Both boat speed and apparent wind speed are shown as a fraction ( or multiple ) of true wind speed, which is represented by the vertical vector at the left of the diagram.
32.
Also on a run an inexperienced or inattentive sailor can easily misjudge the real wind strength since the boat speed subtracts directly from the true wind speed and makes the apparent wind less.
33.
If the velocity of the boat and the velocity and the angle of the apparent wind are known, for instance from a measurement, the true wind velocity and direction can be calculated with:
34.
As the boat is going downwind, the apparent wind is less than the true wind and this, allied to the fact that the sails are not producing aerodynamic lift, serves to limit the downwind speed.
35.
If the boat can accelerate until the apparent wind is alpha degrees off the bow, then it can be seen from the table above that it can make good downwind faster than the true wind.
36.
From the detailed data provided for the 2009 record set by a sand yacht, it can be seen that the record was achieved when the yacht's course was about 120 degrees off the true wind.
37.
And so can sand yachts : during the 2009 land speed record, the yacht Greenbird was proceeding at about 3 times the speed of the wind on a course about 120 degrees off the true wind.
38.
Indeed, it can be seen from the polar chart for the Skiff that it can make good about downwind at a windspeed of, by jibing back and forth at about 140 degrees off the true wind.
39.
Suppose that a boat is at a standstill, then starts to sail on a course that is 135 degrees off the true wind ( the value 135 is chosen for this explanation in order to simplify certain calculations ).
40.
Note that, if a boat can accelerate until it is sailing at 45 degrees off the apparent wind when sailing 135 degrees off the true wind, then its speed will be 1.41 times the speed of the true wind.
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