Why do aluminium ferries have to go fast?

Austal’s Mike Wake discusses the reduced fuel consumption and pollution of aluminium ferries

Why do aluminium ferries have to go fast?
Molslinjen’s new catamaran will use little fuel when she begins operations in Denmark thanks to her aluminium hull

By Guest |

This article was first published in the Autumn/Winter 2018 issue of International Cruise & Ferry Review. All information was correct at the time of printing, but may since have changed.

For decades, naval architects have done their best to optimise the shape of hulls on ferries, but the weight of the steel has always been a major inhibitor that has increased displacement, hull resistance and fuel consumption.

Building hulls in aluminium can offer significant weight savings and in turn, lower fuel consumption and drastically reduce emissions. Although aluminium ferries cannot compete with the very large steel ro-pax vessels that serve longer routes, they offer significant benefits when it comes to short sea operations.

Take for example the 109-metre-long catamaran that Austal is building for Danish operator Molslinjen. This high-speed ferry has a deadweight capacity of 1,000 tonnes and will achieve speeds of around 40 knots with 36.4 megawatts (MW) of power, consuming around seven tonnes of fuel per hour. However, if the power is reduced to 12MW, the ferry will operate at a speed of 26 knots and only use 2.3 tonnes of fuel per hour. If the power is dropped to 8MW, the ferry will travel at 18-19 knots and expend just 1.25 tonnes an hour. 

Importantly, engine and propulsion installation weights can be decreased every time the power is dropped, which could enable a ferry company to have a 1,500-deadweight-tonne vessel that can accommodate more than 1,000 passengers and 425 cars (or 610 lane metres and 232 cars), while running on very little fuel. 

The same logic applies to trimarans. Fred. Olsen’s 127-metre-long Benchijigua Express, for example, is a trimaran that has operated year-round on long routes for more than 13 years without any service disruption. Benchijigua Express has a maximum deadweight of 1,200 tonnes and similar to the Molslinjen catamaran, she has 36.4MW of power and a top speed of 40 knots. By taking out two of the four engines and running the remaining two at 75% power, the vessel will achieve around 26 knots with 600 tonnes deadweight and consume a competitive 2.59 tonnes of fuel each hour. At full deadweight, the speed is still circa 20 knots. Taking out two of the engines entails the removal of propulsion machinery, which saves on installed weight and makes it possible to replace the remaining high-speed diesel engines with medium-speed units to lower fuel consumption even further.

Currently, the industry has not agreed on the maximum possible size for an aluminium vessel. However, Austal believes that it would be realistic to build catamarans and trimarans measuring 130-150 metres in length, with deadweights of up to 3,000 tonnes.

There is, however, a slight stumbling block with the current legislation surrounding the High Speed Craft Code because it doesn’t accommodate slower ferries. Although ferry companies can use the code to construct a high-speed ferry and then operate it at a low speed, they cannot build what is essentially the same ferry but with less power. Ideally, the code would be modified to become a ‘Lightweight Craft Code’ and we could save a lot of fuel and associated pollution. 

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