Advantages of controllable pitch propellers (CPP)

• Ship is manoeuvred and reversed by changing blade pitch while the engine runs continuously at constant (or optimum) speed and direction, giving instant astern capability without stopping or reversing the engine.
• Full power is available ahead and astern; crash manoeuvring and quick response are better, valuable in ferries, tugs, trawlers and naval vessels.
• Engine can be kept at optimum efficiency, reducing specific fuel consumption and wear; fuel saving applies over a range of loads.
• The main engine can be run at constant rpm, simplifying speed control; with combinator control both pitch and rpm are set together.
• Better manoeuvrability at low speeds; can hold a slow bollard pull or zero-speed positioning by feathering pitch.
• Allows a free-running or two-stroke engine to be used with a CPP, and permits reverse power without reversing gear.
• Remote and bridge control is simplified, and manoeuvring can be operated by a single combined lever.

Disadvantages

• Greater initial cost and more complex hub with moving blade mechanism, pitch servo cylinder and control system.
• The hub is larger, increasing boss/pod diameter and creating hydrodynamic drag, and making blade sealing more difficult.
• More maintenance: blade seals, pitch mechanism, hydraulic cylinder and servo system require careful attention; seals can leak.
• Slightly lower propeller efficiency at design condition than a fixed pitch propeller with optimally matched blades.
• If hydraulic pitch control fails, pitch may default (or stay), requiring emergency locking arrangements.
• Pitch control adds interlock and safety complexity to the machinery control system.

Combinator control for CPP

A single-line diagram shows: bridge combinator lever -> electronic RPM/pitch follower/control unit -> engine governor signal (fuel pump rack / engine speed) and CPP pitch servo control -> servo amplifier -> electro-hydraulic pitch control block -> hydraulic pump -> pitch setting cylinder in propeller hub -> feedback transducer returning actual pitch.

In combinator control the one lever sets BOTH engine speed (rpm) and propeller pitch together on a pre-programmed relationship. The control unit compares demanded lever position with computed rpm/pitch set points and, through followers, positions both the engine speed governor and the pitch servo. The programme is chosen so that for a given lever position the combination of rpm and pitch gives the ship the best thrust and economy without overloading the engine, matching the engine's optimum propeller curve. Feedback from actual rpm and actual pitch is fed back so the servo balances the demand; when the lever is in the astern region pitch is taken to negative without altering engine direction. An overriding feature permits operation with fixed pitch or with rpm/pitch set independently should the combinator programme fail.