Diesel Inboard RPMs

[Plymouth Sailor]

Dear Fellow Sailors,

I have a new-to-me Irwin 28 with a 25hp Kubota diesel engine. Wondering if anyone can recommend the proper RPMs for cruising speed if there is no wind. Also, if the throttle is as far as it can go, (haven’t tried) is this damaging to the engine. I’m a neophyte when it comes to diesel engines.

Thank you,
Mark

 

[kappykaplan]

Diesels like to be run at 80-85% of wide-open throttle.

 

[dlochner]

The worst thing you can do to a diesel is not run it fast enough. Long periods of idling cause unburned fuel to build up in the exhaust system and in the cylinders. ~80% of WOT is standard with periodic runs at WOT. If you have the manual it should list the designed WOT, if no manual run the Diesel wide open and note the RPMs, use ~80% of that.

The most fuel efficient RPM is usually a little lower and at a slightly lower boat speed.

 

[jssailem]

Mark
You know specifics on your engine influence the quality of responses.

The Kabota engines with a 25 or so HP rating are several. They have a range of top RPM ratings. From 2600 to 3600 rpm’s.

A general selection is about 80-85% of WOT (wide open throttle). You can run near or at WOT for short periods 30-60 minutes. Watch for temp issues.

 

[dkinzer]

The most fuel efficient RPM is usually a little lower and at a slightly lower boat speed.

There was an entry in the log book that came with the boat when we purchased it last year showing approximate fuel consumption at various RPM.

RPM Gallons Per Hour
1750 0.7
2000 1.0
2250 1.5
2500 1.8
2750 2.6
3000 3.2

I don't know if the previous owner took this data from Yanmar's graphs for the 4JH4E or if they measured it. If correct, increasing the RPM from 2000 to 3000 more than triples the fuel consumption rate in exchange for a 50% increase in boat speed assuming a linear relationship between RPM and boat speed.

 

[Ralph Johnstone]

Also, if the throttle is as far as it can go, (haven’t tried) is this damaging to the engine.

Depends.

I’m a neophyte when it comes to diesel engines.

No reason to not start learning. You actually get obsessed with this crap once you start playing with it..

First off, make sure the boat tach. is reasonably accurate. Maybe pull out all the stops and invest in an optical tach. to calibrate.

Also, if the throttle is as far as it can go, (haven’t tried) is this damaging to the engine.

Now you're getting to the touchy-feely part of the inspection. I don't know your engine from an egg beater so download the manual, which I'm reasonably sure is on this site. Check the max. RPM of the engine. Warm up your engine until hot and then ensure the Xmission is in neutral and make sure it will reach the spec'd max. RPM.
Next, take the boat out where you can run full throttle and then see if you can get to max. RPM in gear. If you can't make max. RPM, this can be concerning to the engine and time to post another question.

As others have said, you want to cruise at aqpprox. 80% of max. RPM

 

[Barnacle Bill]

Much of the info above is spot on. Diesels like to be run and really are built to go full blast without any issues, but 80% is a good cruising speed. As mentioned above, going slow all the time is not good for a diesel either. Old diesels (found in most sailboats) are not electronic when it comes to feeding the fuel. They are mechanical and the rings in the cylinders expand with heat, which is what you want to create the required seal. If you run an engine too cool (slow) all the time, this can hurt the engine in the long run. 212 is a good running temp for a diesel inboard. It is actually best to get the rpms and temp up right after you leave the marina to get things sealed with heat rather than slowly running the boat at low speeds and cooler temperatures.

 

[garymalmgren]

Hi Plymouth
RE; Wondering if anyone can recommend the proper RPMs for cruising speed if there is no wind.

I come at your questions from a different because I don't have a tacho.
I have a log /speed meter though.

Let's start with a few assumptions.
1. Your hull and prop are pretty clean.
2. Your engine is running well.
3. Your theoretical hull speed is 6 ( plus a little) knots.

Head out.
Run the engine up to max revs (for me this is when she gets the shakes).
At this stage the boat will be going as fast as she can but the engine is going too fast and wasting fuel.
Keep your eye on your speed.
Throttle back slowly (in increments) till you lose (about) 0.2 knots.
Listen to the engine and become familiar with that sound.
You have reached the proper RPMs for cruising speed.
As mentioned before , if you are into fuel economy, reduce revs and speed a little more.
For really good fuel economy, sail.

Re: Also, if the throttle is as far as it can go, (haven’t tried) is this damaging to the engine
This will not damage the engine because you will get sick of the noise and vibration pretty quickly and slow her down.


Have fun working this and other (thousands of) little things out.

gary

 

[FastOlson]

Some basics will help readers to help you.
While a few mariners have purchased and then marinized Kubota engines, most of these durable little diesels come by way of a few boat engine manufacturers. From back in the late 70's, Universal/Westerbeke in the US was building their "Universal" diesels with Kubota engines/blocks. There are some other builders with less presence in the US, and in the last 20 years Betamarine has become a major engine supplier, basing their diesels on Kubota blocks.

Did your boat come with any paperwork about your specific engine and transmission? Parts and manuals can be sourced once you know the model number of your particular engine, and there is a semi-hidden model number near the high pressure injection pump on most of the ones I have heard of.

Can you post some pictures of your engine?
Anyhow, please try to help others here help you.

