7
WHY SAIL TRIM MATTERS
Get your sail trim right, and you won’t only sail
faster, you’ll also heel less, point higher upwind
and make less leeway. Twist is one of the key
aspects of a well-trimmed sail be it a mainsail
or headsail. Too little twist, an over-tight leech
and the wind will stall on the aerofoil. Too much
twist and the sail will be spilling wind at the top
and so losing power. Most sailors understand
that to be the case, but creating or reducing twist can be done
in a number of different ways and each has a corresponding
effect on a sail’s performance.
‘Put simply, adding twist spills power and reducing twist adds
power,’ says Bill Gladstone, director of North U – North Sails’
education programme. ‘In addition, reducing twist improves
pointing ability, so the less twist you have the higher you can
point upwind. As a counter, though, the less twist you have the
smaller groove you have to sail in, as the whole aerofoil section
can easily stall.’
It is tempting to think of the mainsheet as the main twist control
on a boat. After all, when we sheet a mainsheet all the way in, the
boom is pulled down and the leech is hardened, reducing twist.
However, it can be much better to control twist and mainsail shape
through the vang, leaving the mainsheet as an in/out control. This
means that mainsheet trimming is easier – as you are not having to
sheet the sail in and down – and quicker. It also means you have
more precise control of your boat.
If you have a traveller this effect is better achieved by using the
mainsheet to pull tension in the leech and using the traveller as
the in/out control. Although the kicker does work to control the
leech, because it also pulls the boom forward and into the mast it
can cause some mast bend low down and so distort the shape of
the sail lower down, flattening it.
‘It is worth noting, that many cruisers these days do not have
adequate vang or traveller systems in place,’ notes Gladstone.
‘So for the most part twist is going to be controlled via the
mainsheet, even though that might be an imperfect system.
‘Twist is correctly thought of as one of the three aspects of
power – the other two being angle of attack and depth – but it
should be considered for sea-state purposes too. If you are sailing
in relatively flat water in medium wind, then you would sheet your
mainsail in until the leech telltales are just flying, as this will give
you the most power. However, if you have the same wind
conditions but there is some seaway then you would want to let
the mainsail twist a bit more as this will give you a wider groove
upwind and it improves acceleration too, so when the boat is being
knocked around by the waves, you get back up to speed quicker
and you are less likely to stall the sail.’
REMEMBER: Twist is not only used for power but also to improve
acceleration and help the boat stay on its feet in wavy conditions.
Experiment with the three mainsail variables of twist, depth and
angle of attack, and watch what impact it has on your angle to the
wind, boat speed, and course over the ground and heel.
8
UNDERSTANDING SAIL DEPTH
Depth is the second key power control for a sail.
‘Broadly, the depth of a sail is controlled by outhaul
and mast bend – though some boats will not be
able to easily bend
their mast,’
explains
Gladstone.
‘When you are
underpowered, adding depth
increases power, so easing the
outhaul in lighter winds is the best
way to do this. As the wind increases
you pull on more outhaul. If outhaul
is all the way in and you still need
to reduce power then you move on
to bending the mast.
‘If you have a backstay this is the
best way to induce mast bend, but
remember, mast bend has several
effects, not just flattening the sail. As you bend the mast several
things happen. The sail gets flatter as the middle of mast goes
forward. Secondly, you increase twist in the sail as the top section
of the leech no longer has as much tension and the third thing is that
the draft moves aft. As you bend the mast
to keep the same twist profile you might
need to adjust the mainsheet and then
pull on more halyard or downhaul. The
downhaul or halyard increase in tension
both pull draft forward in a sail, so pulling
on a bit more when the mast bends
means that the draft of the sail moves to
where it is designed to be, just forward of
the middle of the sail.’
REMEMBER: Bending the mast to
flatten the sail moves draft aft
so often needs to be countered
with halyard tension or downhaul.
Da
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EXPERT ON BOARD
Gr
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Mast bend flattens a sail but
moves the depth aft; downhaul
counters this
Twist, or how open the leech is,
shown by the telltales, affects your
boat’s pointing and its ability to
power through choppy conditions
