Principles of naval engineering - Historic Naval Ships Association

Chapter 5-FUNDAMENTALS OF SHIP PROPULSION AND STEERINGFigure 5-7.— Cavitating propeller.71,23(147B)static water pressure at submarine operationaldepths, cavitation sets in when a submarine isoperating at a much higher rpm than when nearthe surface. For obvious reasons, a submarinethat is under attack will immediately dive deepso that it can use high propeller rpm with theleast amount of noise.A certain amount of vibration is alwayspresent aboard ship. Propeller vibration ,however,may also be caused by a fouled blade orby seaweed. If a propeller strikes a submergedobject, the blades may be nicked.Another propeller phenomen is the "singing"propeller. The usual cause of this noise is thatthe trailing edges of the blades have not beenproperly prepared before installation. Theflutter caused by the flow around the edges mayinduce a resonant vibration. A "singing" propellercan be heard for a great distance.BEARINGSFrom the standpoint of mechanics, the termbearing may be applied to anything which supportsa moving element of a machine. However,this section is concerned only with those bearingswhich support or confine the motion of sliding,rotating, and oscillating parts on revolving shaftsor movable surfaces of naval machinery.In view of the fact that naval machinery isconstantly exposed to varying operating conditions,bearing material must meet rigid standards.A number of nonferrous alloys are usedas bearing metals. In general, these alloys aretin-base, lead-base, copper-base, or aluminumbasealloys. The term babbitt metal is oftenused for lead-base and tin-base alloys.Bearings must be made of materials whichwill withstand varying pressures and yet permitthe surfaces to move with minimum wear andfriction. In addition, bearings must be held in93