MAINSAIL TRIM
While the mainsail provides a large
portion of power, it also affects the boat's directional control. The main sail
helps to steer the boat by functioning like a trim tab on the keel.
The leech has the most influence on the direction 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. An open leech allows the air to flow
easily off the mainsail without developing as much sideways force, reducing
windward helm.
The five basic steps of main sail trim are Twist, Outhaul, Draft Position,
Helm Balance, and Fine Tuning.
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 over-bend wrinkles, ease the backstay tension or
tighten the checkstays/runners to straighten the mast.
2.Outhaul
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. 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 (see below). 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 traveler position
The main sheet traveler controls the angle of the mainsail to the boat's
centerline 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.
Boat-speed 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.
De-power the mainsail by bending the mast, opening the leech, easing the sheet,
dumping the traveler, 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 get windward helm down into the acceptable range.
Sources:
(1) www.arklowsc.ie
(2) Annapolis Guide to Seamanship