Cal,

Besides your other calculations, I would think about the useful life of
the diesel (or at least the time to a rebuild). An early rebuild will
cost a lot more than the likely fuel savings based on engine choice.
Modern turbocharged high-RPM lightweight diesels share little in common
with traditional shipboard engines (except the fuel).

Look at the following link to David Pascoe's Boat Survey site (this
whole site is a pretty depressing commentary on boat quality these days
- you might want to have a few drinks before visiting). I understand
that some surveyors believe his opinions are overly alarmist and
negative.

http://www.yachtsurvey.com/GasNdiesel.htm

One of his opinions is that you ideally want a diesel that has a CUI/HP
(CubicInchDisplacement divided by HorsePower ) of less than 0.7. A
ratio over 1.0 is not good.

He agrees with some others posters that you can get dangerous carbon
buildup if you don't run your diesel at high enough RPM's. It can
lead to catastrophic engine failure (that sounds like fun!)

Also, make sure you get to HEAR your engine at harbor speed and
cruising speed before making a decision. I find some turbochargers
sound like a dentist's drill - others have a less grating pitch. It
also seems some engines are just plain quieter - although it's hard
to compensate for the quality of the engine room insulation.

Carl

Hi Cal:
I have thought of this problem also, of not having enough load on
a diesel to have it run efficiently, and have come to think that a Diesel-
Electric system would be the way to go. You can have a small generator
running for light loads, and slow running, have a larger generator for
running at cruising speed, and run both generators for War Emergency
Power!
Don't know if this is an option in your situation, but here are some
links that you might find interesting reading.

OSSA:
http://www.ossapowerlite.com/index.htm
Fast Electric Yatch System:
http://www.feys.org/
--

SeeYaa Harbin Osteen KG6URO

When American Citizens with dual citizenship pledges allegiance
to the flag, to which flag do they pledge allegiance too?

-
"Cal Vanize" wrote in message ...


As we continue to look at trawlers and motor yachts, they seem to come in a wide range of diesel engines. We've seen some 44' -
47' boats that use engines ranging in horsepower from a single 125 to twin 450s. They seem to have only small "relative"
differences in LWL (to me meaning similar hull speeds). They have differences in displacement, but the differences don't seem to
track very closely to engine HP.

If these boats are operated below planing speed as displacement hulls, then shouldn't they require around the same amount of power
to push them through the water?

In reading engine specifications (power and fuel consumption) it appears that large diesel engines running at a relatively low
RPMs use around the same of less fuel than smaller diesels that are run harder. Or at least the horsepower to the prop seems to
cost about the same fuel consumption regardless of engine size. The engine RPMs for a given HP seems to fall at different points
on the torque curve for larger HP engines than for smaller ones, especially for turbocharged engines. (I don't know if this makes
any difference or not.)


(OK, thanks for following. This is what I'm leading up to....)


So is it more economical to operate large diesel engines at low RPM or small diesel engines at higher RPMs?

Is there a long term problem with either running a large diesel under gentle RPMs or a smaller diesel closer to its continuous
duty capacity?

Is there a maintenance advantage (extended oil, belt, filter, etc., longer engine life) to running a diesel slower?


Are these questions even meaningful to boat ownership and maintenance?

TIA and happy New Year to all!!

"Harbin Osteen" wrote in
:

Hi Cal:
I have thought of this problem also, of not having enough load on
a diesel to have it run efficiently, and have come to think that a
Diesel- Electric system would be the way to go. You can have a small
generator running for light loads, and slow running, have a larger
generator for running at cruising speed, and run both generators for
War Emergency Power!
Don't know if this is an option in your situation, but here are
some
links that you might find interesting reading.


