Controllable pitch propeller
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A controllable pitch propeller (CPP) or variable pitch propeller is a special type of propeller with blades that can be rotated around their long axis to change their pitch. If the pitch can be set to negative values, the reversible propeller can also create reverse thrust for braking or going backwards without the need of changing the direction of shaft revolutions.
Aircraft
Such propellers are used in propeller aircraft to adopt the propeller to different thrust levels and air pressures so that the engines don't stall or overturn. Also for cruising the engine can operate in its most economic range of rotational speeds. With the exception of going into reverse for braking after touch-down, the pitch is usually controlled automatically without the pilot's intervention. A propeller with a controller that adjusts the blades' pitch so that the rotational speed always keeps the same is called a constant speed propeller.
- For more details on this topic, see propeller#Aircraft propellers (airscrews).
Ships
Controllable pitch propellers or CPP for marine propulsion systems have been designed to give the highest propulsive efficiency for any speed, at any loading condition. When the vessel is fully loaded with cargo the propulsion required is much higher than when the vessel is empty. By controlling the pitch of the blades, the optimum efficiency can be obtained and fuel can be saved.
The CPP also improves manoeuvrability of a vessel. When manouvring the vessel the advantage of the CPP is the fast change of propulsion direction. The direction of thrust can be changed without slowing down the propeller and depending on the size of the CPP can be changed in approximately 15 to 30 seconds. A reversing gear or a reversible engine is not necessary anymore, saving money to install and service these equipments. Sometimes this better manouvrablility of the vessel can mean that the vessel won't need a tug to dock at her berth.
CPP also has the advantage of reducing wear on the propeller shaft. Because the pitch of the propeller (and therefore the speed of the ship) can be altered, the engine itself stays at a steady speed. Because the engine is not constantly slowing down or speeding up, wear on the shaft is reduced.
Despite the fact that the CPP is designed to give the highest propulsive efficiency for any speed, at full power a fixed propeller will have a slighter better efficiency then a CPP. Therefore you will not find a CPP on large vessels that make long trips at a constant service speed, for example crude oil tankers or the largest containerships.
A CPP can mostly be found on harbour or ocean-going tugs, dredgers, cruiseships, ferries and cargovessels that sail to ports with limited or no tugassistance.
At the moment the range of CPP goes up to 44000 kW (60000 hp).
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