Sailing yacht - what RPM to run engine at?

[Caroline Joan]

I have a 50ft Beneteau sailing yacht.
I have read that Diesel engines last longer if they are run at higher RPM. Obviously the longer the engine lasts the better as that is a major expense when it needs replacing.
My engine has just gone over 2000 hours - so to me quite low hours and I have read of engines with 10000 hours and much more.
I have a 2007 YANMAR 4JH4-TE turbo 100hp Diesel engine
The manufacturer recommends a minimum RPM of 2200.
When even I run at 2200 I feel the engine

I have 2 questions:
1 - When manufacturer says minimum recommended RPM is 2200 - is this all the time.
Sounds like a silly question but could I cruise at 1800 rpm for an hour then 2200 for an hour.
Really just to save fuel cost, plus 2200 sounds like the engine is working too hard, probably becuase its below the helm and I notice it more?
The engine certainly sounds and feels more comfortable at 1800-2000 rpm
Obviously paying a bit more for fuel now is better that replacing an engine sooner.
What harm is running at less than 2200 doing.

2 - We will be sailing in Pacific for a couple of seasons and there will be prolonged periods of time when we will have no wind.
Rather than sit idle mid ocean for a week or 2, we may choose to use our engine (hate to use engine - love to sail!).
Given that distances can be great between islands and the need to conserve fuel I was wanted to know how low rpm I could run the engine at for several days.

I have 400 litres of fuel.

My crude test below (ignores wind, current, swell) shows that at 1400 rpm my 400 litres will allow my to motor up to 1667 miles, whereas 2200 will allow me to motor only 584 miles.

I did a quick RPM test to show RPM vs Knots.
Litres per hour are ex Manufacturer

RPM Knots L/hr Litres Hours Potential Distance
1400 rpm 5.0 1.2 400 333 1,667
1600 rpm 2.7 2.1 400 190 514
1800 rpm 6.4 3.0 400 133 853
2000 rpm 6.8 4.0 400 100 680
2200 rpm 7.3 5.0 400 80 584

I realise litres per hour will vary according to conditions and from my experience manufacturers per hour are always less than reality.

Your thoughts especially on the use of my engine during long ocean passage are much appreciated.

[StuM]

Something is very wrong with your figures.
5 knots @ 1400 but you slow down to 2.7 knots if you increase to 1600?

[Attikos]

Are the knots you show speed through water or speed over ground?

[malbert73]

I think people over think this all the time. Anytime you are motoring in gear the engine is under a load so generally ok. Running in neutral to charge batteries- ie no load- is where engines can be harmed long term.
If I were cruising at low RPMs, like 1600-1800, id run it up to 2200 every 4 hours for 10 min to clean things out.

[sailorboy1]

Check your owners manual. Mine acturally says that if you run low rpm to run up high every few hours. I run my engine at 2000-2400 rpm because I've found that is the best fuel efficiency overall. I like 2000 rpm for the combination of speed/noise best as it is a lot quieter than at 2400 rpm if I'm not really in a hurry. But if I run at 2000 rpm every few hours I push the throttle to the stop every few hours for a minte or so. It definitely seems to clean out the engine and when I back down the engine sounds better than it did before I did it.

[Kenomac]

Yanmars like our 100hp model in our 53 like higher revs, cruising along at 2000rpms is quite normal and fuel efficient but requires higher revs each hour to clear out the turbo soot. Our Perkins Sabre 185hp is just the opposite and is quite happy going along at 1000-1200 all day long.

[Peeew]

My Yanmar owners manual suggests full power for a while every few hours and a few minutes up and down to 3000rpm then allow to cool at idle before shut down.

[svspirited]

The primary reason they say that is at lower revs you will start to get some build up that would normally burn off at higher revs and higher temperatures, this is especially true when a turbo is involved as they tend to not spin up until 1800 or so rpm, if you have ever seen what happens to a turbo on an engine that does not regularly run up to the proper speed and temperature you will understand why manufacturers say this.

If you run low revs under load for long periods you should bring the engine up to the recommended rpm for 30 minutes or so every few hours. This will ensure everything burns off properly and will allow you to burn less fuel for those long doldrums passages.

[Attikos]

Originally Posted by Peeew

My Yanmar owners manual suggests full power for a while every few hours and a few minutes up and down to 3000rpm then allow to cool at idle before shut down.

Mine, too. I think their engineers suggest it in order to blow out the dirt from head and exhaust.

[GordMay]

Originally Posted by svspirited

The primary reason they say that is at lower revs you will start to get some build up that would normally burn off at higher revs and higher temperatures, this is especially true when a turbo is involved as they tend to not spin up until 1800 or so rpm, if you have ever seen what happens to a turbo on an engine that does not regularly run up to the proper speed and temperature you will understand why manufacturers say this.

*If you run low revs under load for long periods you should bring the engine up to the recommended rpm for 30 minutes or so every few hours. This will ensure everything burns off properly and will allow you to burn less fuel for those long doldrums passages.

Indeed.
You're probably OK with lower RPM (Note *), if your engine temperature is up to operating specification.

[Caroline Joan]

oophs should read 5.7 kts at 1600rpm

[Caroline Joan]

knots are SOG

[DEAN2140]

You said knots are SOG. That is the wrong measurement since it will vary depending on current, wind, sea state, etc. Speed through the water is a constant.

[Scrimshaw4]

I've always been told that if the correct prop is on the boat and the engine is properly sized for the boat that the optimal engine is speed is 80% of max neutral RPM. Ie, if engine is rated at 3000 RPM then 2400 RPM is the optimal run speed.

[Cocaptains]

Always thought that diesels could run at 80% forever.