Fuel Consumption:

- Dependant on load, however, this unit is running 5-6 hours a day
pushing an average of 55 amps dc to charge batteries (Peaks are
substantially higher). During this time, AC current from the AC Generator
supplies the house and 1/4 gallon an hour usage is typical. If the
battery bank weren't so big, far less time would be spent charging and
fuel consumption would drop dramatically.

http://psychicworldusa.com/Lister_Gen/Lister1.html

No need of starting batteries, either! Just turn the crank and away she
goes! Watch the videos at the bottom...(c;

What's that you say? Noise?? What noise? Huh? I can't hear you!
Speak louder!...Yes, it DOES sound like a pile driver, doesn't it?...hee
hee.

I wonder how many hours are on the Listers that light up all kinds of
villages in India??

Thanks to all for their help, explanations and useful links. The input
provided is helping us narrow our search.

Happy new year to all.

On Dec 31 2006, 9:18 pm, Cal Vanize
wrote:
As we continue to look at trawlers and motor yachts, they seem to come
in a wide range of diesel engines. We've seen some 44' - 47' boats that
use engines ranging in horsepower from a single 125 to twin 450s. They
seem to have only small "relative" differences in LWL (to me meaning
similar hull speeds). They have differences in displacement, but the
differences don't seem to track very closely to engine HP.

If these boats are operated below planing speed as displacement hulls,
then shouldn't they require around the same amount of power to push them
through the water?

In reading engine specifications (power and fuel consumption) it appears
that large diesel engines running at a relatively low RPMs use around
the same of less fuel than smaller diesels that are run harder. Or at
least the horsepower to the prop seems to cost about the same fuel
consumption regardless of engine size. The engine RPMs for a given HP
seems to fall at different points on the torque curve for larger HP
engines than for smaller ones, especially for turbocharged engines. (I
don't know if this makes any difference or not.)

(OK, thanks for following. This is what I'm leading up to....)

So is it more economical to operate large diesel engines at low RPM or
small diesel engines at higher RPMs?

Is there a long term problem with either running a large diesel under
gentle RPMs or a smaller diesel closer to its continuous duty capacity?

Is there a maintenance advantage (extended oil, belt, filter, etc.,
longer engine life) to running a diesel slower?

Are these questions even meaningful to boat ownership and maintenance?

TIA and happy New Year to all!!

=====

Cal: Some points to keep in mind:

1. Despite the countless variations in diesel engine designs and sizes,
there are common principles and fundamentals that apply. For example,
every diesel engine likes to run hard and long at its engineered
"cruising RPM", otherwise known as its "sweet spot." At cruising RPM,
the engine vibrates less, is quieter, and is most economical in terms
of fuel burn. That's why it's called the sweet spot. There's nothing to
be gained, and sometimes much harm is done, by "going easy" on a
diesel. Long-term, a diesel designed to run at high RPM will strangle
itself at low RPM -- carbon up the exhaust system, clog injectors with
unburned fuel, etc. So, as you ask, there is no maintenance advantage
to running a diesel slower than its designed cruising RPM.

2. Follow the mfr's recommendations regarding lube oil changes, filter
changes, etc. The biggest single killer of marine diesel engines is
unchanged lube oil and lube oil filters, and paradoxically, this is one
of the easiest maintenance chores to perform.

3. In my diesel engine maintenance workshops ("Diesels for Dummies"),
and aboard boats where I train new owners, I encourage familiarity with
their diesel engines. Routine preventive maintenance, which is neither
difficult nor time-consuming, ensures reliable performance and long
engine life. Presuming the engine is properly spec'ed for the boat, if
you give it clean filtered lube oil and clean filtered fuel, plus
abundant clean filtered air, it will be there to serve you every time
you need it.

Hope this helps,


Captain Bernie Weiss
www.AtlanticYachtDelivery.clm

"Captain Bernie" wrote in news:1167950935.170607.170430
@i15g2000cwa.googlegroups.com:

clog injectors with
unburned fuel

Huh? All my injectors are all "clogged with unburned fuel"....otherwise,
she don't start in the mornin'!

