On Wed, 20 May 2015 05:19:05 -0700 (PDT), Tim
wrote:

Maybe I'm not thinking right, but how can running only 1 engine save fuel? To achieve any speed to get to
plane and you having to over tax that one engine, causing higher fuel consumption?

===

You don't run on one engine if you want to get on plane. The fuel
savings comes from running slowly at less than hull speed which
minimizes drag. The problem with running that slowly is that both
engines end up at or slightly above idle speed. That's particularly
bad for diesels since they are prone to developing carbon deposits and
sticky rings when they idle for a long time.

The fuel savings, which is minimal based on my experience, comes from
operating the remaining engine at a more efficient RPM range. Internal
combustion engines require a certain amount of power at idle speed
just to overcome friction, run pumps and alternators, spin cam shafts,
etc. That's all wasted power that does not show up at the flywheel or
contribute to forward motion. It's not all gravy however since the
prop for the engine which is shut down is now creating drag, as do the
rudders which have to compensate for the off center thrust.

I have done controlled tests with our boat, running slowly at around 7
kts on a single engine, and letting the idle prop spin freely. The
fuel save was 20% at best.