184 seamanship secrets
header and a header resulting from a change of true wind direction are diff er-
ent animals. A velocity header will cease to be a header once your boatspeed
drops in response to the lull. As your boatspeed falls—and assuming the true
wind direction remains steady—the apparent wind will move back aft to its
previous position. Th us, tacking will gain you nothing unless sailing the other
tack will get you into a stronger breeze. Look at the wind on the water ahead
and to windward. Darker streaks denote areas of stronger breeze. Look at other
sailboats too—do they seem to be heeling more and moving faster to windward
or ahead? Weigh the evidence and decide whether the wind is more likely to
fi ll back in if you hang with your current tack or switch tacks.
Remember these two facts about gusts and lulls:
If true wind speed increases, the apparent wind moves away from the bow
(a velocity lift ).
If true wind speed decreases, the apparent wind moves toward the bow
(a velocity header).
True wind direction changes. Th e Chapter 4 discussion of true wind lift s and
headers focuses on close-hauled sailing. But a change in the true wind direction
has an equally signifi cant though more subtle eff ect on off wind sailing. One thing
that makes sailing downwind diffi cult is that a small change in true wind direction
results in larger changes in apparent wind direction. Keep in mind that sailing ves-
sels sail to the apparent wind. According to Steve Colgate, founder of the Off shore
Sailing School, a 16-degree change in the true wind makes a 28-degree change in the
apparent wind when you’re sailing downwind. Th at’s one reason boats sometimes
roll from side to side when running before the wind. Th e person at the helm might
be oversteering (moving the wheel or tiller too much) to try to keep the boat on
a straight course. Oversteering in combination with a big apparent wind change
could result in an accidental jibe, in which the boom swings across the boat with
great force, possibly damaging the rig or causing serious injury. In gusty downwind
sailing conditions, keep the wind over the stern quarter rather than directly over
the transom, and rig a boom preventer (see “How to Prevent an Accidental Jibe”
on page 198) or lower the mainsail and sail under headsail alone.
Remember this when sailing downwind:
When well off the wind, a small change in true wind direction causes a
large change in apparent wind direction.
How to Determine Wind Forces
You’re sailing in a light 5-knot southwesterly breeze with the wind on the
quarter. The wind slowly increases to 15 knots throughout the day, but you
hardly notice. With the boat level and the crew relaxed and content, what