This system involves generators producing electricity that drives an electric motor unit that turns the propeller shaft or other propulsion device such as azipods (see below). The system that drives most multi-directional tugs uses a modification of this system whereby the electric propulsion system drives the so-called “paddle-wheels” that create a water thrust.  (See below.)

Azipods

This system involves generators producing electricity that drives an electric motor unit that turns the propeller shaft or other propulsion device such as azipods (see below). The system that drives most multi-directional tugs uses a modification of this system whereby the electric propulsion system drives the so-called “paddle-wheels” that create a water thrust.  (See below.)

Note :

1.  The term azipod is actually the trade name of a product of a Finnish company that developed this form of propulsion. The term has become widely used for similar products developed by other companies. However, the term usually applied to this form of propulsion is Podded Rudder Propulsion Unit.
2. Not all such products are powered by electric motors. Those with Schottel or Voith-Schneider propulsion systems are driven by diesel engines via shafting.
3. Interesting examples of vessels that have a combination of an electrically-driven pod up forward and two diesel-driven pods down aft for steering and propulsion are the three Unicorn bunkering tankers currently operating in Durban (2) and Cape Town (1). Each of these pods can turn through 360 degrees.

Fitted below the hull of the ship, a pod consists of an electrically-driven (or diesel-driven) propeller on a mounting that can turn 360 degrees around the vertical axis. This means that the thrust of the propeller can be directed in any direction required by the officer of the watch on the bridge. Some larger ships – such as cruise ships that have to manoeuvre in confined areas without tugs – have several pods, making turning easier. Pods can be used in the bow of a ship with a conventional rudder and propeller at the stern; in other ships, pods have replaced the conventional rudder and propeller as both propulsion and steering can be combined with a set of pods.

An azipod below the Stern of the ship.  Source of photograph unknown.

Dynamic Positioning System (DPS)

By coupling the Global Positioning System (GPS) to a computer-controlled system that, in turn, controls the pods on the ship, the vessel can hold her position for a long time. The computer activates the pod, governs the speed of the propeller and its direction of thrust. This has the effect of moving and even turning the ship as required to keep her in position without anchoring, despite the effects of wind or current. The system can of course propel the ship ahead or astern as required. Modern oil rigs, drillships, survey ships or anchor-handling vessels are among those with a dynamic positioning system.

Using her dynamic positioning system, the drillship Discoverer Luanda is seen holding her position off Robben Island without anchoring.   Photograph : Nils Warner

Multi-Directional Propulsion

Multi-directional propulsion – epitomised by the Voith-Schneider system – allows a vessel to move in any direction, a most useful capability for harbour tugs that need to move in any direction at a moment’s notice to push or pull a ship in a particular direction. This system has been used in South African harbour tugs since 1974 when the first four Voith-Schneider tugs were built in Durban. Apart from two that were built in France and three in Japan, all other local harbour tugs that entered service after 1974 were built in Durban and have similar propulsion systems. The Voith-Schneider propulsion system has generators that provide electricity to drive horizontally-orientated “paddlewheels” that create a water thrust whose direction is altered according to the direction in which the tug has to move. If the tug is to go ahead, the water jet will need to be jetted out astern. If the tug is to move to wards the port side, the jet of water is jetted to starboard.

The three River-class mine counter-measures vessels of the South African Navy also have this propulsion system.

Mvezo, the first of nine harbour tugs built in Durban as part of a new tug-building programme for Transnet National Port Authority, has a multi-directional Voith-Schneider propulsion system. Photograph : TNPA