Chapter 5-FUNDAMENTALS OF SHIP PROPULSION AND STEERING 



The purpose of the bearings which support 

 the shaft is to absorb radial thrust and to main- 

 tain the correct alignment of the shaft and the 

 propeller. 



The reduction gears shown in figure 5-2 are 

 used to allow the turbines to operate at high ro- 

 tational speed while the propellers operate at 

 lower speeds, thus providing for most efficient 

 operation of both turbines and propellers. 



The propellers, bearings, shafting, and re- 

 duction gears which are directly or indirectly 

 involved in the development and transmission of 

 propulsive power are considered in more detail 

 following a general discussionof power require- 

 ments for naval ships. 



POWER REQUIREMENTS 



The power output of a marine engine is ex- 

 pressed in terms of horsepower. One horse- 

 power is equal to 550 foot-pounds of work per 

 second or 33,000foot-poundsof work per minute. 

 Different types of engines are rated in different 

 kinds of horsepower. Steam reciprocating en- 

 gines are rated in terms of indicated horsepower 

 (IHP); internal combustion engines are usually 

 rated in terms of brake horsepower (BHP); and 

 steam turbines are rated in terms of shaft horse- 

 power (SHP). 



Indicated horsepower is the power measured 

 in the cylinders of the engine. 



Brake horsepower is the power measured at 

 the crankshaft coupling by means of a mechani- 

 cal, hydraulic, or electric brake. 



Shaft horsepower is the power transmitted 

 through the shaft to the propeller. Shaft horse- 

 power can be measured with a torsionmeter; it 

 can also be determined by computation. Shaft 

 horsepower may vary from time to time within 

 the same plant; for example, a plant that de- 

 velops 10,000 shaft horsepower at 100 rpm on 

 one occasion may develop 12,000 shaft horse- 

 power at the same rpm on another occasion. The 

 difference occurs because of variations in the 

 condition of the bottom, the draft of the ship, the 

 state of the sea, and other factors. Shaft horse- 

 power may be determined by the formula 



27rNT 



SHP 



33,000 



where 



SHP = shaft horsepower 

 N = rpm 



T= torque (in foot-pounds) measured 

 with torsionmeter 



The amount of power which the propelling 

 machinery must develop in order to drive a ship 

 at a desired speed may be determined by direct 

 calculation or by calculations based on the meas- 

 ured resistance of a model having a definite size 

 relationship to the ship. 



When the latter method of calculating power 

 requirements is used, ship models are towed at 

 various speeds in long tanks or basins. The most 

 elaborate facility for testing models in this way 

 is the Navy's David W. Taylor Model Basin at 

 Carderock, Maryland. The main basin is 2775 

 feet long, 51 feet wide, and 22 feet deep. A 

 powered carriage spanning this tank and riding 

 on machine rails is equipped to tow an attached 

 model directly below it. The carriage carries 

 instruments to measure and record the speed of 

 travel and the resistance of the model. From 

 the resistance, the effective horsepower (EHP) 

 (among other things) maybe calculated. Effective 

 horsepower is the horsepower required to tow 

 the ship. Therefore, 



R 



EHP = 



6080V 

 T 60 

 33,000 



where 



R = tow rope resistance, in pounds 

 V - speed, in knots 



The speed in knots is multiplied by 6080 to 

 convert it to feet per hour, and is divided by 60 

 to convert this to feet per minute. We have then 



6080 R^V 



60 



EHP = 



33,000 



6080 R^V 

 60.33,000 



608 R^V 608 R^V 

 ' 6.33,000 98 X 103 



= 3.0707 X 10-3 R^v 



: 0.0030707 R^V 



The relationship between effective horse- 

 power and shaft horsepower is called the pro- 

 pulsive efficiency or the propulsive coefficient 

 of the ship. It is equal to the product of the pro- 

 peller efficiency and the hull efficiency. 



87 



