Sailing yacht - what RPM to run engine at?

[Alita49DS]

I have a similar Yanmar, in my case 4JH3, fitted to my Jeanneau 49DS.

A not dissimilar boat and I rather question how fast you say yours goes at a set revs.

However fast, if you look at the official Yanmar Power Curves sheet (I would attach it if there was a way) the fuel consumption is approximately (the sheet does not do tenths):

<2000 Not shown
2000 3.5l/hr
2200 4.5
2400 5.5
2600 6.5
2800 7.5
3000 9.00
3200 10.5
3400 12.5
3600 14.5


I sail considerable distances in the Med. There is often no wind. I keep a record of engine hours and litres filled to the top. The above is very much in line with my own experience. I now run at @2000rpm as fuel is very expensive in Europe and if I was in a hurry I would not be on a sail boat.

Every now and again I give the engine a burst of revs but it is usually pretty clean.

I installed a NASA Exhaust temperature monitor and the engine seems to run at a happy temperature. Despite the heat in the Med. My understanding is that such a monitor is much more likely to save your engine than relying on the engine block temperature. A friend of mine who has just had to have his engine rebuilt due to overheating would concur.

[a64pilot]

I have never understood the theory of Diesels like to have the snot run out of them.
I don’t think that is true for any engine type. Diesels in particular I’d say don’t like to be run unloaded, they stack or slobber then and that can lead to bore glazing.
However if under load and at operating temp they seem to do well, even at 50% power or lower.

However like any motor, it’s likely best that maybe towards the end of your motoring run or at least once a day, to run one up and let it build some heat to clean itself out.

The longest lived Diesels are invariably the ones that run at low RPM and produce the least power per cu in or weight, not the high performance high RPM Diesels, and if Diesels ran the longest and best if run hard, then this wouldn’t be true.

[David M]

The big dilemma with Diesels (unlike gasoline) is that they really will last longer if run towards their upper RPM range. The worse thing you can do to a Diesel is to allow it to idle for long periods of time. The downside is that the faster you move through the water the worse your fuel efficiency. There is no way around this dilemma.

Cummins requires that their marine Diesels be ran at 100%, for 30 minutes every 10 hours. This blows accumulated carbon out of the combustion chambers.

[sailing mike]

Look at the maximum torque output and at what RPM is it achieved. That’s the RPM to run the engine at. Do give a full blast run for a couple of minutes every few hours if you like.

[MJH]

I take it that you are not the original owner.

READ THE ENGINE MANUAL for your EXACT model engine; get it off the Internet if you don't have a hard copy. Hopefully there is a graph that will tell you what the best fuel RPM or range is...use that. Save the higher RPM for emergencies.

Also note what the MAX RPM is for that Make/Model. On a calm day with zero current do a test run...you should be able to come close (+/- a bit) to that MAX RPM without any shudder or prop cavitation. If you get either then something is wrong and needs looking into but that's a question for another time.

Good Luck.

~ ~ _/) ~ ~ MJH

[cyan]

Originally Posted by a64pilot

I have never understood the theory of Diesels like to have the snot run out of them.
I don’t think that is true for any engine type. Diesels in particular I’d say don’t like to be run unloaded, they stack or slobber then and that can lead to bore glazing.
However if under load and at operating temp they seem to do well, even at 50% power or lower.

There is test data floating around [SIC] that identifies low operating temperature as the main issue, with cylinder glazing, blow-by, ring failure, and oil contamination as secondary effects. As I understand it, many(most?) marine diesels at idle or low RPM with little/no load will not EVER reach proper operating temperatures, even with their mechanical thermostats bypassing the heat exchanger.

[a64pilot]

Originally Posted by cyan

As I understand it, many diesels at idle will not EVER reach proper operating temperatures, even with their thermostat bypassing the heat exchanger.

That is correct and why many Diesels oil coolers are often oil heaters as they heat exchange with coolant.
However, many Diesels have been idled overnight for decades and yet live long lives. Big trucks, but almost always they are run hard enough to get up to and maintain operating temps after those long idles.
I think it’s fair to say that Diesels will tolerate higher sustained RPM runs better than gasoline motors, but I don’t think it’s correct to say that they last longer if run at the upper range of their RPM limits, or 3600 RPM generators would last longer than 1800 RPM gensets and that is not true.
If you look at the average European spec generator, they only turn 1500 RPM, never more and never less, yet generators on average last longer than prime mover motors, and they are not special designed motors either. They are not blown out every few hours by being taken to high RPM etc.

Diesels are RPM limited motors due to the fuel injection requirements, they are not lower RPM motors for any other reason, so it’s likely safe to say that a 3600 RPM max motor can safely be run at 80% of max RPM for longer than a 6500 RPM motor, due or of course the difference in RPM.

[Seighlor]

Back to your question on "long ocean passages": My program was to motor or motor-sail at engine speeds that would produce 5-knots through the water. I would not, however, exceed 1200 rpm. Additionally, my engine speed might be less than 1200 rpm with a boost from the breeze to produce a 5-knot speed. My objectives were to continue to make headway while extending the range of my fuel as best possible.

If the life of my engine was diminished by this practice, it was imperceptible to me.

[sailcrazy]

We run a Yanmar 4JH-TE(55hp). The most trusted Yanmar mechanic we've ever run into told me that (Yanmar) diesels like to be loaded up...much more so than just loafing along. He told us, then showed me a Yanmar report, that mentioned our Yanmar should typically be run at 70%-80% max rpm when cruising. And, if we didn't/couldn't do that, we should rev it up to max (or near max)rpm for about 20min each 4-5 hrs of ops below that 70%-80%. He had lots of reasons why, including cyl wall glazing, carbon buildup, turbo ops, etc....but I'm not a mech-I just took him at his word. Our max rpm is 3600, we typically cruise at 2000-2300rpm for noise and fuel economy-and because our transom tucks significantly at higher rpms- and then rev it up to 3400-3500rpm for 15-20 min after 4-6 hrs of cruising. Seems to work just as advertised.

[Tillsbury]

Same advice I’ve always heard. Find out it’s max revs and sit at near 80% of that as a cruising speed. Occasionally gun it full throttle for an “Italian tune-up”.

[tarian]

Just Had my perking 4236 in for a full service and maintenance check up with a new mechanic been doing these big engines fro 20 years on boats and plants , tractors etc , said to me that the engine needs to be 80% for this type of diesel and it is a common mistake of people not to run them like that

[rickdelz]

I have a pair of YANMAR 4JH4-TE on my catamaran. But they are only 75hp???

[a64pilot]

So how do these mechanics explain smaller high RPM motors that run for very long lives, do so in 1500 RPM generators?
Lot of Yanmar, Kubota, Mitsubishi etc powered generators

[model 10]

I found that the windvane would steer the boat with full sail at 900 rpm. Any more than that and the Aries goes goofy. It's a Perkins 107 and the temp gets to 165-75 and fuel burn is pretty low. I can average about .3 gph. It's pretty quiet too.

[hamburking]

Its not strictly about RPM. Its about temperature. You want the engine to be running at operating temp, not cold. So if its under load, you should have no trouble. I like to cruise at 1500-1800 rpm, which is comfortable and quiet.

You get into trouble with glazing on the cylinders if you are just idling, not in gear, like to charge the batteries or run a watermaker.

And if in doubt, rev up ONCE PER DAY (not once per hour) for about 5 minutes.