186 seamanship secrets
Looking down at this wedge from above the
mast, we can slice a cross section through at
any height and draw a straight line from luff to
leech (the after, trailing edge of a sail), parallel
with the boom. Look along this line, which is
called a chord, and find its point of maximum
separation from the sail membrane. That point
is the position of maximum depth, or draft,
often called simply the draft position.
A mainsail’s draft position should stay be-
tween 40% and 50% of the chord length aft of the
luff. Use this table as a rough guide for setting
draft in a mainsail:40% (breezy to heavy winds)
45% (light to moderate winds)
50% (very light winds; “ghosting”)
Think of the wind as a giant hand that presses on a mainsail. The pres-
sure of the hand is greatest at the point of maximum draft, and in heavy air,
if our draft is too far aft, the hand pushing on the sail tries to round the boat
up into the wind. Weather helm—the tendency of the boat to round up into
the wind—increases, and the boat heels more and becomes more difficult
to steer.
In light air you want a fuller, more powerful sail (see below), and a deeper
draft combined with a forward draft position would make it difficult for a
light breeze to remain attached to the sail with laminar flow. The air would
tend to detach and become turbulent, reducing the sail’s effectiveness, so the
best draft position is farther aft in light air.
Three Secret Systems to Give You Perfect Draft Control
Fortunately, most sailboats have three powerful control systems that enable
us to move and shape draft to match any condition of wind or sea. Th ink of
each sail edge—the luff , leech, and foot—as having a separate “control system”
toolkit. You want a full, powerful shape in light air and a fl at, depowered shape
for heavy air.
- Mainsail luff control system. Your sail’s luff takes it on the chin. Aft er all,
this is where the wind fi rst makes contact, so you’ll need to control the luff
more than any other sail edge.
Adjust the draft in a mainsail according to wind and
sea conditions.