Sail Control
To many of us, sails are a mysterious source of power…they can look good but slow and other times, ratty but fast. Often the difference between the top sailors and the rest is the ability to recognise when they are sailing fast and be able to reproduce fast settings for different wind and sea conditions.

Most sailors can change settings often enough to hit on good speed, but the top sailors also know when to stop and start sailing…all the adjustments in the world won’t make any difference if you don’t start properly, pick the right windshifts or keep the boat in correct trim.

Generally sails should be rather flat in very light winds (to reduce drag), fuller in moderate winds and flat in heavy winds.

Sail Camber and Sail Twist.
Camber and twist (leech profile) are equally vital to boat speed.

Sail Camber:
Camber has two aspects which you can vary…depth ie. How full the sail is, and position along the chord of the deepest point. You can see how full a sail is by looking up at the curvature of the seams.

Controlling camber.
The depth of curvature is varied by increasing the tension of the cloth across its chord - by vang, outhaul and by mast bend.

The position of the deepest curvature is varied by tensioning the downhaul (Cunningham Eye) or the vang.
The point of greatest curvature moves towards the area of tension.
Increasing pressure on the luff via the downhaul moves the centre of pressure forward; increasing the pressure on the leech via the vang will move the centre of pressure aft.

Downhaul (luff tension)
Basically, increased luff tension does not flatten the sail; it moves the point of maximum camber forward and straightens out the aft section of the sail…it frees the leech. It has an important effect on weather helm and therefore pointing ability.
Free leech - equates to light helm, less pointing, more speed.
Tight leech - equates to heavy helm, more pointing, less speed.
Easy adjustment to the downhaul is important.
Note: not keeping the boat flat will cause weather helm, so get it flat before making adjustments.

Outhaul
Easing the outhaul increases low down fullness. It also increases weather helm by powering up the bottom battens and exhausting the air more to windward.
Changing this setting by 10 cm can increase pointing by 2 or 3 degrees.

Mast bend
Sailmakers have two basic methods of cutting draft into a sail.
1. By cutting the sail profile with round to the luff. This excess material in the sail converts to draft when the sail is attached to a straight mast. If draft was only achieved by this method, the flow would be too far forward for efficient sailing.

2. By using seam taper to bring the flow aft. Round is cut into the edge of panels prior to sewing them together. Draft is positioned towards the centre of the sail. The balance of the luff round to seam taper is a very fine line.

Mast bend changes sail camber by sucking out material from the luff of the sail.

Shape for conditions.
Light wind - sail should be flat with the draft/flow (maximum depth) positioned around 45%-50% aft of the mast. This allows the weak breeze to move easily across the sail surface without stalling. Don’t worry about wrinkles down the luff.

Moderate wind - sail should be full and the draft brought forward to around 40%-45% to provide more power.

Strong wind - sail should be flattened with the draft at about 33% aft to reduce heeling and make the boat more manageable.

Twist:
Twist in aerodynamic terms, is angle of attack and is more difficult to appreciate than camber. The amount that the leech twists away from clew to head is vital to speed.
Too little twist is a sure way of going slow.

Controlling Twist
Twist is controlled by mainsheet, vang and traveller.

Mainsheet
Controls the rate at which the leech falls away to leeward. A good starting point is to trim the top batten parallel to the centreline of the boat.

Vang
The vang does a couple of things and is one of the most important controls. Firstly, it bends the mast low down by pushing the mast forward at boom level. With the vang and outhaul, the lower camber can be finely controlled.
In heavy wind, a large amount of vang can flatten the bottom sections. A good indicator of the required vang tension is the appearance of one or two creases from the clew to the middle of the sail towards the point where the second top batten meets the mast.
This throws the bottom batten off to leeward, reduces excessive weather helm and generally frees the boat from that clogged up feeling.
The vang also controls twist, and it is this for which it is mostly used…especially downwind.

Vang Settings
As a starting point, tension so that the upper leech telltale (between top and 2nd battens) just streams aft, while the lower telltales are just on the verge of lifting.
The upper leech telltale is the most important and if the boat feels chocked or speed drops off, check that it is not stalled…immediately ease the mainsheet until it flows and then tension until it is just flipping off.

Traveller (mainsheet hawse)
The traveller controls the overall angle the sail meets the wind…angle of attack. The lift generated by a sail is varied by changing draft or the angle of attack.