Originally Posted by CSY Man
As for the Max continous RPM
, you say 85%.
Is still a special recommedation for Yanmar
due to their designs, or is it general for most diesels?
I have heard 70% of max contious as the target, and have been running my Perkins
4-108 at that speed:
4000, max contious 3000, so I have been running at 2100 RPM and the engine
seems to like it..No smoke, fuel
consumtion 0.55 gallons per hour, minimal oil consumption
and no vibration.
85% would be 2550 RPM, and it seems kind of "busy" at that speed.
Will take a moment to get to the 85% bit but bear with me
manufacturers describe their various ratings for their engines based on the service
they are targeted to - so, for instance, the same engine may be governed to more revs so more fuel
and power in an intermittent type duty such as a ferry
with many stop starts, or for a patrol craft which may only require to run at full and high power sometimes and then only for short periods but run at slow cruise
(compared to max speed) with low power
for the rest of the time, than it would be for more onerous service
. The less onerous the service the higher the rating for basically the same engine.
The highest rating is normally for engines targeted at pleasure boats and that will include a statement along the lines of must not be run at more than X rpm for more than Y hours in any 24 hours - Y is often around 2 hours. The lowest rating is for vessels that run continuously or are subject to heavy drive requirements such as tugs - normally referred to as continuous rating but one has to be careful in that some engine manufacturers (MTU and Cat for example) call the maximum rated revs of their non-continuous rated engines as being their Maximum Continuous Rating (or MCR) so can lead to confusion.
Across all these ratings the figure of operating at 85% maximum revs is a generally accepted rule
for balancing engine service life and cruising the vessel at its required design cruise
speed - so a design brief for a new vessel may state something along the lines of operational speed to be X knots at 85% (say) of the engines maximum rated revs for the particular service.
But one has to keep in mind that running a pleasure rated engine at 85% max revs may be the same as running a lower rated but otherwise pretty much the exact same engine (but governed to slower revs, less fuel and less power) at full revs. Small sailboat engines are basically all pleasure rated and have no alternative lesser commercial
service type rating but one should keep the aforegoing in mind and so it is not a good thing, in my view, to run a pleasure rated engine at anything close to its maximum revs.
Similarly, in my view, it is not a bad thing to run a pleasure rated engine at cruise at revs less than 85% max as those could be the revs the engine would normally be run at if it had a more conservative rating for a commercial
type service. In fact I suspect if you run it at somewhat less than 85% a longer and more reliable life will accrue but I have no evidence of that and furthermore most engines in pleasure boats die of old age or lack of care rather than use. But I do know that pleasure rated engines in commercial vessels (sometimes done for lower initial engine capital cost reasons or for more power from lighter weight engine, or simply because the boat is actually a production boat whose builder
meant it for the pleasure market not the commercial one) which are run for long periods every day, even at cruise, don't last.
In our own case we find we cruise at around 72-75% of max revs, that getting us to the point where many more revs (and hence fuel) is required to make significant increases in boat speed. It also causes the steering
to rattle if we use higher revs (the turbulence from the prop on a semi balanced spade rudder
feeds back through the light steering)