CHAPTERS 



FUNDAMENTALS OF SHIP PROPULSION AND STEERING 



The ability to move through the water and the 

 ability to control the direction of movement are 

 among the most fundamental of all ship require- 

 ments. Ship propulsion is achieved through the 

 conversion, transmission, and utilization of 

 energy in a sequence of events that includes the 

 development of power in a prime mover, the 

 transmission of power to the propellers, the de- 

 velopment of thrust on the working surfaces of 

 the propeller blades, and the transmission of 

 thrust to the ship's structure in such a way as 

 to move the ship through the water. Control of 

 the direction of movement is achieved partially 

 by steering devices which receive their power 

 from steering engines and partially by the ar- 

 rangement, speed, and direction of rotation of 

 the ship's propellers. 



This chapter is concerned with basic prin- 

 ciples of ship propulsion and steering and with 

 the propellers, bearings, shafting, reduction 

 gears, rudders, and other devices required to 

 move the ship and to control its direction of 

 movement. The prime movers which are the 

 source of propulsive power are discussed in de- 

 tail in other chapters of this text, and are there- 

 fore mentioned only briefly in this chapter. 



RESISTANCE 



The movement of a ship through the water 

 requires the expenditure of sufficient energy to 

 overcome the resistance of the water and, to a 

 lesser extent, the resistance of the air. The 

 components of resistance may be considered as 

 (1) skin or frictional resistance, (2) wave-making 

 resistance, (3) eddy resistance, and (4) air re- 

 sistance. 



Skin or frictional resistance occurs because 

 liquid particles in contact with the ship are car- 

 ried along with the ship, while liquid particles 

 a short distance away are moving at much lower 

 velocities. Frictional resistance is therefore 



the result of fluid shear between adjacent layers 

 of water. Under most conditions, frictional re- 

 sistance constitutes a large part of the total re- 

 sistance. 



Wave-making resistance results from the 

 generation and propagation of wave trains by the 

 ship in motion. Figure 5-1 illustrates bow, stern, 

 and transverse waves generated by a ship in mo- 

 tion. When the crests of the waves make an ob- 

 lique angle with the line of the ship's direction, 

 the waves are known as diverging waves. These 

 waves, once generated, travel clear of the ship 

 and give no further trouble. The transverse 

 waves, which have a crest line at a 90° angle to 

 the ship's direction, do not have visible, breaking 

 crests. The transverse waves are actually the 

 invisible part of the continuous wave train which 

 includes the visible divergent waves at the bow 

 and stern. The wave-making resistance of the 

 ship is a resistance which must be allowed for 

 in the design of ships, since the generation and 

 propagation of wave trains requires the expend- 

 iture of a definite amount of energy. 



Eddy resistance occurs when the flow lines do 

 not close in behind a moving hull, thus creating 

 a low pressure area in the water behind the stern 

 of the ship. Because of this low pressure area, 

 energy is dissipated as the water eddies. Most 

 ships are designed to minimize the separation of 

 the flow lines from the ship, thus minimizing eddy 

 resistance. Eddy resistance is relatively minor 

 in naval ships. 



Air resistance , although small, also requires 

 the expenditure of some energy. Air resistance 

 may be considered as frictional resistance and 

 eddy resistance, with most of it being eddy re- 

 sistance. 



THE DEVELOPMENT AND TRANSMISSION 

 OF PROPULSIVE POWER 



Figure 5-2 illustrates the general principles 

 of ship propulsion and shows the functional 



85 



