PROPULSION OF VESSELS 



SPEED OF VESSELS 



Powering of Vessels. The exact power required to propel 

 a vessel at a given speed cannot be found very readily by 

 the principles of mechanics. Instead, empirical rules based 

 on the actual performance of vessels are usually relied on. 

 The conditions that influence the relation between power 

 and speed are many, but only a few of the more important 

 ones will be enumerated here. For instance, the area of the 

 blades of the screw propeller may not be sufficient for high 

 speed, owing to a churning of the water when the propeller 

 is revolved beyond a certain speed; and, although the power 

 expended in revolving the propeller faster may be considerable, 

 the increase of the speed of the vessel may be very slight. 

 A similar state of affairs may occur if the area of the buckets 

 of a paddle wheel is too small. It may be amply sufficient for 

 a low rate of speed, and still be entirely too small for a higher 

 rate, thus showing, probably, a high efficiency of the propelling 

 instrument at a low speed, and a very poor one at a higher 

 rate. Again, the efficiency of the engine may vary greatly 

 for different powers developed by the same engine. There- 

 fore, no hard-and-fast rule that will express the relation 

 between power and speed under all conditions can be laid 

 down. 



Admiralty Rule. The PJ!C most frequently used in the pow- 

 ering of vessels is known as the A dmiralty rule. It involves the 

 selection of a proper constant based on actual experience, when 

 this constant, a number of which are given in the accompany- 

 ing table, is properly selected, the results of the rule will be 

 found to agree very closely with the actual performance of 

 vessels powered by the rule, at least under ordinary conditions 

 and for ordinary efficiencies of the propelling apparatus. The 

 formula is 





in which H = indicated horsepower; 



TV = displacement of vessel, in tons of 2,240 lb.; 

 k =a constant taken from the table; 

 5 = speed, in knots. 