 

[Plymouth Sailor]

Some basics will help readers to help you.
While a few mariners have purchased and then marinized Kubota engines, most of these durable little diesels come by way of a few boat engine manufacturers. From back in the late 70's, Universal/Westerbeke in the US was building their "Universal" diesels with Kubota engines/blocks. There are some other builders with less presence in the US, and in the last 20 years Betamarine has become a major engine supplier, basing their diesels on Kubota blocks.

Did your boat come with any paperwork about your specific engine and transmission? Parts and manuals can be sourced once you know the model number of your particular engine, and there is a semi-hidden model number near the high pressure injection pump on most of the ones I have heard of.

Can you post some pictures of your engine?
Anyhow, please try to help others here help you.

Will do, thanks

 

[jviss]

A couple of things. First, it's that diesels like to be run at 80%+ power, not RPM. So, running an unloaded diesel at 80% of rated RPM will glaze the cylinders just as well as running it slower. Second, fuel consumption and RPM are not correlated across the board; fuel consumption and power output are. So, run it at 3000 RPM no load and it will sip fuel. Run it at 3000 RPM driving a load that's in the 80 to 90% power output range and it will use a lot more fuel. Same RPM, vastly different fuel consumption. This is because there's a governor that will try to maintain the set RPM as the load changes - there's actually no throttle on a diesel, it's a speed control.

 

[dlochner]

A couple of things. First, it's that diesels like to be run at 80%+ power, not RPM. So, running an unloaded diesel at 80% of rated RPM will glaze the cylinders just as well as running it slower. Second, fuel consumption and RPM are not correlated across the board; fuel consumption and power output are. So, run it at 3000 RPM no load and it will sip fuel. Run it at 3000 RPM driving a load that's in the 80 to 90% power output range and it will use a lot more fuel. Same RPM, vastly different fuel consumption. This is because there's a governor that will try to maintain the set RPM as the load changes - there's actually no throttle on a diesel, it's a speed control.

You're splitting hairs. True, running diesels unloaded at high RPMs is not good for the diesel. However, ~80% of WOT is pretty close to 80% of power. At 80% WOT my Yanmar 3JH2E produces 85% of its max hp and is running at just about the most fuel efficient speed. (See attached.) There is likely some error in the calculations because it is not a particularly good graph to interpolate from, however it should be good enough.

 

[LakeOntario270]

Much of the info above is spot on. Diesels like to be run and really are built to go full blast without any issues, but 80% is a good cruising speed. As mentioned above, going slow all the time is not good for a diesel either. Old diesels (found in most sailboats) are not electronic when it comes to feeding the fuel. They are mechanical and the rings in the cylinders expand with heat, which is what you want to create the required seal. If you run an engine too cool (slow) all the time, this can hurt the engine in the long run. 212 is a good running temp for a diesel inboard. It is actually best to get the rpms and temp up right after you leave the marina to get things sealed with heat rather than slowly running the boat at low speeds and cooler temperatures.

Where does the 212 running temperature come from? My Westerbeke, which is a Mitsubishi, runs at a constant 160-165 degrees.

 

[Bob S]

Dave, at what rpm do you cruise in your boat? Looks like 3600 is WOT. Are you cruising between 2800/3000?

 

[dlochner]

Dave, at what rpm do you cruise in your boat? Looks like 3600 is WOT. Are you cruising between 2800/3000?

Around 2500 to 2600 with a speed through water between 6.5 to 7 knots. It is a good balance between speed and fuel economy. Once the boat gets up to about 7 knots I just start pushing a lot water without much increase in speed and the swim platform goes way under water.

3600 is WOT without the transmission or in neutral. In gear WOT is closer to 3400, 80% would be 2720.

The important take away here is don't baby the diesel. Running in idle or at higher speeds without a load does no favors to your engine. Does running at 75% WOT a lot worse than running at 80% WOT, I don't think so. If you're always running at 40% or 50% that may be different, be good to run a little higher.

 

[odaydokay]

Where does the 212 running temperature come from? My Westerbeke, which is a Mitsubishi, runs at a constant 160-165 degrees.

My Universal's top recommended is 195. I also wonder where the 212 came from. That would be NG on most Universals I think.

 

[Ralph Johnstone]

My Universal's top recommended is 195. I also wonder where the 212 came from

It came from the internet !



This came from a Universal manual.

[/QUOTE]

 

[jviss]

You're splitting hairs. True, running diesels unloaded at high RPMs is not good for the diesel. However, ~80% of WOT is pretty close to 80% of power. At 80% WOT my Yanmar 3JH2E produces 85% of its max hp and is running at just about the most fuel efficient speed. (See attached.) There is likely some error in the calculations because it is not a particularly good graph to interpolate from, however it should be good enough.



View attachment 207927

No, Dave, I am not splitting hairs, you are misinterpreting the graphs you shared.
With no load, e.g, the gear in neutral, the engine only need make enough power to overcome the internal friction of the engine at that speed. If you spin it up to "80% of WOT" it will run faster, for sure, but it's still only making enough power to overcome the internal friction of the engine - which is negligibly more than at idle. The "throttle" on a diesel is a speed control: it tells the governor the speed at which you want to operate. The governor puts enough fuel into the engine to maintain that speed throughout the range of loads, until it goes out of regulation at either end; either bogging down, at the high end, or idling at the low end. So, at a given speed, let's say, mid-range, there can be vastly different amounts of power produced, and fuel consumed, for a given RPM.

Those charts most likely reflect dynamometer tests.

jv