The fairlead cars are moved back and forth along the track to get the optimum shape / twist of the sail. The position of the farilead car is adjusted so that the sail has the optimum 'twist'. Twist is needed because the speed of the air is faster the higher above the water
(due to friction) AND the actual speed of the boat. If you dont set the correct amount of 'twist' the sail will be unstable and wont perform well .... and the amount of twist needed is 'different every time you sail' because of differing wind
sea-state conditions. If you dont set for proper twist there will be sections of the sail that 'will not be working' or some sections become very unstable (flapping).
Here's how to set the proper 'twist' (trim) and fairlead position of a headsail:
On a beat (close winded). slowly turn the boat into the wind while watching the entire luff (forward section of the headsail) from the highest (head) to the bottom (tack). If the luff section of the headsail starts to 'flutter' at the head before it starts to flutter anywhere else (too much twist), it means that the fairlead position on the rail is too far aft; if the luff starts to flutter at the bottom / tack (too little twist) it means that the fairlead position is too far forward. The goal, for the day and the conditions at the time, is to set the correct fairlead position so the front section (luff) 'breaks' simultaneously along the entire luff section (all at the same time) when you slowly luff-up to weather
. If the entire luff section 'breaks' when slowly luffing, it means that the fairlead is correctly set, the sail is properly 'twisted' and the whole forward section of the sail is 'working'. Every day is different and everyday you re-adjust the jib/genoa fairlead position (.... or any time the wind much changes velocity or density).
Once you know (and mark on the rail) your basic twist setting, in flat water
you'll want 'less' twist aloft to get higher boat speed and less power you move the fairlead a bit further forward while watching to see that the luff 'breaks' along the entire luff simultaneaously as you 'feather up' to begin to luff the sail. In more heavier wind conditions and especially those conditions that also have high/steep 'choppy' waves where you need *more power* and less speed, you move the fairlead back from the 'basic' position ... while watching that the entire luff section 'breaks' at the same time when you are feathering up to weather
. All these settings will ensure that the sail is properly twisted and that you are using ALL of sections of the sail. If ALL sections of the sail are not 'working', then you will get 'flapping' and shaking in these non-working sections.
For very heavy weather, you can 'depower' the sail by moving the fairlead further back from the 'basic' setting by promoting more twist aloft than needed and by simultaneously 'flattening' the foot of the sail -- by moving the fairlead further back - for less heel and more 'boat control'. All this is like 'shifting gears' in a car ... 1st, 2nd, 3rd ... the proper balance of speed or power.
What 'cruisers' DONT do:
When reaching (or any angle less than beating) and where the clew moves forward (when sheets
are eased) .... and especially for 'optimum' shape/trim .... the fairlead should be moved forward, so that the *amount of twist is the same or remains constant* as when beating. Cruisers hardly ever have well trimmed sails
when less than beating (and they usually dont have well trimmed sails
when beating either) ..... simply because they never move the jib/genoa fairleads forward when the genoa sheets
are eased and clew of the sail goes forward.
Aerodynamics is NOT an intuitive science; most 'sailing trim' books/instructions are dead-wrong and have been wrong since the time that Wilbur and Orvil Wright first flew. The crap about air travelling faster on the backside of a wing .... is not even close to being right, and they still teach such 'mystical crap' in USA high schools. There however is a 'simplicity' that you can observe of how a sail and its 'trim' / 'shape' is performing ---- the use of TELL-TALES.
You need tell tales near the leading edge (luff), at the position of maximum draft
, AND (most importantly) at the leech of the sail. Suffice it to say that the tell tales should be all flying flat against the sail and streaming straight back to insure that the sail is not 'luffing' (stalling) and also that the airstreams are not 'separating' from the surface of the sail. All trim and shaping is done to make such tell-tales - 'stream straight back'. If all the tell-tales are streaming 'straight back', then the aerodynamics (of trim and shape) will be correct. I strongly advise the usage of tell-tales (on luff, at midcord and at the leeches).
Then there is a 'boat designer's compromise' that should be considered - especially when beating into the wind: Most boats are too FAT, Beneteaus are especiallly and notoriously too FAT (broad beamed). Boats, nowadays are way too broad beamed because the modern purchaser is more interested in 'comodious below decks accomodations' (and dockside entertaining) than in sailing. This compromise of FAT boats usually results in the fairlead track being placed too far away from the centerline. When pointing/beating, the angle that an imaginary line from the stem/tack to the clew on a headsail should be about 10 degrees from the boat's centerline; the clew being on this 10 degree line will ensure that the boat will 'point'. If the angle is more than 10 degrees from the centerline then the boat will not 'point' well.
What to do if the boat's beam and its genoa/jib fairlead position is 'too fat': - Barberhauler !!!!!
A barberhauler is a simple 'rope'/line that is attached to the jibsheet at the clew (with a snatchblock) and is pulled-in perpendicularly to the boat's centerline until the tack-clew angle is about 10 degrees. In practice with a barberhauler, you begin to pull-in on the barberhauler (while slightly 'easing' the jibsheet) while watching the speedo; when the speedo begins to show a slight drop of speed, then open the barberhauler an inch or so .... and then you will be able 'point' like a banshee and the boat will be FAST.
FAST is not only for racing
; but, is for SAFETY
---- Fast away from storms, fast away from a lee shore, to be able to choose to go through a squall where 'you desire' to be and not in a position totally dependent on the 'winds', fast away from a freighter that is now on a collision
course, etc., etc., etc., etc.
I wish there were some 'good' books
on sail trim and shaping that I could recommend ..... after all these years (of aerodynamics) there just aren't any, or aren't any for the non-aerodynamicist.
One of the 'seminal' authors of aerodynamic based sail trim and shaping is Arvel Gentry. His articles although written for the 'layman' are extremely 'technical' and must be read several times over & over in order to begin to make 'sense'. I suggest (if you have a 'technical' ability) go to: www.arvelgentry.com
and especially go to the "magazine articles" section in that website. These 'magazine articles' are the seminal articles that set the sailing world entirely on its ear during the 1970s: "How a sail REALLY works"; PROPER USE OF TELL-TALES: Checking Trim on the Wind - November 1973; Achieving Proper Balance - December 1973; Sailing to Windward - January 1974; Are You at Optimum Trim? - March 1974
hope this helps.