16 AERODYNAMICAL EXPERIMENTS UPON A YACHT'S MAINSAIL. 



The trouble with the whole subject of aerodynamical experiments on sails is that it is new, 

 and, therefore, it is hard to state things clearly. That is what I think Professor Everett needs 

 to look out for in his paper — I don't imply that he has reasoned falsely. 



The President: — I am sure I express the will of the Society if I tender to Professor 

 Everett the thanks of the Society for the very interesting and very able paper that he has 

 read to us. 



I am reminded by the Secretary that I neglected to offer to Professor Everett the oppor- 

 tunity of replying to the discussion. 



Professor Everett : — I thank you, sir, but I do not think the discussion calls for any 

 comment, except that it has shown that the matter has awakened interest. I will reply later to 

 the communicated discussions. 



The President : — That, may, perhaps, explain my failure to call on Professor Everett, 

 as there was nothing said in the discussion of his paper except in the way of commendations of 

 it, and there was not very much to reply to. 



Professor Everett (Communicated) : — There has been a surprising unanimity in the 

 comments indicating the desirability of further experiments investigating the effect of fullness 

 or "bag," and that sort of an investigation could readily be undertaken by methods similar 

 to the one outlined in the paper. Such work could be made to also cover the point raised by 

 Mr. Foster, namely, the effect of varying position of gaff with respect to the boom. 



Mr. Lowell's comments as to leaving out factors which have a very important bearing in 

 actual sailing is much to the point. A sail is always used in conjtmction with a hull and the 

 hull injects factors into the action of the sail which are vital to any complete analysis of its 

 performance. A complete analysis of this sort cannot be made in a wind tunnel and no attempt 

 is made in this paper to connect the sail with any specific hull. For this divorcement of sail 

 from hull it is absolutely necessary in carrying out any experimental work on sails to deal 

 with the apparent wind, as this is the one to which the wind tunnel wind corresponds, as 

 emphasized in the paper. 



While it is true that in sailing different courses we do not have constant speed of appar- 

 ent wind this method of presentation has its simplicity to recommend it. Once we have the 

 resistance curves of the hull from the model tank, the relation between the real and apparent 

 winds becomes determinate, but as this is different for each hull the use of the apparent wind 

 as a prime variable was preferred as it applies to all cases. With this in mind, it is obvi- 

 ous that the statements in the paper which were criticized by Mr. Lowell are, after all, correct, 

 as will be seen by the following further analysis of the example which he cites. 



In the first condition Mr. Lowell derives for a real wind of 15 miles per hour and an 

 apparent wind of 7i'2 miles per hour a resultant pressure along the keel of 0.228 pound. In 

 the second condition he obtains for apparent wind of 13 miles per hour the resultant pressure 

 along the keel of 0.3669 pound. These pressures are not comparable unless we increase the 

 speed of the apparent wind of the first condition to that of the second condition, or 13 miles 

 per hour. If this is done, there results for condition one and apparent wind of 13 miles per 



hour a resultant pressure of 0.228 X (^^l = 0.69 pound, which is in excess of the pressure 



for the second condition and is the maximum, as stated in the paper. 



