On Sun, 25 Nov 2007 07:20:49 -0500, "Roger Long"
wrote:

Engines that are idled a lot simply get overhauled a bit sooner. If you
don't have good reasons to idle, why throw away your engine time which =
money?

That's the way I understand it also. Instead of getting maybe 5,000
hours between overhauls, you end up with 3 or 4,000 instead. With a
turbo it can be a lot worse than that.

I have some hard numbers for single vs low speed twins on my own boat,
based on a relatively small number of data points. Running both
engines slowly to achieve about 1.0 x SQRT(LWL), the best fuel economy
I've been able to achieve is 1.4 NMPG.

Running single engine with the other one freewheeling I have been able
to get 1.7 NMPG.

The boat has sight guages on the tanks so that I can measure fuel burn
to within 5 gallons accuracy. The boat is a heavily loaded,
semi-displacement GB49 with total weight in the of range 60 to 70,000
lbs. The engines are 2 stroke DD 6-71s, naturally aspirated, rated at
280 hp each. They will hit their rated max of 2400 RPM at WOT.
Props are 4 bladed 30 x 25, reduction gears are 2.5 to 1.

With both engines the boat will reach 1.0SQRT(LWL) at 1200 RPM,
burning about 5 gph total (about 85 actual hp). Single engine at 1500
RPM, same speed, burns about 4 GPH (68 actual hp) .

Since it seems reasonable to assume that the overhead of running a
large engine slowly is greater than a small engine, it is entirely
possible that a different outcome would be obtained with smaller
engines running closer to their rated power output.