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Running a large diesel slow

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!!

Running a large diesel slow

Cal:

You are going through exactly the analysis that I did before buying our new
trawler.

As you have noted there are true displacment trawlers- Krogens for example
and fast trawlers- Sabres, Mainships, Albins to name a few. The true
displacment trawlers have naturally aspirated diesels- the venerable Ford
Lehman, or lightly turbocharged, low rpm diesels- the John Deere in the
Krogen. The fast trawlers have somewhat larger engines, but mostly they are
heavily turbocharged with high boost pressures, common rail fuel injection,
etc which yields up to 70 hp per liter.

In my view if you don't go overboard, you can operate a fast trawler slow at
displacment speeds and enjoy the benefits of the fast engine when you need
it. Yes, the Krogen is a very efficient displacment hull and the John Deere
is a very efficient modern diesel. But a fast trawler such as my Mainship
34T with its 370 hp Yanmar can operate at displacement speeds just fine. I
run mine about half the time at displacement speeds and another half at
about 35 hp/liter which just gets me into planing territory at 12 kts.

The other consideration is running the high horsepower engine too slow and
at too light of a load. At some point- less than 10 hp per liter in my view,
a modern high output diesel will not reach operating temperatures. Then you
will get sooting, unburned fuel washing the cylinder walls, glazing, high
oil consumption, etc. But if you stay at 10 hp per liter or greater,
particularly if you run at fast cruising speeds for the last ten minutes of
the day, then you will be ok.

Some trawlers, the Sabres and Albins come to mind, put such large twin
engines that you can't really run them a displacment speeds and stay at the
10 hp per liter rule. These might work if you only ran one engine, however.

And back to your original question about engine efficiency. My Yanmar 370
peaks at about .05 gal/hp/hr at 2,800 rpm which is my fast cruising speed.
Slow it down to 1,700 rpm and it drops to .06 gal/hp/hr. I suspect there are
two reasons for this. First it takes a lot of parasitic horsepower to simply
turn over a big block and that parasitic horsepower is a bigger part of the
total at 1,700 rpm (60 hp) than at 2,800 rpm (200 hp). Secondly an injector
that can squirt enough fuel for 370 hp won't be very efficient at 60 hp. The
common rail piezo injectors have improved on this however.

An engine designed for displacment speeds only, say 3 liters and 100 hp
would be about 20% more efficient than the 370 hp Yanmar in my boat.

So, I probably answered more questions than you asked. Enjoy your search for
a new trawler.

David

Running a large diesel slow

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, you hit on a bunch of points, let me see if I can cover them.
First off, trawlers are displacement hulls, adding lots more power does
not equate to lots more speed. I've seen plenty of lobster boats that
crused at 15 knots with say a 200 Hp engine. Someone decides to add a
400 Hp engine, and only gains three knots (provided the hull continues
to operate in the displacement mode, some start to climb up on a plane
on the keel, and fall off, resulting in a scary ride). My point being,
as you noticed, don't expect to see a dramatic increase in speed due to
much higher power, that's just the way displacement hulls behave.

On running engines at low RPM's, diesels, like gas engines, have their
"sweet spot" where they run best. On older style engines, running them
for extended times at low RPM's can cause problems. When you do this
it's best to give the boat/engine a good run to burn off any soot that
may start to build up. Sportfishing boats run into this after extended
periods of trolling, when they make the run home the engines typically
run poorly for a short period of time while everything is burned off.
On newer style common rail engines I suspect this is not as much of an
issue. Diesel's run best under load at rated RPM's. While the engine
speed and economy varries from engine to engine a propeller is most
effeciant at slow speeds, so the best combination is a large propeller
spinning as slow as practical. For a trawler a slower turning engine
makes the most sense, your just not going to be able to use the extra
RPM's a faster spinning engine has, they will gear the engine, or prop
the boat, with less reduction and a smaller prop (beware, they tend to
under prop such engines on purpose, to me that's a dead giveaway that
their not going to hold up).

For your application, a reasonable size engine, turning at lower RPM's
is your best bet, a common rail setup would really give you the best of
both worlds. It will give you good fuel burn numbers, good speed and
long term reliability. I would stick with engine manufacturers like
Cat, Cummins (I would avoid the over 300 Hp 5.9 engines) or Lugger, I
would avoid the lightweight high RPM engines, I haven't seen then hold
up as well. The right engine should outlast you. I hope this helps you.

John

Running a large diesel slow

Cal Vanize wrote:
As we continue to look at trawlers and motor yachts, they seem to come
in a wide range of diesel engines.

As a confirmed old school sail guy I have advocated a specific
"trawler" type to all. its really a buy of a llife time opportuity.

Do a Google search for ship brokers and look at the "gulf shrimper"
vessels. I defy you to compare the following numbers for a MOTOR BOAT:

1997 to about 2000
Steel
75' x 22' x 12' of course may vary up to 85' x 24'x 13'
Single 540 HP Cat.... 3412 rated at 100% continous HD.... Industry
standard.
20 GPH
Got problems???? Just go to any real boat yard that works on real work
boats, not "yachts" and get a fair deal for fair work.

Now get this........................... $100,000 to about $120,000

Why on earth would anyone buy some fiberglass, candy ass, yacht when
yas can get a real boat for 1/5 the price????!?!?!
Bob

Running a large diesel slow

Hi,
The smaller the diesel engine the harder it has to work to achieve the
hull speed of the boat.

(Square root of the waterline length X 1.34)

You need the minimum size diesel that will achieve this. If 50
horsepower does it then that is what you need.

Diesels must be worked hard or there are several issues with glazing and
smoking. A hard worked diesel engine is a happy engine.

If you want a planing fast boat then you must buy more horsepower.
However a Trawler type 'Slow Boat' only needs minimum power.

They are cheaper to run per hour as well.

Oliver Fleming

"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!!

Running a large diesel slow

On Mon, 01 Jan 2007 18:16:19 +1000, Oliver Fleming wrote:

Hi,
The smaller the diesel engine the harder it has to work to achieve
the
hull speed of the boat.

(Square root of the waterline length X 1.34)

You need the minimum size diesel that will achieve this. If 50
horsepower does it then that is what you need.

The rule of thumb for this is 2hp per ton of displacement. So a 20 ton
boat would need 40hp to reach hull speed, and a 50hp diesel to operate at
80% capacity (another rule of thumb). But this doesn't take wind or waves
into account.

Matt O.

Running a large diesel slow

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

Running a large diesel slow

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!!