A sailboat can sail faster than the wind
In order for a sailboat to move, it has to feel wind on its sails coming
from at least 45 degrees off the bow (a little less on some boats) to provide
power. When the boat moves, the boat's speed makes it feel like the wind is
coming from more in front of the boat than it really is (like sticking your
hand out a car window - there seems to be lots of wind from the front because
your hand is moving through the air). Using vector addition, the boat speed
is subtracted from the true wind to generate the apparent wind which the boat
feels. In this example, with a true wind of only 17 knots off the stern
quarter, a boat moving through the water at 23 knots actually feels an apparent
wind of 12 knots at a 45 degree angle off the bow, enough to move a fast hull
(a cat or trimaran, a planing dinghy, sailboard or kiteboard) at that speed.
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