind 25-30knts)

 

intentional. (i hope)

It was very windy and some big waves, so the rig tension was let off making the jib slot open up.   we had max kicker and loads of cunningham opening up the top of the main sail but notice the main was still brought to the middle.  Made us go very fast that day