NOTES ON THE PERFORMANCE OF THE S. S. TYLER. 93 



way movement; and the ship being generally dispatched on schedule, regardless of the amount 

 of cargo offering. 



Had it been possible to obtain a full load each way, each trip, the performance as 

 shown would have been much improved. 



Professor C. H. Peabody, Member of Council: — I congratulate Mr. Rigg in presenting 

 us with the trials of a merchant ship under favorable conditions, and I wish also to express my 

 appreciation of the statement he makes that there is no serious difficulty about preparing 

 merchant ships for such tests. I wish he would kindly inform us what methods were used 

 in this case for producing such a load, because it has come to my knowledge that in many 

 cases it is exceedingly difficult to get a merchant ship properly loaded for trial, that in 

 some cases there is no way of loading except by throwing cargo or ballast into the hold, and 

 I believe that both of these performances, while possible, are frequently inconvenient. 



Captain C. W. Dyson, U. S. N. (Communicated) : — In reading over the article pre- 

 pared by Mr. Rigg, I note that he makes no reference either to the beneficial effect of the very 

 full midship section on the after body lines nor upon the happy choice of revolutions for the 

 propeller. 



While the complete lines of the hull govern the effective horse-power required to tow 

 that particular hull at any given speed, the indicated horse-power necessary to be developed 

 by the propelling engines in order that this effective horse-power may be delivered depends 

 directly upon the form of the after body lines, upon the designed revolutions and the per- 

 missible diameter of the propeller. 



In designing a propeller to deliver a given effective horse-power, and to realize the 

 maximum tank efficiency possible, a certain diameter and a tip speed corresponding to this 

 diameter are required. In addition, the after body of the hull must be of such form as will 

 permit the water from forward to flow to the screw and to enter it in a solid unbroken 

 stream. This I will designate as Condition 1 ; it is met with in bodies having a long, fine 

 after end and a propeller located well clear of the hull. 



For Condition 2, the after body lines are such as to break up the column of water 

 flowing to the propeller, so that it is working in water more or less broken up by eddies 

 and which may also contain considerable quantities of free air. This condition produces the 

 loss that is ordinarily called "thrust deduction" or "augment of resistance," and reduces 

 the propulsive efficiency below the tank efficiency. 



For Condition 3 the diameter may be limited to a dimension smaller than that indi- 

 cated by Condition 1, while the revolutions remain the same. In this case the tip speed will 

 be lower than in Condition 1, the column of water acted upon by the propeller will be smaller 

 in cross-sectional area, the velocity of flow of the water in the column will be increased, the 

 indicated horse-power per revolution will be increased from that of Condition 1, but the ap- 

 parent slip per revolution will be increased over that existing for Condition 1, the pitch and 

 surface of the propeller also being increased over that of Condition 1 , a net loss in efficiency 

 of propulsion resulting. 



By scanning the above conditions it is seen that a propeller may operate under any one 

 of the following conditions : — 



1. Under Condition 1 — maximum efficiency. 



2. Under Condition 2 — maximum efficiency decreased by thrust deduction. 



