propeller, the rotating screw of a power-driven vessel by which it is forced through the water. The first mechanical propulsion of a ship had been a
paddle steamer's stern wheel or paddle wheels, but these had disadvantages which led to efforts to design a means of propulsion which would be permanently submerged and thus capable of being driven without straining the engine. The principle of the Archimedes screw was well enough known, and it was an adaptation of this principle which eventually produced the answer. Four engineers are usually credited with its invention, all at about the same period, between 1833 and 1836. They were the Englishman Robert Wilson, the Frenchman Frédéric Sauvage, the Swede John Ericsson (1803–89), and another Englishman,
Francis Pettit Smith, whose invention was finally awarded a patent. It was first tried out in 1838 in the 237-ton steamship
Archimedes, but it was Ericsson who was responsible for the first propeller-driven warship when in 1843 he fitted a propeller to a ship for the US Navy which became the USS
Princeton, a ten-gun
sloop.
Propeller design proceeded reasonably rapidly and with generally increasing horsepower. With the development of the original
reciprocating engine into the
compound and
triple expansion types, propellers increased in numbers from the single one driven by a shaft in the centreline of the ship, first to two, with one on each
quarter, and then, for the largest ships, to four, with two on each quarter. These arrangements were efficient but did not always increase steering control. The introduction of the turbine (see
steam propulsion) concentrated the attention of designers towards greater efficiency in the shape and
pitch of the propeller blades. Later, in order to absorb the ever-increasing power needed to propel single-screw
tankers, five-bladed and six-bladed propellers were introduced, their size closely approaching the 7.3-metre (24-ft) diameter of the
Great Eastern's four-bladed one.
For sailing
yachts with
auxiliary power, propellers are usually made to fold, to
feather, or to rotate freely when the vessel is under sail, in order to minimize drag as it goes through the water. Each type has its advantages, and it is usually a matter of personal choice as to which type is fitted.
Modern Propeller Systems by Denis Griffiths
Controllable Pitch Propeller.
The conventional propeller has a fixed blade pitch which means that the blades are at a set angle to the axis of the shaft rotation. In order to change the speed of the ship the rotational speed of the propeller shaft must be changed, usually by changing the engine speed. In order for the ship to go
astern, the rotational direction of the propeller shaft must be changed. An alternative to changing the speed of the engine and to reversing the engine is to fit a controllable pitch propeller. In this case the individual propeller blades are mounted on the boss of the propeller in such a way that the pitch of each blade can be changed by means of an operating mechanism located within the propeller boss. The propeller shaft is rotating at all times when at sea or when the ship is manoeuvring. If the ship is to be stationary the blades are given zero pitch which means that they are at right angles to the axis of the propeller, and they exert no thrust. To move the ship forward, the blades are given a forward pitch and the larger the pitch the faster the ship will move. To move it astern, the blades are given a reverse pitch.
Thruster.
This is a propeller unit located in a tunnel at the bow or stern of the ship. Called bow thrusters or stern thrusters, they are driven by an electric motor mounted above the thruster tunnel, the drive being through a bevel gearbox. They exert a sideways thrust on the ship to increase manoeuvrability, so that they can often do the work of
tugs when a ship is berthing or undocking. Their propellers, of the controllable pitch type, allow the magnitude and direction of the thrust to be varied, and using them in conjunction with the ship's propulsion propeller further increases manoeuvrability. Because they work by forcing water from one side of the ship to the other it is essential that they are placed well below the waterline to avoid air being drawn into the tunnels. They only function when the ship is at rest or moving very slowly. For illus. of bow thruster see
trawler.
Propulsion Pod.
This has a controllable pitch propeller, which allows for variation in propulsive thrust, and is driven by an electric motor in the pod. The pod is positioned for maximum efficiency, usually at the stern for larger ships but may be at other locations. Because there is no direct drive from an engine within the ship the pod is able to be located anywhere on the hull, and it may be fixed or it may rotate. If a pod is steerable (can be rotated) it will provide a thrust in any direction and can therefore also act as a side thruster and as the
rudder, and it is rotated by a hydraulic motor unit located where the pod connects with the ship's hull. The hull at this point must be strengthened because of the thrust from the pod's propeller; this area effectively acts as what is technically called the thrust bearing. The power cables driving a fixed pod's electric propulsion motor go directly to the pod, but a rotating pod's electrical connection is with slip rings, similar to the system used in electric motors.
Apart from the advantage of acting as thrusters and rudders, pods provide for better propulsion efficiency. This is because the hull form can be designed to suit the needs of the water flow into the propeller, there being no conventional propeller shafts to consider in designing the hull form. Propulsion pods are fitted to many ships including, supply vessels for
offshore oil and gas rigs,
ferries, and
cruise ships. The
Queen Mary 2 has four propulsion pods, two fixed and two steerable, which act as thrusters and rudders. For illus.
see
tug.
