PRINCIPLES OF NAVAL ENGINEERING 



Series-parallel units are being used on some of 

 the more recent naval combatant vessels, such 

 as DEs, DDs, and CVAs. 



6. Type IV (cruising geared and vented 

 unit).— The Type IV propulsion unit consists of 

 a crusing element, HP element, and LP element- 

 each contained in a separate casing. The cruis- 

 ing turbine is connected in tandem through a 

 cruising reduction gear to the forward end of 

 the HP turbine. The cruisingturbine contributes 

 power to the propeller shaft for powers up to 

 the most economical point of operation, and for 

 higher powers it is idled in a partial vacuum 

 and supplied with cooling steam to prevent over- 

 heating. For cruising power, steam is admitted 

 to the cruising turbine and then exhausted to the 

 HP turbine inlet, and thence from the HP tur- 

 bine exhaust into the LP turbine through a cross- 

 over pipe. The arrangement of the HP and LP 

 turbine is identical to the Type II units. It is 

 possible to disconnect the cruising turbine to al- 

 low for repair. Once disconnected, the HP tur- 

 bine may be placed in service. 



All of these six types of propelling units con- 

 tain an astern element for backing or reversing. 

 An astern element is located in each end of a 

 double flow LP turbine casingor canbe in either 

 end of each single- casing turbine or single flow 

 LP turbine. 



Figure 12-23 illustrates the flow of steam 

 in a Type IV propulsion unit when the cruising 

 turbine is in use; figure 12-24 illustrates the 

 flow of steam at higher rates of operation. 



Astern elements in noncombatant ships are 

 usually velocity-compounded impulse stages 

 (Curtis stages) mounted in the exhaust end of 

 the ahead turbine. Astern elements in com- 

 batant ships are velocity-compounded (Curtis) 

 stages installed at each end of the low pressure 

 turbine. Each astern element has its own steam 

 inlet but the admission of steam to both ele- 

 ments is controlled by one astern throttle. The 

 astern elements exhaust through the low pres- 

 sure turbine exhaust chamber to the condenser. 

 Figure 12-25 illustrates the flow of steam for 

 astern operation in a Type IV propulsion unit. 



Figure 12-26 illustrates a typical highpres- 

 sure turbine. This turbine has one velocity- 

 compounded impulse stage followed by eleven 

 pressure-compounded impulse stages; hence it 

 is a pressure-velocity-compounded impulse 

 turbine. 



A low pressure double-flow turbine is shown 

 in figure 12-27. Note the astern elements. 

 This particular lowpressure turbine is a straight 



reaction turbine. In some ships, double-flow 

 pressure- compounded impulse turbines are used 

 as low pressure turbines. 



A typical cruising turbine is shown in figure 

 12-28. This is an eight-stage impulse turbine. 

 The first stage is a velocity-compounded (Cur- 

 tis) stage; the remaining seven stages are pres- 

 sure-compounded (Rateau) states. The turbine 

 is therefore a pressure-velocity-compounded 

 impulse turbine. 



Unlike the geared turbine propulsion plants, 

 the turboelectric drive installations have a single 

 turbine unit for each shaft. Figure 12-29 shows 

 the general arrangement of a turboelectric pro- 

 pulsion unit. As may be seen, the plant includes 

 a turbine, a main generator, a propulsion motor, 

 a direct-current generator for supplying excita- 

 tion current to the generator and the propulsion 

 motor, and a propulsion control board. 



Although the speed reduction is brought about 

 electrically, rather than by the use of reduction 

 gears, the speed reduction ratio between turbine 

 and propeller in the turboelectric drive is ap- 

 proximately the same as it is in the geared 

 turbine drive. 



One of the outstanding differences between 

 the geared turbine drive and the turboelectric 

 drive is that the turboelectric drive does not 

 have an astern element. In the turboelectric 

 drive, the direction of rotation of the propulsion 

 motor controls the direction of rotation of the 

 propeller. Hence there is no need to reverse 

 turbine rotation for astern operation. 



PLANT OPERATION 



Operating a ship's propulsion plant requires 

 sound administrative procedures and the coop- 

 eration of all engineering departmental person- 

 nel. The reliability and the economical operation 

 of the plant is vital to the ship's operational 

 readiness. 



A ship must be capable of performing any 

 duty for which it was designed, A ship is con- 

 sidered reliable when it meets all scheduled 

 operations and is in a position to accept un- 

 scheduled tasks. In order to do this, the ship's 

 machinery must be kept in good condition so 

 that the various units will operate as designed. 



In order to obtain economy, the engineering 

 plant, while meeting prescribed requirements, 

 must be operated so as to use a minimum 

 amount of fuel. The fuel performance ratios 

 are good overall indications of the condition 

 of the engineering plant and the efficiency of 



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