Studies Reveal Significantly Lower Operating Costs of Aluminium Vessels

Owners can reap major savings even at moderate speeds

Rising fuel prices have had a clear impact on all forms of passenger transport and the ferry industry is no exception. It is already well established that, when considering high speed marine transportation, the lighter weight of aluminium construction provides clear economic benefits over steel. Now new design studies undertaken by Australian shipbuilder Austal have highlighted that aluminium vessels also have clear advantages for vessels operating at relatively moderate speeds.

Austal carried out a study on a 65 metre catamaran Ro-Pax ferry with a target operating speed of 14 knots at 350 tonnes deadweight. Two construction alternatives were compared, namely a combination of steel hulls with an aluminium superstructure and all-aluminium construction.

Fitted with four Caterpillar 3508 diesel engines each driving a fixed pitch propeller, the steel hulled vessel could achieve the target speed with the engines operating at 85 per cent of the maximum continuous rating (MCR) of 746kW.

“By contrast the much lighter aluminium vessel requires only 68 per cent of MCR to provide the same performance. This means that the steel ferry will use over 23 per cent more fuel when operating at the same speed,” explains Austal’s Sales and Product Development Manager, Glenn Williams.

If these ferries were to operate six voyages per day, 49 weeks per year on a 35 nautical mile shuttle service this could equate to an annual fuel saving of some 630,000 litres.

In addition to substantial savings in fuel and lubricants, the lower engine power output required for the aluminium vessel produces notable engine maintenance cost benefits. Analysis by engine manufacturer Caterpillar using a total fuel usage, condition monitoring based maintenance regime shows the time between overhauls (TBO) is extended and scheduled maintenance costs, including replacement of filters and other parts, are reduced (see Figure 2 in downloadable PDF version of this media release).

“The net effect is that the costs of maintaining the engines on the steel vessel over a 10 year period would be around 12 per cent higher than keeping the same engines in service on the aluminium alternative,” says Glenn Williams. “These savings, and the increased TBO, are further notable rewards for selecting aluminium over steel.”

The benefits of the increased TBO are clearly seen in Figure 2 which shows the operator of the steel hulled ferry would incur the costs of the first major engine overhaul in the fourth year of operation – a year earlier than if an all-aluminium vessel was in service.

When the combination of fuel and planned engine maintenance is considered the through-life savings to operators choosing the all-aluminium vessel are very worthwhile (see Figure 3 in downloadable PDF version of this media release). Based on fuel, parts and labour costs applicable in Australia the total propulsion-related costs of the steel hulled ferry would be over 21 per cent higher than for the aluminium vessel over the first decade of operation.

“Notably, these savings will become more pronounced if fuel prices continue to rise in the future,” Mr Williams points out. “The more the vessel is used the greater the overall savings which means aluminium is particularly attractive in situations with high annual operating hours and where owners plan to keep their vessels in service for many years.”

It is worth noting that these outstanding results are achieved even without taking into account the possibility of fitting the aluminium ferry with smaller engines operating at a more typical percentage of MCR. Such an installation would result in even greater through-life savings as well as reducing the purchase price.

Indeed previous studies carried out by Austal during the tendering stage for the Royal Australian Navy’s Armidale Class Patrol Boat Project (in which Austal's all-aluminium monohull design was ultimately successful over designs in steel and composites offered by other shipbuilders) have demonstrated this principle. For the same performance, the aluminium vessel requires approximately 20 per cent less fuel than the same design in steel and, importantly, the difference in power required meant smaller, lighter, less expensive engines could be fitted. As a result the overall construction costs of the aluminium and steel vessels were virtually identical.

Material selection can open up new possibilities

In some circumstances the lower displacement and reduced power requirement of aluminium vessels can also open up entirely new and attractive options for owners. For example, it was found that an alternative propulsion arrangement could be developed for the 65 metre aluminium catamaran ferry design. Whereas the required power and draft limitations meant that a four engine, four propeller configuration was the only option for the steel hulled variant, the choice of aluminium construction placed the vessel into a different design space in which propulsion with two engines became feasible.

The 14 knot operating speed could be achieved with two 1118kW Caterpillar 3512 engines operating at 87 per cent MCR. Analysis carried out by Austal and Caterpillar found that the costs of fuel and oil for the four engine steel-hulled ferry would be 32 per cent higher than the two engine, aluminium vessel, and the engine maintenance and overhaul costs would be 35 per cent higher over the 10 year period. This reflects the significantly lower costs of performing preventative maintenance and overhauls on only two engines. Overall, the propulsion related costs of the four engine steel vessel were found to be nearly one-third higher than those of the lighter aluminium catamaran fitted with two engines (see Figure 4 in downloadable PDF version of this media release).

The benefits of selecting aluminium over steel are not limited to the propulsion-related savings highlighted above. Aluminium vessels also require less ongoing maintenance, particularly in the time and labour intensive ship husbandry tasks such as surface preparation and coating of internal structures (not required on aluminium vessels). This results in a further reduction in total vessel maintenance costs.

“The simple message is that if operators are interested in reducing operating costs then they should definitely seriously consider an aluminium vessel, regardless of the application and regardless of the speed at which they want to operate,” Mr Williams concluded.

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Note to Editors:
High resolution versions of the Figures referred to in this News Release are available for download by registered media at http://www.austal.com/go/news-and-images/media-access