AERODYNAMICAL EXPERIMENTS UPON A YACHT'S MAINSAIL. 



By Professor H. A. Everett, Member. 



[Read at the twenty-third general meeting of the Society of Naval Architects and Marine Engineers, held in 



New York, November 18 and 19, 1915.] 



The following experiments were carried out in the Aerodynamical Labora- 

 tory of the Massachusetts Institute of Technology in January, 19 15, and were 

 undertaken primarily to determine the possibility of obtaining by this means re- 

 liable information concerning the action of sails in the wind. The work resulted in 

 the determination of the location of the true center of pressure, the normal pressure 

 per unit of area for a given wind velocity, and the proper angle of boom to center 

 line for fastest sailing on a given course (Plate 12). 



The experiments were carried out with a single sail (a mainsail), as the adop- 

 tion of two (as jib and mainsail) would have injected an indeterminate feature, the 

 interrelationship between them due to variations in sheet trimming. The sail 

 used (Plates i and 2) was an exact reproduction of a successful sail as used 

 on one of last season's, class P, racing yachts, and was made accurately to the 

 scale of 3/s oi an inch to i foot out of light, firm China silk. It was made out of 

 a single piece of silk, no attempt being made to reproduce seams, but the roach at 

 the leach of the sail was the maximum which the rule permits and was held out 

 by miniature battens. The sail was carefully cut and made by Wilson & Silsbee 

 in exact conformity with their cutting plan for the full-sized sail. 



In carrying out the tests it was considered undesirable to have any sort of 

 body under the sail to which the main sheet could be made fast, as this would 

 inject a disturbing factor into the action of the wind on the sail by disturbing the 

 air flow to the sail, therefore the boom (of ^^^-inch steel tubing) was rigidly at- 

 tached to the mast (of ^^-inch steel tubing) at an angle which represented the 

 normal setting of the sail. The gaff was free to swing off and take up whatever 

 position the wind drove it into (see Plate 2). The sail was set by throat and peak 

 halyards as nearly as possible in exact conformity with the setting of the actual 

 sail. The sail was provided with battens correct to scale, the attachment to the 

 boom and gaff was by the customary lace lines, and the luff was held to the mast 

 by small brass rings. The sail was vertical at all times, no attempt being made to 

 simulate heel. 



The experiments were made possible by the facilities of the aerodynamical 

 laboratory or "wind tunnel" recently established at the Institute in connection with 

 the Department of Naval Architecture under Prof. C. H. Peabody. Their suc- 

 cessful conclusion was largely dependent on the hearty cooperation of Assistant 

 Naval Constructor J. C. Hunsaker, who is in direct charge of the aeronautical in- 

 struction and this laboratory. 



