MY BASIC MAINSAIL TRIMMING PROCEDURE:
My racing experience is limited and from long ago, so the following likely contains errors. Anyone who has ever met a layer is specifically prohibited from using this information, in any manner whatsoever.
The main sail helps to steer the boat by functioning like a trim tab on the keel
The leech, has an important influence on the directional tendency of the boat. A closed leech directs the airflow to windward, creating a large side force to leeward at the stern of your boat. This creates weather helm
and tends to push the bow of the boat to windward.
Similarly, an open leech allows the air to flow easily off the mainsail without developing as much sideways force This will result in less windward helm.
1.Set twist with mainsheet tension.
On a mainsail, twist is controlled by the amount of mainsheet tension, as well as the amount of tension on the vang. The mainsail leech is the best indicator of how much the sail is twisted. To set the proper sail twist, trim the mainsheet until the top batten is parallel to the boom.
When the sheet is eased, the main has a very twisted shape, with the top batten falling off to leeward . As you trim the sheet, the top batten angle narrows until it is parallel with the boom. Trimming harder will take away all the twist and close the upper leech.
The best average setting for the top batten is parallel to the boom. With the top batten in this position, the top batten telltale should stream aft between 50 and 90% of the time. This tell tale, attached to the aft end of the top batten and extending 8 to10 inches beyond the leech, indicates whether the upper leech is stalling. When the leech is stalled, the telltale curls around to leeward of the main. Twisting the main sail more will open up the leech and re-establish the airflow. In choppy conditions, after tacks and in light airs, ease the sheet to open up the leech slightly and prevent stall.
The primary means for adjusting depth
in the upper two-thirds of the main is mast
bend. Bending the mast moves the luff away from the leech, which does several things simultaneously - it flattens the sail, opens the leech and moves the draft
aft. If you see overbend wrinkles, ease the backstay tension or tighten the checkstays/runners to straighten the mast.
The best way to control depth
in the lower third of the main is with the outhaul
. The tighter the outhaul
, the flatter the bottom of the sail. If the waves are big for the wind, ease the outhaul slightly to give more power. If the waves are small for the wind, as in an offshore
breeze, pull on the outhaul to flatten the sail and reduce drag.
Besides depth, the outhaul also changes the tightness of the lower leech. Easing the outhaul adds depth to the foot of the main. Conversely, tightening the outhaul opens the lower leech.
The tighter the lower leech, the more windward helm you have. That's why it makes sense to tension the outhaul in heavy air to open the leech and reduce the helm forces.
3.Set draft position with luff tension:
Once you've set the overall depth of the sail, the next step is to position the area of maximum draft - at about 50% (centreline ‘twixt luff & leach). This is usually done with the cunningham tension.
The cunningham applies tension to the luff of the main, and this controls draft position. Tighten the cunningham to move the draft forward; ease it to let the draft move aft. In general, the more you bend the mast, the tighter you need to pull the cunningham to get the draft in the right place. You'll also have to pull the cunningham harder on an older main, because a sail's draft moves aft with age.
In light airs, keep the cunningham quite loose. In light airs, lower the main halyard
(especially downwind) to get the proper luff tension.
4.Set helm balance with traveller position:
The main sheet traveller controls the angle of the mainsail to the boat's centreline and to the wind This has a large effect on helm. 3 to 5 degrees of windward helm is the ideal setting.
5.Fine-tune the total power of the main with the above controls:
The final step in mainsail trim is continual evaluation of the sail's power. The main trimmer must keep track of the boat's heel angle, speed and pointing ability.
The most obvious indication of over-powering is the angle of boat heel. Boatspeed and the amount of windward helm are actually more sensitive and accurate indicators of over powering.
Measure the rudder
angle required to sail in a straight line. The ideal angle is about 3 to 5 degrees of windward helm. If you have more than this, you are overpowered.
Depower the mainsail by bending the mast, opening the leech, easing the sheet, dumping the traveller, and reefing if necessary. These adjustments are simply changing the total power being exerted by the mainsail. Since most of the main's power is side force, adjusting the amount of this power affects windward helm. Ideally you should reduce windward helm down into the acceptable range.
***E. & O. E.***