Chapter 5-FUNDAMENTALS OF SHIP PROPULSION AND STEERING 



71,23(147B) 



Figure 5-7.— Cavitating propeller. 



static water pressure at submarine operational 

 depths, cavitation sets in when a submarine is 

 operating at a much higher rpm than when near 

 the surface. For obvious reasons, a submarine 

 that is under attack will immediately dive deep 

 so that it can use high propeller rpm with the 

 least amount of noise. 



A certain amount of vibration is always 

 present aboard ship. Propeller vibration , how- 

 ever, may also be caused by a fouled blade or 

 by seaweed. If a propeller strikes a submerged 

 object, the blades may be nicked. 



Another propeller phenomen is the "singing" 

 propeller. The usual cause of this noise is that 

 the trailing edges of the blades have not been 

 properly prepared before installation. The 

 flutter caused by the flow around the edges may 

 induce a resonant vibration. A "singing" propel- 

 ler can be heard for a great distance. 



BEARINGS 



From the standpoint of mechanics, the term 

 bearing may be applied to anything which sup- 

 ports a moving element of a machine. However, 

 this section is concerned only with those bearings 

 which support or confine the motion of sliding, 

 rotating, and oscillating parts on revolving shafts 

 or movable surfaces of naval machinery. 



In view of the fact that naval machinery is 

 constantly exposed to varying operating condi- 

 tions, bearing material must meet rigid stand- 

 ards. A number of nonferrous alloys are used 

 as bearing metals. In general, these alloys are 

 tin-base, lead-base, copper-base, or aluminum- 

 base alloys. The term babbitt metal is often 

 used for lead-base and tin-base alloys. 



Bearings must be made of materials which 

 will withstand varying pressures and yet permit 

 the surfaces to move with minimum wear and 

 friction. In addition, bearings must be held in 



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