AERODYNAMICAL EXPERIMENTS UPON A YACHT'S MAINSAIL. 5 



square foot. Eiffel's formula for the resistance of curved surfaces (camber i in 

 7) at 90° with the wind is o.oo^^AV where A is the projected area in square 

 feet and V is the wind velocity in miles per hour. This gives 



_ _ _ Q 



/•= 0.0033 X ^'' X 15.1^ =0.825 pound per square foot 



which is an interesting agreement lying well within the accuracy claimed for 

 either experimental work. It would seem that this formula of Eiffel's was appli- 

 cable to the determination of maximum wind pressure for yachts, provided the 

 area of spars, etc., is included in the area. 



The graphical representation of the resultant forces on the sail as shown by 

 Figs. 4 to 15, Plate 8, can be made to give a further analysis of the action, if we 

 apply to it the condition of a phantom boat sailing with different angles of boom to 

 keel line. That is, if the angle between the boom and the wind is maintained con- 

 stant and a hull is placed at varying angles with the boom, it will then be possible to 

 resolve the resulting pressure into two forces, one at right angles to the keel and the 

 other along the line of the keel. The first does no useful work and produces leeway. 

 The second may be called the drive and produces the useful work of overcoming the 

 resistance of the hull and driving the boat ahead. By putting the boat at different 

 angles for a given angle of boom to wind (superposing the diagram of Fig. 16, 

 Plate 8, and turning it to the different angles), the curves of Figs. 17 to 28, Plate 9, 

 are derived. As an analysis of the action of the sails this is probably not as directly 

 useful as one which involves sailing a given course and using different angles of 

 boom to boat corresponding to different trimming of the main sheet. 



An analysis along these lines may be obtained by taking cross curves from 

 the curves already derived (Figs. 17 to 28). For instance, if we take a cross curve 

 at 20°, we shall have the effect which the sail produces when the boat is sailing at 

 20° with the wind and the boom carried at different angles with the center line of 

 the boat. The curves derived in this way are given by Figs. 29 to 40, Plate 10, and 

 all for the constant apparent wind velocity. 



It is interesting to consider on what direction of sailing the boat is driven the 

 fastest by this sail. If we assume that the hull resistance is the same for all 

 courses and is not affected by different angles of heel, the course which is at 190° 

 with the wind (Fig. 39) is the one on which the boat will go the fastest. For this 

 course the best angle which the boom should have with the center line of the boat 

 is shown to be about 88°. It should be remembered that this applies to the single 

 sail rig only. 



Figs. 41 to 51, Plate 11, show the action which takes place for the different 

 courses, each arranged so that the boom makes the angle with the boat which is 

 most effective for speed, in other words with the main sheet properly trimmed. 

 A very interesting point to be noted here is that, for courses from 45° to 160° 

 with the apparent wind (shown by the fly at the mast head), the angle between the 

 boom and center line of ship for best sailing, with this sail, should be approximately 

 one-half the angle between the fly at the masthead and the center line of the ship. 



