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Malcolm, Peter and others, thank you for your kind responses to the questions I raised. Re running diesels at light loads my understanding is this... as a rule of thumb if you spin a prop 30% faster it will absorb twice as much hp. If an engine makes 400hp at 2600rpm and the prop is matched, then the same prop will take 200hp to turn at 2000rpm and 100hp to turn at 1500rpm. Light loading comes from diesels being able to produce more hp than the prop can absorb. At 1500rpm the prop absorbs 100hp but the diesel is probably capable of 170hp or more. The engine does not run at optimal temp and runs rich. Hence as 400hp diesel which is propped for correctly will not be as efficient as a 200hp diesel when 100hp is required. The following figures are from an Onan 40kw genset which runs at 1800rpm regardless of output http://www.cumminsonan.com/marine/products/diesel/compare?gensetId=68 25% load , ie 10kw output 1.2 gal/h so 8.3kw per gal/h 50% load , ie 20kw output 1.9 gal/h so 10.5kw per gal/h 75% load , ie 30kw output 2.6 gal/h so 11.5kw per gal/h 100% load, ie 40kw output 3.4 gal/h so 11.8kw per gal/h You can see that 75% load is circa 30% more efficient than a 25% load. This will be different for each engine and the distributor should be able to provide the facts, if not find someone else. The above is the key reasons why some twin engine boats find it more economically to run one engine at lower speeds. Lightly loaded diesels produce high levels of emissions and new laws have forced diesel engines manufactures to improve their product. Tier II common rail diesels are a great improvement. Tier III and beyond will hopeful further improve things. The efficiency gains of Tier II diesels has eroded much of the benefit that diesel/electric offered, i.e. prop and engine rpm not directly linked and engine can be correctly loaded. Malcolm it would be nice to get some firm figures for your long distance designs. Cheers John Seahorse 52 ____________________________________________________________________________________ Never miss a thing. Make Yahoo your home page. http://www.yahoo.com/r/hs