340 LOGARITHMIC SPEED-POWER DIAGRAM. 



(b) The eflfective power delivered by the propeller is not equal to the 

 effective horse-power required to tow the ship with the propeller present and 

 operating, but is usually less, due to the wake. 



(c) The power required to drive the ship is not the same as if the pro- 

 peller were not there, but is usually greater, due to pressure effects upon the 

 hull, or stream line effects. 



The thorough analysis of these three and other effects cannot be made 

 for every case, however valuable it might be were it possible. It seems, 

 then, that rather than make three involved guesses it might be as well to 

 determine, from previous tests, a single coefficient. This might be termed 

 an apparent wake. Superpose the propeller sheet upon the power sheet for 

 the ship, and the whole upon the foundation sheet, bearing in mind that the 

 shifting lines are not the same for the two. From trial, the speed and actual 

 revolutions are known. From the model the effective horse-power is known. 

 For the known speed of the ship find the effective horse-power, and for the 

 propeller find the speed of advance which both gives this same horse-pov/er 

 and at the same time gives the number of revolutions known to have existed 

 on trial. The difference between these speeds, divided by the speed of the 

 ship, may be termed apparent wake, a quantity which may be used upon 

 somewhat similar designs at corresponding speeds. 



Following are the results from such a calculation upon the U. S. S. 

 Manning and propeller by means of this diagram, together with results 

 of more careful analysis by the same means, on the assumption of 90 per 

 cent hull efficiency, as in Professor Peabody's paper. 



Speeds (knots) 16 14 10 



Revolutions per minute 152 127.7 86 . 3 



Effective horse-power from model 1,360 775 231 



Apparent wake (per cent) 3.8 5 9 



Corresponding propeller efficiency (per cent) 68 . i 68 . i 67 . 9 



Corresponding shaft horse-power (A) 1,998 1,139 34° 



Assuming 90 per cent hull efficiency : — 



Wake (per cent) 6.2 7 9 



Propeller efficiency (per cent) 67 . 8 68 . i 68 . i 



Power delivered by propeller 1,410 795 232 



Power delivered by shaft (B) 2,080 1,169 34^ 



Per cent excess of (B) over (A) 4.1 2.7 .3 



