STANDBY PUMP.—The standby pump is of the same type as the main hydraulic pump, but it  is  mounted  at  the  forward  end  of  the  main reduction gear housing and is driven through a disconnect coupling. The suction strainer and suc- tion gauge for this pump are mounted separately. The primary function of the standby pump is to assist  the  main  pump  in  effecting  pitch  changes. When the control pitch (C/P) unit is in the holding pitch  position,  the  standby  pump  discharge  oil  is unloaded back to the sump through the hydraulic block. But, whenever a pitch change is ordered, the pump discharge oil is directed to the hydraulic block high pressure passage. LOWER OIL TANK.—The lower (sump) oil tank is usually located aft and below the OD box assembly. The oil capacity of the sump varies depending on the type and class of ship. Two pumps, the main and standby hydraulic pumps, take suction on the lower oil tank through a foot valve, which permits the oil to flow from the tank but does not allow it to return through the suc- tion  line. UPPER GRAVITY OIL TANK.—This tank is located above the maximum draft line. Its main purpose is to maintain hub oil pressure above that of the surrounding seawater when the C/P unit is   secured.   In   the   Kamewa   installation,   the upper gravity oil tank serves an additional pur- pose. During C/P unit operation the tank assists in maintaining the sliding ring chamber pressure. PRINCIPLES OF OPERATION The CRP propeller provides the ahead and astern propulsion thrust for a vessel by a change in the pitch of the propeller blades. Such changes can  be  obtained  even  when  the  main  propulsion machinery,  including  the  propeller  shaft,  are turning at a high rate of speed. Blade pitch con- trol  permits  a  full  range  of  ahead  and  astern thrusts.  Maximum  ahead  thrust  is  provided  with the blades in the full ahead pitch position, and maximum astern thrust is provided with the blades in the full astern pitch position. When the pro- peller blades are set at zero thrust, the propeller shaft may be turning at any speed without im- parting thrust to the vessel. When  a  change  of  propeller  pitch  position  is ordered, a pitch position command from the pro- pulsion control system is fed to the controls. This command signal activates the electrohydraulic ser- vocontrol valve which, in turn, activates the flow of control oil to and from the OD box to change the   position   of   the   valve   rod   actuator.   The hydraulic power oil flows to the OD box and is admitted to the valve rod via the annular chamber in the OD box and the ports in the valve rod. The oil flows within the bore of the valve rod to the hub servomotor, and returns from the hub via a passage formed between the valve rod and the propulsion shaft bore. ‘The oil leaves the OD box via ports in the OD box shaft and the annular chamber to return to the sump tank. Control oil is  regulated  by  a  set  of  sequencing  and  reducing valves in the hydraulic system which maintain the required pressure level. Control oil is supplied to the electrohydraulic servocontrol valve. From the servocontrol valve, the control oil flows to one side of the low pressure (LP) chamber of the OD box to drive the valve rod actuator. Control oil returns to the sump through the OD box manifold from the other side of the LP chamber. When the propeller is operating at the desired blade  pitch  position,  the  OD  box  valve  rod actuator  is  hydraulically  locked  and  the  hub  ser- vomotor is hydraulically held in a stationary posi- tion. The configuration of the regulating valve pin in the hub servomotor allows hydraulic power oil to  circulate  continuously  through  the  hub  servo. The oil pressure developed on each side of the hub servomotor  piston  is  balanced  and  established  at the level necessary to counteract blade loading which would tend to change pitch position. A hydraulic  pitch  change  signal  from  the  elec- trohydraulic servo control valve moves the valve rod actuator and the valve rod. This movement changes the size of the oil passages to each face of the hub servomotor piston, thereby creating a differential pressure in the circulating oil to each face of the piston. The regulating valve pin then supplies high pressure oil to one face of the piston and  connects  the  other  face  to  the  return  oil passage.  The  high  pressure  oil  develops  the necessary pressure on the piston face to overcome blade  loading  and  move  the  turning  mechanism and the blades to the desired pitch position. Blade pitch will continue to change until the oil port openings equalize and the oil pressure developed ENGINEMAN  1  &  C 4-14