Advantages associated with maneuvering:
- Where extensive low speed maneuvering is required for a diesel powered vessel.
- When the operating conditions vary widely and maximum thrust is desired throughout operating conditions.
- Where shaft reversing capabilities are not provided by engine design.
- When improved maneuverability and a minimum ship stopping distance are desired.
- Where a constant shaft rpm is an advantage over a wide range of operating powers.
- The main engine may be started earlier prior to sailing thus giving time for the temperatures to settle down, to reduce thermal shock.
- When there is a need for a quicker fuel type changeover.
- Cost and maintenance.
Brief operation description:
The input signal from an operator acts on the diaphragm in the valve housing and directs hydraulic oil via one piston valve through the tube to one of two sides of the servo piston (ahead movement or reverse). The movement of the servo piston through a crankpin ring and sliding block rotates the blades and varies the pitch. The piston valve however must be restored to its neutral position, at the correct pitch position. When propeller blades reached a desired angle hydraulic oil stops to push the servo piston and a valve on an oil return line closes, thus fixing the pitch. Among other things a hydraulic oil system has safety valves to relief an oil pressure if the propeller blades somehow be immobilized from the outside.
In my practice I almost never had any problems with CPP. There was a curious situation when the vessel had two independent medium speed engines and accordingly two separate controllable pitch propellers. We were sailing on one engine. The bridge called to start the second one. Engineer at watch gone to prepare the engine and cranked it on air. Since we were sailing at sea the engine did not stop to rotate. In order to stop it we had to gently change the propeller pitch angle from the engine room control station. Then it stopped.