Japanese CRP pod ferry order

Mitsubishi Heavy Industries (MHI) is to build two high-speed ropax ferries, featuring CRP Azipod propulsion, for Osaka-based Shin-Nihonkai Ferries. It is the first application of the innovative contra-rotating pod propulsion solution devised by ABB.

Both ships will be built at MHI?s Nagasaki yard and are slated for delivery in June 2004.

At around 17,000g, the ferries will be the largest ropax ferries in Japan. They will also be the fastest with a maximum design speed of 31.5 knots. The ferry operator plans to run both vessels on the Maizuru (north of Kyoto and Osaka) to Otaru (on Hokkaido, near Sapporo) route, in place of existing 21.5 knot ships. The new 224.5m long by 26m wide ferries will be able to carry 158 trailers, 66 passenger cars and 820 passengers.

Propulsion system

The single-skeg ferries will be equipped with an azimuthing pod unit, supplied by Finland?s ABB, installed in a contra-rotating mode immediately behind a standard mechanically driven main propeller. The concept was first revealed in The Motor Ship in May 2001 and was developed by ABB as a solution to powering ultra-large post-Panamax boxships and fast monohull ropax vessels without resorting to using larger and more powerful diesel engines. Located on the same axis, but without any physical connection, the pulling propeller of the pod will contra-rotate in relation to the shaft driven main propeller.

The theoretical efficiency gain of the contra-rotating propeller principle has been touted for many years, but it has rarely been used in commercial vessels. In high-power applications with co-axial shafts rotating in opposite direction, the complexity of gears and bearings has been an obstacle to further commercial success. Azimuthing propulsors, with their shaftless electric drive, offer a solution to this.

Compared with a conventional twin-shaft configuration, ABB reckons total installed power can be significantly reduced with the CRP Azipod solution. It says the concept gives an improvement of some 10-15 per cent in propulsion efficiency, mainly because the absence of shaft-brackets reduces hull resistance and improves propeller efficiency with the gain of the contra-rotating principle.

The aft propeller uses the rotative energy left in the slipstream of the forward propeller when it turns in the opposite direction. The single-skeg hull also leads to a favourable wake field for the propellers. In addition to the fuel savings and reduced emissions, ABB says the steerable Azipods will provide a strong thrust in all directions allowing the vessels to achieve increased manoeuvrability when berthing.

The power plant will be of the combined diesel-electric and diesel-mechanical (CODED) system. This consists of two Wärtsilä 12V46C medium speed diesel engines with a combined output of 25.2MW used for mechanical drive, through a reduction gear, of the main controllable pitch propeller. The electrical power for the 17.6MW azimuthing pod drive and the hotel load is supplied in each ferry by two similar Wärtsilä 12V46C engines as main generating sets and one smaller harbour generating set. The 27MW main electrical power plant will be linked to three main alternators and a 6.6 kV main switchboard.

Tests and fine tuning

Talking to The Motor Ship, a spokesman for ABB reveals that most of the tests on the CRP pod have been successfully completed although the system still has to be fine-tuned. In particular, there is the perceived problem of some cavitation with the CRP pod during high-speed manoeuvres. A solution to this may be to install a small rudder. ABB is also investigating covering the exposed parts of the pod strut, at the height where the propeller vortex hits it, with stainless steel. The company is confident any unresolved design issues will be resolved soon and should not have any adverse impact on the completion dates of the two Japanese ferries.