Oyster News 49 - Oyster Yachts

SAIL TRIM
Welcome to the second in
our series on getting the
best from your sails. In the
first article we looked at
headsails, the tension
applied to them and the
effect on your yacht's
handling and performance.
In this issue we are taking
a closer look at mainsails.
We will consider their
settings and adjustments,
relationship to the headsail
and overall sail care.
Windward sailing is where
attention to detail in sail
trim creates most impact.
This applies particularly
to mainsail trimming.
To windward the main
needs to be set relatively
flat (compared to the
headsail). Adjusting the
halyard, outhaul, backstay,
and to a lesser extent the
battens, will have a
noticeable effect on
performance.
26 www.oystermarine.com
By Matthew Vincent of Dolphin Sails
HALYARDS
As with the headsail, once the main is up, the halyard tension will be the first control you need to
check. The tension should be enough to just remove the horizontal wrinkles that occur in the luff.
This should set the draft at the correct position, which is approximately 45-50% aft in the sail.
You can check this by looking up the sail from the centre of the boom, where you can judge the
draft position by the seams or draft stripes.
Older sails, and in particular those made from Dacron, tend to lose their shape. As a result the
draft will increase and move further aft. This means you will find it difficult to flatten the mainsail
sufficiently and consequently a reduction in overall performance will occur.
OUTHAUL
When windward sailing in a breeze (Force 3-4+), you need to apply firm outhaul tension. Doing so
will maintain a desired 5-9% draft in the lower sections of the main. More draft will increase
backwind in the sail, likely to produce weather helm and a decrease in performance.
BACKSTAY
Increasing the tension in an adjustable backstay will straighten the forestay and flatten the
headsail. Depending on your rig type, this may also have a flattening effect on the mainsail, which
is desirable for windward performance.
BATTENS
Battens are best considered as relatively passive in your sail. A good sail is cut so that the battens
will blend into the shape of your main. However, a sign that increased batten tension could
improve your performance is when vertical wrinkles occur across the battens. (Note: The windward
side of your main, which is in compression, will tend to look more wrinkly than the leeward side). In
vertical furling mains the battens will be sewn into the sail and will not be adjustable.
For off wind sailing, in order to deepen the sail and therefore generate more power in your main,
a good rule of thumb is to decrease the halyard tension by approximately 0.5% of the luff length,
and the outhaul by up to 1.5% of the foot length. You will also need to reduce any applied
backstay tension.
Once the overall shape has been set using these controls, the sheet, vang (kicker) and traveller
are then used to position the sail relative to the centreline and headsail.
MAINSAIL AND HEADSAIL INTERACTION
A bird's eye view of a yacht (diagram) shows us that when sailing to windward, the headsail is set
at a relatively wide angle from the centreline at 11-13 degrees (slightly less on performance
boats), compared to the mainsail, which is set at an angle of 0-10 degrees. This indicates that the
headsail provides more of the forward drive, whilst the main, which operates in a permanent
header (from the headsail), is set closer to the centreline, and should be thought as more of a
steering and helm balancing sail.
TRAVELLER SHEET AND VANG
The best way to judge if the sails are well trimmed is to feel it through the helm balance. You
should be seeking to minimize rudder movement and keep it within a 0-5 degree band. For
example, say you are sailing to windward in about 12 knots of breeze; the sails are well set, the
main is near the centreline; the helm is nicely balanced, then a strong gust strikes your boat. She
could be kept on track by applying excessive helm. However, a much better move is to ease
down the traveller. Enough traveller should be eased to keep the helm within the 0-5 degree
band. You will lose a little pointing ability for a moment. More importantly though you will be able
to keep the helm straight, your boat on track and you will probably increase boat speed. As the
gust passes, you can trim the traveller back up. Easing the traveller is preferable to easing the
sheet or vang, as pointing ability is more easily maintained. A consequence of easing the traveller
down may be to increase the backwind in the main. Moderate backwinding is preferable to
excessive helm use.