Backwind (an erroneous term) is caused by the following:
Check the GENOA shaping first
• Insufficient forestay tension - causes the genoa to go 'draft aft' and simultaneoously closes the leech (hooks it to weather) and brings the leech closer to the mainsail thus closing the so-called 'slot'.
Set up the backstay (reacts with forestay) for 15+% initial tension for 15kts. windstrengh and look to see if the leech exit shape of the Genoa(s) at the upper and middle panels are parallel to the boats centerline. If exceeding normal backstay tension ....
• increase genoa halyard tension to relocate the postion of max. draft more forward which will also simultaneously open
the leech - to make the leech exit PARALLEL with the boats centerline and to open the 'slot'.
Caution - increased halyard tension can also make the luff shape 'too rounded' so beware of 'helm pressure changes' or lowering of boatspeed when you do this --- the aim should be FLAT luff entry shape on both (racing cut) sails.
• Too much jibsheet tension - does the same as Insufficient forestay tension. On a beat, the jibsheets should be tensioned only to the point of where the leech exit is PARALLEL to the boat centerline. If the jib leech exit is too far away from the main (check by slowly dropping the main traveller as you watch for the development of 'backwinding') then BARBERHAUL the clew of the genoa INBOARD until the main 'begins' to backwind and/or the boatspeed 'just begins' to drop ... then very slightly ease the barberhauler. Too much jibsheet tension causes shape problems of draft-aft and hooked-up / 'closed' leech. Most winch grinders typically put too much load into jibsheets and that results in closed leech and draft aft shape - restrain the 'winch gorillas'.
(add 2/22 - another way to do is to have parallel 'stripes' on the outboard undersurface of the spreaders and while watching the approx. distance between the stripes and the genoa leech move in/out (via jibsheet tension or barberhauler) to attain max. boat speed ... sometimes allowing the leech to open an inch or two is all it takes to increase speed or eliminate so-called 'back-wind' ... the max. speed as read on the speedo
will tell you how far in or out to set the leech distance or amount of barberhauling. When barberhauling in expect the boat to point higher and with no loss in speed ... but dont get carried away or youll 'choke the slot', etc.
My sport boat has athwart ship tracks for the jib leads ... every day and every condition requires a different slot opening distance and clew/leech distance from the spreaders. Boats that sheet to the rail and without barber-haulers cant do this 'vital' adjustment.)
Aim - Get ALL the 'full set' of tell tales (including 'gentry tufts') flying perfectly in power-pointing mode (all straight back)
You simply cant set/shape the main until the genoa is flying/shaped 'perfectly'; once SHAPED and trimmed to perfection, THEN ....
• Aim for perfect set/shape ... all telltales streaming parallel / straight back, especially the leech tales. Set MAX draft via the outhaull while watching the speedo while on a 'power-beat' (all tails straight back for 'power pointing').
* Adjust for neutral helm pressure, with traveller at center, (added 2/22 - and second batten from the top - aft end parallel to the CL) by adjusting main halyard and cunningham tension, then ease a wee-bit until the boat s-l-o-w-l-y heads up when the helm is released. Add as little additional halyard strain to keep the luff entry area of the main FLATISH - too much halyard strain will cause the position of max. draft to go forward and the forward position may be too close to the jib/genoa
(shows as 'backwinding') and if you cant adjust out the backwinding (by fore/aft draft position), then ease the genoa barber hauler or ease the jibsheets. (added 2/22 - if you cant achieve neutral helm by the previous; then and only then, consider to change the mast rake .. and start all over again with the 'set-up' and 'shape-up').
Its OK to have a 'happy bubble' of backwinding ... just a small section in the mid luff to show backwind = but verify with the speedo!
• Mast 'prebend' - allows a flatter
mainsail luff entry shape - less prone to 'backwinding'. The C&C 34 is a double spreader rig so you should have ~ 1-1/2" prebend in the mast, even with rod rigging. A sailmaker EXPECTS the proper prebend when he cuts the main and if you dont have it, the mainsail will be too full and with a ROUNDED (backwind vulnerable) luff entry.
• If this is a woven DACRON mainsail - "How to properly RAISE a Woven Dacron Mainsail" - How to properly RAISE a woven dacron mainsail - SailboatOwners.com
THEN once the main is shaped to perfection ..... go back and reset all the shape/trim/ of Genoa !!!!!!!!! and if needed come back and readjust / reshape the main, again. One sail influences the aerodynamic flow over the other ... requires many adjustments and checks !!!!!!!
Forestay to Luff Shape Matching.
I have an article posted on "matching forestay tension to luff shape" at: MatchingLuffHollow.gif picture by svAquila - Photobucket
.... as if this 'match' between forestay tension and luff curve shape (luff hollow) isnt set up perfect, its probably one of the prime causes of 'lousy pointing ability' ... and sometimes too much 'backwinding', closed leech shapes, etc.
Backwinding as most sailors assume is NOT what is happening when the luff of the main 'seems to collapse due to too much wind in the 'slot'', in actuality its the 'timing' of the interaction between the genoa and the main (aerodynamically called 'bootstrapping') is off because the CIRCULATION flow around both main and genoa are not correct and are canceling one another in the region of the 'slot'. (Sorry folks, your high school 'science teachers' have been 'dead wring' ... for almost 100 years!! no big surprise there, huh.).
Suggest you also go to: ArvelGentry.com
---> Magazine articles
---> then the 'sequence' of articles:
Checking Trim on the Wind,
Achieving Proper Balance,
Sailing to Windward,
Are You at Optimum Trim?,
Gentry is the sailor/aerodynamicist who debunked the 'slot effect', backwinding, and wrote the 'sailing world shaking' article - "How Sails Really Work", etc. etc. ... all based on firm aerodynamic principles. His Mag. articles plus watching the speedo as you 'set up for the day' will Im sure correct ALL of your 'backwind' problems.
... hopefully also my article on 'matching forestay tension to luff shape' will also help solve your 'backwind' problem (of incorrect aerodynamic 'bootstrapping').
Also, suggest you go back review all the elemental basics of trim, etc. A good 'reference' for the 'basics' would be: DonGuilette's "Sail Trim Users Guide" Sail Trim Users Guide (US delivery)