Controllable Pitch Propeller - Ships

Ships

Controllable pitch propellers (CPP) for marine propulsion systems have been designed to give the highest propulsive efficiency over a broad range of speeds and load conditions. When the vessel is fully loaded with cargo the propulsion power required at a given ship speed is much higher than when the vessel is empty. By adjusting the blade pitch, the optimum efficiency can be obtained and fuel can be saved. Also, the controllable pitch propeller has a "vane"-stance, which is useful with combined sailing / motor vessels as this stance gives the least water resistance when not using the propeller (e.g. when the sails are used instead). This is the marine equivalent of "feathering" an aeronautical propeller.

A fixed pitch propeller (FPP) is more efficient than a controllable pitch propeller under a specific rotational speed and load condition. At that particular rotational speed and load, an FPP can transmit power more efficiently than a CPP. At any other rotational speed, or any other vessel loading, the FPP will not be more efficient, either being over pitched or under pitched. A correctly sized controllable pitch propeller can be efficient for a wide range of rotational speeds, since pitch can be adjusted to absorb all the power that the engine is capable of producing at nearly any rotational speed.

The CPP also improves maneuverability of a vessel. When maneuvering 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 40 seconds. The increased maneuverability can eliminate the need for docking tugs while berthing.

A reversing gear or a reversible engine is not necessary for ships utilizing CPP, saving money to install and service these components. Depending on the main engine rotational speed and the size of the CPP, a reduction gear may still be required. A CPP does require a hydraulic system to control the position of the blades. A CPP does not produce more or less wear or stress on the propeller shaft or propulsion engine than an FPP. Therefore this will not be an argument to choose between an FPP or a CPP.

Large vessels that make long trips at a constant service speed, for example crude oil tankers or the largest container ships, do not utilize a CPP as the capital and running costs far outweigh the small improvement in manoeuverability and reduced requirement for starting air. Uljanik shipyard in Yugoslavia produced a series of four VLCCs with variable pitch propellers in the mid-seventies. The first three vessels were ore/oil carriers, the fourth a pure tanker. They were each powered by two 20,000 bhp B & W diesel engines directly driving Kam-e-Wa variable pitch propellers. Due to the high construction cost none of these vessels ever returned a profit over their lifetimes.

A CPP is usually found on harbour or ocean-going tugs, dredgers, cruise ships, ferries, cargo vessels and larger fishing vessels that sail to ports with limited or no tug assistance. Prior to the development of CPPs, some vessels would alternate between "speed wheel" and "power wheel" propellers depending on the task.

Current CPP designs can tolerate a maximum output of 44000 kW (60,000 hp).

Bruntons, an engineering firm in Essex, has patented the "AutoProp", a marine propeller where the blades swivel freely and automatically set to the correct angle. The Autoprop is suitable for small to medium yachts and boats, and is particularly beneficial for motor-sailers.