SAILING A BOAT 137 



100. SAILING A BOAT 



The kite and the aeroplane both act in a single medium; 

 that is, they move in air only. A sailboat moves in water, but 

 obtains its energy from the motion of the air. The theory of 

 sailing a boat, however, does not greatly differ from the theory 

 of the kite. 



The force of the wind, as it strikes against the sails, may be 

 resolved into one component perpendicular to the sail, and 

 into another component parallel to the sail. The latter has 

 no effect if the sail is flat. The perpendicular component can 

 be resolved into 'one component acting in the direction of the 

 boat's motion, which tends to send the boat ahead, and into 

 another component acting at right angles to the boat's motion. 

 This causes leeway and may be prevented to some extent by 

 a deep keel, or by a centerboard. 



The windmill acts very similarly to a sailboat when the 

 latter is sailing across the breeze, with the exception, of course, 

 that the windmill is constrained to revolve. 



It may be well at this point to state again that this theory 

 is only part of the whole theory and does not take into con- 

 sideration the fact that with sails of boats and with blades of 

 windmills the curved surface plays a prominent part in the 

 production of motion. Reaction also enters into the problem 

 of transforming the energy of the wind into useful work. 



References : 



a. 1607:531-534. The Windmill. 



b. 1804:63. The Theory of Sailing. 



c. 1809 : 78-79. Boat Sailing. 



d. 1811:368-369. The Theory of Sailing. 