What do you do, Bernie, blow air in 'em when you're shutting down?

Come on Larry, you've got to allow for a few word choice slip ups in a group
like this.

I'm sure he meant "un(completely)burned" fuel. It's the ash of partly
burned fuel when the engine is too cold to promote full combustion that
causes the problems. It doesn't even have to clog the injector enough to
stop the flow of fuel. A little clot of carbon on a nozzle can break the
spray pattern and turn it partially into a stream which has less surface
area for its volume and thus burns less completely leading to further carbon
build up.

--
Roger Long

"Roger Long" wrote in
:

I'm sure he meant "un(completely)burned" fuel. It's the ash of partly
burned fuel when the engine is too cold to promote full combustion
that causes the problems. It doesn't even have to clog the injector
enough to stop the flow of fuel. A little clot of carbon on a nozzle
can break the spray pattern and turn it partially into a stream which
has less surface area for its volume and thus burns less completely
leading to further carbon build up.



Sorry. I've been running diesels for lots of years and NEVER had an
injector clog because of the fuel. Injectors clog because of crap IN the
fuel, things high pressure injection pumps can't force past the orifice.
I've never seen any carbon on the outside of an injector clog it against
the hundreds of PSI of injection pump. I'm sorry. A little clot of
carbon could, I suppose break the spray pattern. I'll buy that. But,
it's never happened to any vehicle or boat or genset diesel I've had
contact with....and that's a lot of diesels.

Two things would cause the carbon....too much injection on too little
air...or...too much LOAD, which is normally what's up with that black
cloud of smoke at full throttle. In a boat, too much prop. There's no
reason for a diesel to make lamp black, especially not now with
controlled injection.

What diesel do you know of with this carbon problem, anyway? Carbon in
the pipes is normal, black, gooey snot saturated with unburned fuel a
diesel always produces. Diesels make soot because they run out of air,
usually about the time the exhaust port comes uncovered in a big 2-stroke
beast, or the exhaust valve opens opens, if it has one. But, that
doesn't "clog the injectors".

http://people.bath.ac.uk/ccsshb/12cyl/
I just wanna see and feel it run....(c;

Well, your direct experience may well be more valid than my hearsay. I've
spent 30 plus years around people who run big diesels for a living and I'm
just inferring from the gripes I've heard.

It may also be that fisherman, tug boat operators, and the like run their
engines a lot harder and longer. I certainly see more black smoke coming
out of the exhausts of commercial boats than yachts. The ones that stick in
my memory may also simply have been wrong about what was ailing their
engines.

I've never had the opportunity to see the inside of an abused diesel
cylinder at overhaul but the impressive build up's I've seen in aircraft
engines make it pretty easy to believe that carbon could effect the spray
pattern out of an injector without reducing it. You would think that
nothing could survive in the environment of an internal combustion engine
cylinder but physics are strange. If there is dust on the wings of your
airplane before you take off, a lot of it will still be there when you land.

--
Roger Long

In article ,
Larry wrote:

"Captain Bernie" wrote in news:1167950935.170607.170430
@i15g2000cwa.googlegroups.com:

clog injectors with
unburned fuel

Huh? All my injectors are all "clogged with unburned fuel"....otherwise,
she don't start in the mornin'!

What do you do, Bernie, blow air in 'em when you're shutting down?



Bernie just doesn't understand how modern Injection Pumps, with Builtin
Governers work. It is a very simple case of someone told him something,
and he repeats it every time the question comes up, wheather it is
correct or not.

If a Diesel engine is operating "At Design Temprature" it really doesn't
matter if it is loaded, unloaded, fast Rpm, slow Rpm, or whatever.

Bernie, ever wonder how all those diesel engines, at Truck Stops,
Idle all night long in cold weather, and still are able to pull BIG
Loads, the next morning, day in and day out, and still have 40K hours
between InFrame Rebuilds?

Bruce in alaska
--
add a 2 before @