|Controllable Pitch Propellers|
Controllable pitch propellers are used in some naval ships and give a ship excellent maneuverability, allowing the propellers to develop maximum thrust at any given shaft rpm. Ships with controllable pitch propellers require no reversing gear since the direction of the propeller thrust can be changed without changing the direction of shaft rotation. Controllable pitch systems are widely used on diesel-driven ships, while gas turbine powered ships use controllable pitch as the only means available for providing reverse thrust. Controllable pitch propeller systems may be controlled from the bridge or from the engine room through piping inside a hollow propulsion shaft to the propeller hub. Hydraulic or mechanical controls are used to apply the actuating force required to change the position, or angle of the pitch, of the propeller blades.
A hydraulic system is the most widely used means of providing the force required to change the pitch of a controllable pitch propeller. In this type of system, a valve-positioning mechanism actuates an oil control valve. The oil control valve permits hydraulic oil, under pressure, to be introduced to either side of a piston (which is connected to the propeller blade) and at the same time allows for the controlled discharge of hydraulic oil from the other side of the piston. This action repositions the piston and thus changes the pitch of the propeller blades.
Some controllable pitch propellers have mechanical means for providing the blade actuating force necessary to change the pitch of the blades. In these designs, a worm screw and crosshead nut are used instead of the hydraulic devices for transmitting the actuating force. The torque required for rotating the worm screw is supplied either by an electric motor or by the main propulsion plant through pneumatic brakes.
In most installations, propeller pitch and engine power are controlled through a single lever. Movement of the lever causes both engine speed and pitch to change to suit the powering condition ordered. In emergencies, and in ships without single lever control, the propeller pitch may be changed independently of the engine power setting. Under this condition, overspeeding of the engine can result if the pitch is set too low, or overtorquing of the engine can result if the pitch is set too high.
Instructions regarding the limitations of engine speeds at different propeller pitches have been issued to all ships equipped with controllable pitch propellers. In addition, the special operating and maintenance instructions for these propellers should be consulted before any overhaul or repairs are undertaken.
A fixed propeller, designed for maximum speed, cannot give maximum power at low speed, while a fixed propeller designed for power, cannot achieve maximum speed.
With a controllable pitch propeller it is always possible to obtain full utilization of the engine, irrespective of the purpose of the vessel.
Maximum horsepower can be taken from the engine â€“ without overloading â€“ by changing the pitch.
Even if the engine drives a winch or a shaft generator, the number of revolutions can be kept constant â€“ and the speed of the vessel can then be regulated by means of the propeller pitch.
At lower than the maximum speed, the engineâ€™s fuel consumption can be reduced considerably by increasing the pitch and lowering the number of revolutions while maintaining the required speed. In this way the loading of the engine, and the total efficiency of the unit is increased.
Using fully reversible propellers, only a reduction gearbox is needed.
When using sail and engine power at the same time, the correct pitch can always be obtained so that wind and engine together are utilized in the best possible way.
Prompt and inexpensive replacement of individual blades in case of damage to the propeller.
Engine idle speed on a fixed pitch propeller vessel often propells the vessel too fast for docking maneuvers, whereas a CP propeller offers supperior low speed vessel control.
Hundested Propeller A/S is one of the best know manufacturers of Controllable Pitch Propellers. Their product information follows:
Hundested Propeller A/S specializes in the manufacture of controllable pitch propellers. They are located in Hundested, a city on the island of Zealand, not far from the Danish capital of Copenhagen.
The propeller head
The Hundested propeller head is made of manganese-bronze and is attached to either a flanged shaft or tapered end shaft, depending upon the system size. The propeller blades are moved by a tie rod cross head with stainless pins.
On each pin a movable slipper block is mounted in a milled slot in the blade foot.
The propeller head is lubricated with water emulsifying grease.
The mechanism of the propeller head consists of simple mechanical parts.
- The propeller blades: The blades are made of nickel-aluminium-bronze in 2, 3, or 4 bladed design. They are designed for open running and propeller nozzles, as well as for feathering position.
- The propeller shaft: The standard propeller shaft is supplied in OH-steel. Stainless steel can also be supplied on request. The shaft is hollow bored for a solid tie rod.
- The stern tube: The standard stern tube is of bronze, for grease lubrication. Oil lubricated stern tubes are made of steel. Water lubricated stern tubes are supplied on request.
- Flanges: The flanges are manufactured to suit the particular gear or coupling in use.
This propeller system has a simple and rugged hub/blade design with no bearings or seals to maintain.
These propeller units can be approved by all major classification societies and can be adjusted to individual requirements.
Hundested has extensive experience in the design and manufacture of variable pitch propellers, beginning manufacture in 1929 and supplying more than 8,000 propeller units. They supply complete propeller units including propeller nozzles, line shaft bearings and flexible couplings and carry large stocks of spare parts. Their propeller units have been supplied to the leading engine makers â€“ and to almost all types of vessels, tugboats, ferries, training ships, yachts, etc.