Transportation on Water 



359 



Sailboats may sail more directly into the wind, or the angle formed 

 by the sail with the direction of the wind may be much larger than 

 45. The principle involved is exactly the same. Suppose the force 

 of the wind is 300 pounds, and the sail is set as shown in A, Fig. 127. 

 What is the amount of the effective force? What amount is wasted 

 in the spill of the sail? 



With the results obtained, you may compute the pressure of the 

 sail force utilized in making headway, and that exerted upon the 

 hull and centerboard, which is lost in leeway. Cf. B, Fig. 127. 



A B 



FIG. 127. Boat sailing into the wind. A, relation of forces when the sail 

 and the wind are considered. B, relation of forces when the hull and the water 

 are considered. 



Suppose the wind is south westward and a sailboat has to make a point 

 almost directly northeast of its present position. In what direction 

 will it head on its first tack? What direction on the next? Show 

 graphically the course of the boat in making its destination. 



If the wind exerts a force of 240 pounds, and the angle formed by 

 the plane of the sail and the direction of the wind on each tack is 45, 

 what are the resolved forces ? Resolve then the sail force into the force 

 which drives the boat ahead and that which is wasted. Solve the 

 problem graphically as well as numerically. 



158. Principle involved in resolved forces. The facts 

 developed in the study of the problem of sailing a boat have 

 a wide application. They are promptly recognized in such 

 familiar devices as wall brackets, street lamp brackets, bracket 

 supports of trolley wire, bridges of the truss type, and roof 

 trusses, and in such feats as flying a kite or piloting an airplane. 

 Because of the wide range of application, it has been found 

 necessary to try to state them in a convenient general proposi- 



