Starpath TrueWind INDEX
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copyright © 2000, Starpath School of Navigation
True wind versus apparent wind
In reading a telltale, we need to keep in mind that it shows the direction of the
apparent wind as distinguished from the true wind. The apparent wind is the
combination of the true wind and the effective wind generated by the motion of
the boat. The difference between the directions of the true wind and the apparent
wind depends on how fast you are moving relative to the true wind speed. For
boat speeds less than 10 or 20 percent of the true wind speed, the difference is
negligible, and you can read the true wind direction directly from the telltale.
When you are moving, the direction of the true wind is always aft of the appar-
ent wind. If the apparent wind is on the beam, you must face this apparent wind
and turn aft to be looking in the direction the true wind comes from. This is true
regardless of your point of sail. If the apparent wind is 45° on the bow, the true
wind is closer to the beam. If the apparent wind is on the quarter, the true wind is
closer to the stern.
The exact number of degrees the true wind is aft of the apparent wind depends
on your speed relative to the wind and on your point of sail. At any relative
speed, the difference between the two is largest when you are sailing with the
apparent wind on the beam. The difference is typically somewhere between 10°
and 40°, where, generally speaking, the higher the performance of the sailboat,
the bigger the shift can be.
When relying on wind shifts to tell us something about the weather, it is ulti-
mately the true wind direction you care about. Apparent and true wind directions
will not always be different enough to matter, but in order to read the wind —
for maneuvering or for weather analysis — we must keep their potential differ-
ences in mind.
Tips, tricks, and special cases
(1) With the wind on the beam, the true wind can be well aft of the apparent
wind. For example, in 5 to 10 knots of true wind, a typical sailboat travels at
some 0.6 to 0.7 times the true wind speed. At speeds of 0.65 times the true wind,
when the apparent wind is on the beam, the true wind is 41° aft of the apparent
wind.