On Sun, 25 Nov 2007 20:10:04 -0500, Wayne.B
wrote:

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.


Interesting. Have you tried other fractions of "hull speed" to see if
the same ratio of one engine fuel consumption to twin engine
consumption remains the same?

As an aside all 6-71's weren't rated at the same power and one of the
differences was the injector s. They came with different colored tags
as I remember. If you are interested in improving fuel economy (and
losing horse power) talk to an experienced Detroit diesel mechanic
about changing injectors. It might give you a bit better economy, if
you are interested.


Bruce-in-Bangkok
(Note:remove underscores
from address for reply)