Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGSTABILIZINGNOZZLESPERIPHERALNOZZLEPROPULSIONDUCTSCUSHION FAN DUCTSPILOTHOUSENOZZLECONTROL VANESFLOTATIONCOMPARTMENTSFigure 25-4.— Air cushion system for SKMR-1).3.265using the waterjet as a prime mover in smallcraft such as the one shown in figure 25-5.The general principle of waterjet propulsionis illustrated infigure 25-6. The "prime mover"consists merely of a water pump. The rotatingimpellers deliver thrust by accelerating a largevolume of water at a high velocity through a nozzle.The velocity of flow through the nozzle is directlyproportional to the flow rate and inverselyproportional to the volume of flow times the velocityof flow.One of the outstanding advantages of thewaterjet mode of propulsion is that the waterjetproduces much less underwater noise than aconventional propeller-driven craft of the samesize and general configuration. Preliminarystudies of the waterjet indicate that a comparableconventional propeller-driven craft could be detectedapproximately ten times farther away thanthe waterjet.Quite a different system of jet propulsion hasbeen suggested as a possibility for the propulsionof underwater vehicles. This propulsion system(fig. 25-7) is generally known as an underwaterramjet system . The fuel in the combustionchamber reacts with the water; hence the waterthat enters the combustion chamber may be regardedas a co-propellant. The products of thereaction are expanded through a nozzle at theafter end of the craft, thus propelling the craftforward. Although a propulsion system of thissort would have many advantages for short- rangeoperation, there are some disadvantages; onepresent disadvantage is that the system is extremelynoisy. Also, it is very inefficient exceptat very high speeds (above 80 knots).DIRECT ENERGY CONVERSIONThe production of power for ship propulsionbegins with the conversion of some stored formof energy. In all present propulsion plants, thestored energy that is the original source of powermust undergo a series of transformations beforeit can be utilized to propel the ship. We have twomajor sources of stored energy: fossil fuel andnuclear fuel. In each case the stored energy mustbe converted into thermal energy which is thenconverted into mechanical energy.'During the past few years, a considerableamount of interest has developed concerning634