Chapter 5- FUNDAMENTALS OF SHIP PROPULSION AND STEERING 



A ship with controllable pitch propellers re- 

 quires much less distance for stopping than a 

 ship with fixed pitch propellers. The controllable 

 pitch propellers are particularly useful for land- 

 ing ships because they make it possible for the 

 ships to hover offshore and because they make 

 it easier for the ships to retract and turn away 

 from the beach. 



Controllable pitch propellers may be con- 

 trolled from the bridge or from the engineroom 

 as shown in figure 5-5. Hydraulic or mechanical 

 controls are used to apply a blade actuating force 

 to the blades. 



A hydraulic system as shown in figure 5-6, 

 is the most widely used means of providing the 

 force required to change the pitch of a controll- 

 able pitch propeller. In this type of system, a 

 valve positioning mechanism actuates an oil 

 control valve. The oil control valve permits 

 hydraulic oil, under pressure, to be introduced 

 to either side of a piston (which is connected to 

 the propeller blade) and at the same time allows 

 for the controlled discharge of hydraulic oil from 

 the other side of the piston. This action reposi- 

 tions the piston and thus changes the pitch of the 

 propeller blades. 



Some controllable pitch propellers have me- 

 chanical means for providing the blade actuating 

 force necessary to change the pitch of the blades. 

 In these designs, a worm screw and crosshead 

 nut are used instead of the hydraulic devices for 

 transmitting the actuating force to the connect- 

 ing rods. The torque required for rotating the 

 worm screw is supplied either by an electric 

 motor or by the main propulsion plant through 



pneumatic brakes. The mechanically operated 

 actuating mechanism is usually controlled by 

 simple mechanical or electrical switches. 



Propeller Problems 



One of the major problems encountered with 

 propellers is known as cavitation. Cavitation is 

 the formation of a vacuum around a propeller 

 which is revolving at a speed above a certain 

 critical value (which varies, depending upon the 

 size, number, and shape of the propeller blades). 

 The speed at which cavitation begins to occur 

 is different in different types of ships; the tur- 

 bulence increases in proportion to the propeller 

 rpm. Specifically, a propeller rotating at a high 

 speed will develop a stream velocity that creates 

 a low pressure. This low pressure is less than 

 the vaporization point of the water, and from 

 each blade tip there appears to develop a spiral 

 of bubbles (fig. 5-7). The water boils at the low 

 pressure points. As the vapor bubbles of cavita- 

 tion move into regions where the pressure is 

 higher, the bubbles collapse rapidly and produce 

 a high-pitched noise. 



The net result of cavitation is to produce: 

 (1) high level of underwater noise; (2) erosion 

 of propeller blades; (3) vibration with subsequent 

 blade failure from metallic fatigue; and (4) 

 overall loss in propeller efficiency, requiring a 

 proportionate increase in power for a given 

 speed. 



In naval warfare, the movements of surface 

 ships and submarines can be plotted by sonar 

 bearings on propeller noise. Because of the high 



PROPULSION CONTROL 

 TRANSFER SWITCH 



pas PILOTHOUSE PITCH 

 CONTROL SWITCHS 



Pas PILOTHOUSE 

 INDICATORS 



ENG RMS. PITCH CONTROL 



pas PROPELLERS 

 AND HUBS 



SERVO MOTORS 



REDUCTION GEAR 



STARBOARD PITCH 

 CONTROL UNIT 



121.24 



Figure 5-5.— General arrangement of pitch propeller control. 



91 



