"backwinding" is quite an incorrect term as whats happening ISNT coming
from the genoa.

Whats happening is either that the leech shape of the genoa is too
straight and or the genoa has too great a LP to match with the
Mainsail. If the mainsail has correct shape (by design and set) then
the remedy is to cut away some of the leech in the middle to affect a
greater 'hollow' of the leech. Try easing the genoa sheet to increase
the distance and see what happens.
Of course if the position of draft on the mainsail is too far forward
(too much halyard tension) then you will get the same result.
"backwinding' for pointing/beating will have virtuallly NO adverse
effect on sailing performance (If the genoa is trimmed correctly).

Most commonly, especially with an older mainsail, is there is an
'ageing' change to the luff dimensions caused by a shinking/shrunken
bolt rope.
A mainsail with a shrunken boat rope will have the following
characteristics: VERY full shape, hooked to windward leech, great
amount of draft that cant be minimized by gross outhaul tension - sail
cant be 'flattened' by extreme halyard and outhaul tension - boat will
heel very easily and the boat speed is usually very Blah! -- the boat
will have more than necessary weather helm (unless you use bar-tight
halyard pressure).
A mainsail 'should' have a somewhat FLAT shape at the luff (entry) and
maximum draft at about 40% to 50% cord length back from the mast. A
sail with a shrunken boltrope will have a 'bulging draft right behind
the mast .... and if you overtighten the halyard (to adjust helm
pressure) the bulge will get even larger (.... and cause what looks
like 'backwinding').

If you have the original dimensions of the sail simply use a steel tape
measure and verify. If you have any PHRF or other racing 'black
bands' embossed on the mast raise the sail with extreme halyard
pressure and see if the sail fits exactly between the bands .... if the
sail luff length comes up short - then the boltrope is shrunken and
needs to be 'eased'.
Boltropes are purposely made shorter (1" shortening for every 10 ft. of
luff length) than the original luff dimensions so that the sail will
keep shape at the higher windranges (15-18kts.) when you put "proper'
halyard tension to it. These ropes are usually three strand dacron.
Every time you stress these ropes they become fatter and shorter until
eventually the sail shape is abysmal. A sailmaker can easily 'ease'
such a shrunken bolt rope (so can you if you have waxed sailtwone and
sailmakers hand needles, etc.) --- but will want to sell you a new sail
first. On dacron sails I usually need to adjust/ease the boltropes
after just about every (hard) sailing season.

;-)




In article , Roger Long
wrote:

OK, I’m finally getting far enough ahead of the curve on the basics
that I can start thinking about sail shape. I can see from the replies
to my post on the backstay that I’ve forgotten a lot over 20 years of
not sailing much except in gaffers.

We have an impressive amount of main backwinding despite a wide
headsail sheeting base on our boat. I haven’t been able to fiddle and
experiment much because we are still spending a lot of time short
tacking in confined waters with a lot of traffic and it’s hard to play
with sail controls when new helmskids are wandering all over the
place.

Our boat seems to go well with the 140 genoa just touching the
spreader tips and the foot along the lifelines. Is this too tight to
expect a smooth main? By the time the wind is strong enough that I
need to ease her in the puffs, the main is often completely depowered
and flogging, even with the traveler nearly amidships.

Is this the proper prescription for backwinding?

More stay and jib luff tension.

Genoa sheet leads farther aft to ease leech.

Main Cunningham (just added but not used yet)

Maximum vang tension to detwist main.

I’ve done some Googeling for "sail trim" but it sounds like there’s
much to be learned here.

BTW our E32 has a modified rudder so I don't know how the stock boats
behave but the handling of ours is superb. I've driven her down hard
and there is never a hint of the sails taking charge. She is a
delight to steer even when way overpowered.