6 AERODYNAMICAL EXPERIMENTS UPON A YACHT'S MAINSAIL. 



In all of the work so far considered no distinction has been made between 

 real and apparent wind, but in applying the work to a yacht the wind which is 

 customarily observed is that indicated by the fly at the mast-head. This is the ap- 

 parent wind, and it is the resultant of the speed of the boat and the true wind. 

 It is obvious that the wind of the tunnel experiments corresponds to the apparent 

 wind. If it is desired to find the relationship between the apparent wind and the 

 real wind or the relationship between the real wind and the courses, an estimate 

 of the speed of the yacht in terms of the speed of the wind must be assumed, and 

 then it will be possible to obtain the direction of the true wind. To illustrate the 

 point, this assumption has been made and the corresponding true wind derived, 

 which is shown by the dotted arrows on Figs. 41 to 51. 



A single curve obtained by plotting the maximum points in the curves of Figs. 

 29 to 40 is given on Plate 12 and may be called the characteristic curve of this 

 sail. 



