Reinventing the propeller

An innovative marine propulsion concept – as old as the invention of the propeller itself – will finally make its commercial debut next year in Japan. Jon Excell discusses the CRP Azipod.

According to E. L. Wisty, Peter Cook’s hilarious park-bench philosopher, the wheel was invented by a caveman called Drodbar. Unfortunately he called his invention the bandanbldderstiddle while his commercially minded friend, Gorbly, copied his idea, called it a wheel and took all the glory.

The screw propeller, another important milestone in mankind’s propulsive history has a slightly less murky background. It was invented in the 1820s by Swedish engineer John Ericsson, and is still the dominant form of marine propulsion today.

In 1836 Ericsson proposed and developed an interesting variation on his brainchild: a system with two propellers behind each other rotating in different directions. The advantages of a contra-rotating system (not least the ability for a downstream propeller to absorb energy from the main propeller) have never been disputed, but for a variety of reasons the concept has been left undeveloped.

Almost 200 years later this often discussed but rarely used propulsion technology will be making its commercial debut on a pair of Japanese ferries. The ferries, which are being built by the Shin-Nihonkai Ferry Company, will be the fastest and largest ever built in Japan. The expected efficiency, speed and reduced vibration of the vessels’ propulsion systems will largely be thanks to a modern reworking of Ericsson’s original ideas.

Designed and developed by the marine arm of engineering Leviathan ABB, the Contra Rotating Propeller (CRP) Azipod is a greatly enhanced version of ABB’s earlier Azipod concept.

When this first Azipod was introduced in the 1990s, it too represented a something of a revolution in marine propulsion. Conventional ship propellers, propeller shafts, reduction gear, rudders and steering gear were replaced by a cable connection from the main diesel-electric generator to an electric motor in a gondola installed directly behind the propeller.

This system had a number of benefits including reduced vibration/noise levels and improved fuel economies, but perhaps the chief advantage was the massive improvement in manoeuvrability allowed by a unit that was capable of turning through 360 degrees. Azipod has been used on many different types of ship, including ice-going vessels, which must be capable of changing direction quickly. Now the concept has been updated, and the list of advantages increased thanks to the incorporation of the contra-rotating principle. Said to improve the hydrodynamic propulsion efficiency of ships by between 10 and 15 per cent, the units will be installed on the Japanese ferries in place of a conventional rudder.

The CRP Azipod combines two separate propulsion systems: a conventionally driven main propeller and a downstream propeller aligned on the same axis that rotates in the opposite direction but has no physical connection to the main propeller. The main propeller is mechanically linked to the main engine while the downstream propeller is driven by an electric motor within a submerged pod that can be steered through 360 degrees.

Previous applications of the concept have been held back by the requirement for extremely complex shafting and gearing. However, once these complex mechanical linkages had been made redundant by the original Azipod design, the main stumbling block to the development of a viable CRP system was removed.

With the shipping industry constantly on the look-out for extra speed but lower noise and vibrations, ABB has said that the system will be ideal for a range of vessels. However, it is expected to be particularly well suited to vessels with propulsion systems that exert a very high load on the propeller or where restrictions are imposed on the size of propeller that can be fitted. This includes ferries, container vessels and liquefied natural gas (LNG) carriers.

As well as exploiting the benefits of contra-rotating propellers, the system enjoys the advantages common to all pod-based systems, specifically excellent manoeuvring characteristics and the ability to turn through 360 degrees for increased thrust in all directions.

Also, because two propellers share the load, propellers with smaller diameters can be produced. This reduces the clearance between the propeller tip and the hull and reduces vibration levels on the hull. The Azipod hull itself acts as a rudder that directs the flow downstream and adds some extra thrust.

Anticipating an enthusiastic reception to the CRP Azipod system, ABB is optimistic about its application elsewhere in the shipping industry. ‘This concept sees a great future in several types of ship, such as large container vessels, roll-on roll-off ships, as well as in LNG carriers,’ said a company spokesman.