Innovative fuel-saving concepts

Navies have looked at the combined-cycle concept on many occasions, notably the US Racer programme

Brian Cowell asks why combined-cycle gas turbines are not used in merchant shipping and offers the prospect of an efficiency of 50-55% (Letters, PE December).

The propulsion machinery of large merchant ships already achieves that level of efficiency. The specific fuel consumption of the cross-head two-stroke diesel which dominates the market is typically 165-170g/kWh. Further economy achieved by exhaust gas waste heat recovery is standard practice, and the engine designers continue to offer various power take-off/power take-in schemes connecting turbochargers and alternators to the main engine, further enhancing plant efficiency.

It is many years since a thermal efficiency of only 40% was the norm! These large diesels also have the ability to run on cheap, very low grade residual fuel oil, though shipping is under growing restrictions on the use of high sulphur content fuels in some parts of the world.

Navies have looked at the combined-cycle concept on many occasions, notably the US Racer programme. A major stumbling block has always been the need to accept sudden and violent load changes, a common situation for a warship in action.

The development by Rolls-Royce of the WR71 engine in the Type 45 destroyer demonstrates that the naval world does look for innovative fuel-saving concepts.

Jim Thomson, Glasgow

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