On Nov 24, 5:23 pm, Bruce in Bangkok wrote:
... This "running one engine to save fuel" story has been popular for
ages, but I have always wondered about it. If it takes X horsepower to
drive a hull through the water at Y speed then is there any real gain ...

I suspect it depends on the engines and the props. I spent some time
with a gent in Niuatoputapu who had been a commercial fisherman and is
a shipyard owner and was totally convinced that running one engine
used less fuel. But, when he installed fuel flow meters he found that
it was slightly less efficient to run a single than a double at
cruising speed and he got noticeably better mileage using both engines
at a slightly slower speed on his 50 foot trawler yacht... In his
case I suppose the drag of the prop was more costly than running the
engines with a light load. YMMV, as they say, but if I had engines
that were big enough to fit with flow meters I'd certainly install
them and test the possibilities.

-- Tom.