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)

On Mon, 26 Nov 2007 23:03:52 +0700, Bruce in Bangkok
wrote:

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?

I have tried 1.0SQRT(LWL) and 1.1 - as Beebe and others predict, 1.0
is more efficient. 1.0 is about as slow as I'm willing to go, and
then only over longer distances where I'm already committed to running
day and night.

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.

Yes, I've considered replacing the injectors and may do it at some
point.

Injector N55, Fuel Output Min.24 - Max.30
''''''' N60 "''''''''''''' 30 - '' 36
'''''''' N65 ''''''''''''''' 40 - '' 46
'''''''' N70 ''''''''''''''' 38 - '' 44
'''''''' N80 ''''''''''''''' 44 - " 50

I believe that I have the N80s at this point, so in theory I could go
all the way down to N55s. That would clearly reduce power and torque
but it's not obvious what the impact on economy would be. It could
also have an adverse impact on the present reduction gear ratio and
prop pitch.

On Mon, 26 Nov 2007 19:36:09 -0500, Wayne.B
wrote:

On Mon, 26 Nov 2007 23:03:52 +0700, Bruce in Bangkok
wrote:

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?

I have tried 1.0SQRT(LWL) and 1.1 - as Beebe and others predict, 1.0
is more efficient. 1.0 is about as slow as I'm willing to go, and
then only over longer distances where I'm already committed to running
day and night.

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.

Yes, I've considered replacing the injectors and may do it at some
point.

Injector N55, Fuel Output Min.24 - Max.30
''''''' N60 "''''''''''''' 30 - '' 36
'''''''' N65 ''''''''''''''' 40 - '' 46
'''''''' N70 ''''''''''''''' 38 - '' 44
'''''''' N80 ''''''''''''''' 44 - " 50

I believe that I have the N80s at this point, so in theory I could go
all the way down to N55s. That would clearly reduce power and torque
but it's not obvious what the impact on economy would be. It could
also have an adverse impact on the present reduction gear ratio and
prop pitch.


True, but this raises the question of whether you are actually
developing " X" amount of horsepower at "Y" RPM. I assumed from a
previous post that you were calculating H.P. from fuel consumption
figures which is really just a ballpark calculation.

Given that the 71 series are no longer made what are rebuilt injector
prices like? Or have they gone out of sight because a lot of people
would rather have the old all mechanical engines then the new
electronic models?


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

On Tue, 27 Nov 2007 09:45:46 +0700, Bruce in Bangkok
wrote:

True, but this raises the question of whether you are actually
developing " X" amount of horsepower at "Y" RPM. I assumed from a
previous post that you were calculating H.P. from fuel consumption
figures which is really just a ballpark calculation.


Without some way of measuring torque, that is the best you can do. 17
hp per gph is a pretty good approximation.

Given that the 71 series are no longer made what are rebuilt injector
prices like? Or have they gone out of sight because a lot of people
would rather have the old all mechanical engines then the new
electronic models?

Newly manufactured parts are available from various sources including
DD dealers. I've heard reports of some difficulty getting new
cylinder kits but the DD service center that I used in North Carolina
had no problem.

On Mon, 26 Nov 2007 23:52:29 -0500, Wayne.B
wrote:

On Tue, 27 Nov 2007 09:45:46 +0700, Bruce in Bangkok
wrote:

True, but this raises the question of whether you are actually
developing " X" amount of horsepower at "Y" RPM. I assumed from a
previous post that you were calculating H.P. from fuel consumption
figures which is really just a ballpark calculation.


Without some way of measuring torque, that is the best you can do. 17
hp per gph is a pretty good approximation.

No. I wasn't casting aspirations about calculating H.P. from fuel
consumption, other then meaning that it is an approximation. I was
more talking about your comment referring to propellers and gearboxes.

Given that the 71 series are no longer made what are rebuilt injector
prices like? Or have they gone out of sight because a lot of people
would rather have the old all mechanical engines then the new
electronic models?

Newly manufactured parts are available from various sources including
DD dealers. I've heard reports of some difficulty getting new
cylinder kits but the DD service center that I used in North Carolina
had no problem.

Given the umpteen million of those things that must have been
manufactured I assume that parts will be available for some time to
come. I worked on a pair of original Detroit Diesels installed in a
landing craft. They started on the first turn of the starter and ran
about 10 PSI of oil pressure most of the time. It didn;t seem to
bother them at all. Finally the owner did an overhaul on them and got
the oil pressure up to a more healthy figure but they didn't seem to
actually run any better.

As another aside I worked on some Russian truck cranes that had a
exact copy of the 6-71 engine in it. It had the old single spring
governor that used to run away and they parked them outside our
office. At idle the engines wold sit there surging and we used to make
bets on which one would run away and crater. None of them ever did
though. They were such an exact copy that GMC injectors were an exact
drop in fit.


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

In article ,
Bruce in Bangkok wrote:

Given that the 71 series are no longer made what are rebuilt injector
prices like? Or have they gone out of sight because a lot of people
would rather have the old all mechanical engines then the new
electronic models?

71 Series Injectors are still being built in the After-Market Parts
Business, by a whole passel of OEM's. The same is true for MOST of
the Overhaul Parts for ALL of the DD 53, 71, and 92 Series engines.
About the only thing you can't get NEW, is the Block itself.

Bruce in alaska
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