242 PROPULSION OF VESSELS . 



total forward force and total resistance become equal at a 

 lower speed of the vessel, which then continues under way 

 at a uniform, but lower speed. Now, the horsepower of an 

 erigine varies (theoretically) directly as the number of revolu- 

 tions, the mean effective pressure remaining constant. It has 

 been shown that the speed of the vessel has been lowered; from 

 this it follows that the revolutions and consequently the horse- 

 power must be less when the resistance has been increased. 

 By adding to the resistance of the vessel, a condition is finally 

 reached similar to that of a vessel moored to a dock; the 

 forward force and resistance are equal and the vessel, as no 

 accelerating force is available, remains stationary. In this 

 condition, the number of revolutions, and, hence, the horse- 

 power, has dropped to the lowest limit. 



Relation of Coal Consumption to Speed. The fuel con- 

 sumption may be said to vary directly as the horsepower 

 developed (this is not exactly true, but only approximately). 

 The horsepower varies about directly as the cube of the speed, 

 whence it follows that the fuel consumption will also vary as 

 the cube of the speed, approximately. Hence, to find the 

 probable coal consumption for a speed different from a known 

 speed, use the formula 



- 



in which c = coal consumption, in tons, at the new speed; 

 5 = new speed; 

 5 = known speed; 



C = coal consumption, in tons, at the known speed. 

 To find approximately the speed of steaming for a new coal 

 consumption, use may be made of the formula 



c 



At sea, owing to an accident, it is often desired to know what 

 speed to maintain in order to reach a given port with the 

 amount of coal on hand. This problem is readily solved by 

 trial and by. application of formula 1. In practice, a good 

 margin of coal should be shown by the calculations as left 

 over, for the reason that the actual coal consumption at the 



