On Wed, 24 Nov 2010 09:48:30 -0600, Vic Smith
wrote:

We have Detroit 6-71 2-cycle diesels which are not efficient at low
speeds. Running single engine gets operating RPMs up to a normal
level. This is useful on long passages in relatively light winds and
calm seas. If the wind kicks up we need more power to maintain speed
and stability.

Tough finding a solution to that.
Just a few thing things that popped into my head, probably none
practical.
Re-propping to get revs up on both engines - non-starter because
you're running 2 engines, and waste heat kills fuel savings.
Transmission that can run both props on one engine input.
Makes sense for cruising speed, but probably too expensive/complex.

Running two engines is always less efficient than one engine,
everything else being equal. An engine in motion has lots of
frictional and parasitic losses - things like pumps, alternators and
valve trains for example. DD 6-71s have an additional parasitic loss
from spinning the blower.

Feathering props. Probably more drawbacks than gain.

Feathering props are good although complex, expensive and difficult to
repair in the boondocks.

Easily detached props. Read to dive far asea, hoist, and get caught
with only one prop when it squalls? Didn't think so.

Not on my boat although I understand that a guy with a GB49 went from
Hawaii to San Francisco that way.

My favorite lame-brain idea is streamlined remote operated prop cowls.
Close when prop not in use, open otherwise.
Insane.

Good out of the box thinking, just a few details... :-)

Do you have trans lube/heat problems when freewheeling a prop?
If you lock it down, will you still get 25% fuel savings?
Lots of drag there.

At slow trawler speeds it is not that big an issue. We start the
other engine once in a while to recirculate the transmission lube. I
also check the temp with a heat gun from time to time when I'm doing
my engine room checks.

In theory locking it down is more efficient than letting it spin.
Locking it down is potentially dangerous however unless a built in
shaft braking system is used.