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