The aim when trimming sails is to adjust their aerodynamic shape and angle to the wind in order to obtain the best possible performance from the boat in the prevailing wind strength and sea conditions.
Correct sail trim is most important when beating to windward, when the sails have to produce their maximum drive to overcome the resistance of the headwind and seas,
The difference in performance between a boat in which the sails are correctly trimmed and one where the crew have simply hoisted and hoped for the best is dramatic.
In extreme conditions, beating against a strong wind and steep seas could be a matter of one boat making progress and the other slipping back due to excessive leeway.

         Telltales
The oldest trimming rule is this: ease the sail until it starts to luff; then sheet it in a bit until it stops luffing. Telltales allowyou to refine your trimming by showing you the airflow over the sails.
A genoa should have three sets of telltales. They should be evenly spaced up the sail roughly a foot back from the luff, at about quarter, half and threequarters of the way up.
Ahelmsman's nirvana is reached when all three sets of telltales stream aft in unison.
When the boat is sailing too close to the wind or the sheet needs to be pulled in, the windward telltales will lift. When the boat is sailing too free, or the sheet needs easing, the leeward telltale will flutter or drop.
Getting the draft right
As a rule orthumb, aim to keep the maximum camber, or draft. In the headsall between 40-50 per cent back from the luff when going to windward. As the wind increases, the draft will tend to drop back towards the 50 per cent mark.
In lightish airs and in flat water a shape with maximum draft close to the 50 per cent mark gives the sail a finer entry and allows the boat to point a little higher. But as the wind increases, if the draft is too far aft the sail will be too full in the leech. The leech hooks to windward, creating drag. and the boat slows down and heels more, making more leeway.
In a seaway, pointing ability is not as important as keeping the boat moving well and giving her the power lo punch through the seas. Adjusting the sail's shape to maintain the draft further forward, giving It a fuller entry, and sailing a few degrees lower, will make life much easier for the helmsman.
Off the wind, draft location is not as crucial, because the wind pressure on the sail is not so readily translated into heeling forces.
It's difficult for the untrained eye to gauge the location of draft In a sail without the aid of camber stripes. which run from luff to leech and provide a quick visual reference, They're a useful and inexpensive modification. Self-adhesive strips are available from sailmakers.

    Controls
There are four ways of controlling the shape of the headsail:
Halyard tension: If we are looking to keep the draft between 40-50 per cent back from the luff, tensioning the halyard as the wind Increases will move the fullness further forward. Conversely, in light airs, when more power is needed to keep the boat moving, the halyard should be eased a little to create a finer entry with more fullness in the sail. Rig tension: If the rig is not correctly set up and tuned, a sagging forestay is inevitable. On the wind you need a tight forestay to flatten the sail- A sagging forestay makes the sail fuller and moves the draft aft, which is the last thing you want. The boat won't point as high and she'll heel more.
Backstay adjusters allowyou to tune easily, but these must always be fitted with a stopper to make sure the backstay cannot be slackened off too much, endangering the rig. Remember, too, that some forestay sag is inevitable. On a cruising fractional rig, backstay tension will tend to bend the mast rather than harden up the forestay. which is supported by the cap shrouds. Sheet tension: On the wind. the last few Inches of sheet tension are vital and will make a noticeable difference to the boat's performance as the sail is flattened and the draft is moved forward. Sheet tension also has an effect on twist - the sail's built-in tendency to have its top half setting at a wider angle to the boat's centreline than its bottom half-Sheet lead position: Most cruising boats have the genoa fairlead fitted to a track which allows fore and aft adjustment of the fairlead- The sheet fairlead position is critical in controlling twist and maintaining a proper headsail shape-There is an ideal sheet lead position for every change in headsall area. There is a simple way of determining this position. Remove the headsall and spread it out flat. Measure the lufflength and mark the halfway point. Stretch a piece of line between this point and the clew. and use It as a reference point for a trim line - use Indelible marker pen or sallmaker's tape - extending 2ft or so from the clew. As long as the sheet forms a straight line with the trim line, the genoa car should be in its optimum position for windward work. When the sail is reefed, and vice vetsa, the car can be moved until sheet and trim line are once again in alignment,
As a rule of thumb for windward sailing, sheet leads should go forward in :
light airs, which slackens the foot of the sail and makes it fuller for more power. As the wind Increases, the lead is moved aft to a position where there is more or less an even pull on leech and foot. If the wind keeps Increasing, you can move the lead further aft, which lets the E top of the sail twist off and spill wind-  !
As the sheets are eased to put the boat | on a reach, the sheetlead should come j forward to Induce the proper sail shape, S When the sail is eased well out. the trim ^ line will give a false reading because the salTsclewwillbesohigh. But if you get the lead as far forward on the track as possible you won't be far wrong.
Telltales are a good guide to correct fairlead position on a reach. When the sheets are cracked foraclose reach, the top of the sail twists off. The bottom two windward telltales, which are usually all the helmsman can see, might be flying perfectly but the lop one will be all over the place. It is telling you that there Is too much tension on the foot of the sail, not enough on the leech, so the top of the sail is luffing first. The remedy is simple:
move the sheet lead forward. If the bottom windward telltale Cutters first, the leech is too tight, the foot is too loose, and you should move the sheet lead aft.
Don't oversheet
A genoa winched in so hard lhat the spreaders are poking into It and the leech is stretched over the shrouds Is a horrible, but not uncommon, sight. It's also an lnefficientwayl.osail.If the sail is sheeted in too flat for the wind angle, it will stall - the wind-flow becomes detached and the sail loses its lift. The boat will be slow and cranky, heel more, and sag off to leeward. Ln light airs. it may even stop dead. If in doubt, ease It out.

Sheeting angles
A performance cruiser with a fine entry to her hull can cany tighter sheeting angles than a displacement cruiser with a fuller entry, which needs a fuller shape in the headsail to keep her moving. A high-aspect ratio jib with little or no overlap can be sheeted further inboard than an overlapping genoa. which would otherwise backwind the mainsail and spoil the slot. Ideally, headsail sheet leads should be moved outboard on a reach. We will discuss ways of doing this in a future article.

The leech line
In light airs a well-cut sail will usually remain stable, but as the wind increases there is a chance that the leech will start to flutter. You don't want this, for two reasons: it upsets the airflow across the sail, causing drag. and over time it will weaken the sailcloth. This is when a little adjustment of the leech line pays dividends. Too often, though, the leech line is tightened up when the flutter is actually caused by apoor sheet lead. so check the telltales first.
You can fiddle with sail trim all day and still end up with a sorry-looking headsail if the boat's rig has not been properly set up and tuned. Nor will any amount of trimming make a tired old genoa set well. If all else fails, enlist the help ofasallmaker.

Quick reminders
*Forestay tension: ease to increase genoa fullness, tighten to flatten.
* Luff tens! on: Increase to move draft forward, ease to flatten the luff.

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