sailboat seamanship 185
could be better? Near the end of the day, the wind shifts abruptly. You’ll need
to head up to make your destination. You change course with full sail set.
Immediately the boat heels over, burying her rail. What happened?
When sailing downwind the apparent wind was relatively light. If the boat
travels at 6 knots and the true wind is 15 knots, the apparent wind while you’re
sailing downwind will be 9 knots (the diff erence between the two speeds). As the
boat turns upwind, the apparent wind increases (see “Clearing Up the Mystery
of Apparent Wind” above) to just under 20 knots on a close-hauled course!
The force exerted on sails and rigging varies with the square of the ap-
parent wind velocity. Thus the apparent wind force doesn’t just triple between
5 and 15 knots, it increases by a factor of nine. Always look to windward for
whitecaps before changing from a downwind course to a reach or close-hauled
course. Reef or reduce sail before you turn toward the wind.
How to Shape the Mainsail, a Sailboat’s Main
Propulsion UnitA balanced sailboat requires less than 4 degrees of wheel or tiller to keep
her course.Th e mainsail serves as a sailboat’s primary propulsion engine. We have all the
tools we need at our fi ngertips to move, shape, and trim that sail into a powerful
machine, capable of taking us anywhere we choose to go. Our magic tools enable
us to build a faster speed shape into the sail or a stronger power shape to meet any
condition. But how do we determine the shape we need? How do we turbocharge
our mainsail for daysailing, cruising, or racing in any weather?
CHOOSING MAINSAIL SHAPE FOR SPEED OR POWER
Th e luff , or leading edge, of a sail greets the wind fi rst. A sailmaker can build
the luff fl at for speed or full for power, depending on where you sail. Flatter
sails generally provide more speed and work well in smooth-water areas such
as Long Island Sound or the Chesapeake Bay. Fuller sails provide more power
to punch through a chop in areas such as San Francisco Bay, the English Chan-
nel, or the Caribbean.
But sail shape isn’t totally in the hands of the sailmaker. Unlike an airplane
wing, a sail is a fl exible membrane, and you can make it fuller or fl atter—within
the limits imposed by the sailmaker—to respond to conditions on the water.
Draft control: The #1 secret to powerhouse sailing. Viewed from leeward,
a sail is a convex curvature of cloth. Viewed from windward it is cupped in a
shape that can be thought of as an invisible wedge. In the case of the main-
sail, the base of the wedge is the main boom and the apex is the masthead.