Diesels deliver economies

Horst Kohler from MAN B&W`s engine technology promotion department argues that diesel engines remain the best option for cruise ship propulsion.

When Royal Caribbean Cruise Lines signed a deal to use turbines as prime movers in its latest generation of cruise ships, many saw this as a breakthrough. Each ship will be fitted with two 25MWel gas turbines and a 9MWel steam turbine. However, a closer examination of the situation shows that some of the advantages claimed for the gas turbine are not as clear-cut.

There is no doubt that turbines are both smaller and lighter than diesel engines of an equivalent output. In these cruise vessels this is said to give room for an additional 50 twin passenger cabins – but it is not possible to position these close to the smaller engine room. Even when the spaces onboard are rearranged, the extra cabins will not be high-class accommodation. Additional space may also be restricted by the requirement to carry spares and equipment to exchange a turbine`s hot section (3m by 1.5m diameter) at sea. Also the air intake and exhaust gas flows are higher, with the turbine necessitating ducting around 15 per cent larger than with equivalent diesel engines.

As modern cruise ships have a lot of superstructure above the waterline in comparison with their draught, the weight of diesel engines positioned low in the vessel can actually be an advantage. While there is no doubt that the naval architect can accommodate this loss of `ballast` in the design, it excludes the use of a common hull form for either diesel engines or gas turbines.