Diesel RPM?

[FPNC]

Kubota engines are used in generator applications where the engine runs at 1800 rpm (60 hz) or 1500 rpm (50 hz,Europe) continuously, for the life of the engine. Which is a long time.

The best thing you can do, is run the engine good and hot on occasion to dry out the oil. Your not ~ blowing out the carbon ~, you're heating up the oil. Then change oil every 150 hours or so.

[Shaneesprit]

Lots of comments here based on Idle speed or maximum / higher running speed.
It has nothing what so ever to do with RPM, it’s just that RPM is generally the only tool the average boat owner has at their disposal.

The issue is load related, so yes in general very low load is bad, Clogged exhaust, coked cylinder heads, Fuel in the oil, in extreme cases leads to engine runaway and I have seen things catch fire when then loaded and burning carbon spitting from the exhaust (although that was not on a boat).
The analogy of a car on a highway being low loaded is not good. That’s a nicely loaded engine.

So on a boat it’s a balance between RPM and prop size, you could have quite low RPM but if your making good headway it’s likely your engine is nicely loaded (much like the car on the highway)
On a boat with a smaller prop, you could run higher RPM and still be under loaded.

[Jimmyhenry]

We have a nearly 50 year old 4-236 Perkins. Cal 46. About 20 tonnes. She likes 1500 rpm. Seems to be her sweet spot. We have motored NZ to Tonga and back. 2000 + NM. Just done 300 miles locally (NZ). Uses very little fuel. Temperature stays steady at 80c.
I have no intention of changing.

[Wandering1]

Originally Posted by FPNC

Kubota engines are used in generator applications where the engine runs at 1800 rpm (60 hz) or 1500 rpm (50 hz,Europe) continuously, for the life of the engine. Which is a long time.

The best thing you can do, is run the engine good and hot on occasion to dry out the oil. Your not ~ blowing out the carbon ~, you're heating up the oil. Then change oil every 150 hours or so.

My Perkins M92B is a commercial engine used in fishing boats. With good oil they recommend 500 hours between oil changes or 1 year. Why would you change every 150 hours? I know my old Yanmar engine in my previous boat had oil changes at 100hrs but that was a high reving light weight engine. The Perkins engine is 4.4 litre and built for commercial use

[Icarus]

Thanks for sorting that one out for all of us...

[01kiwijohn]

Originally Posted by massnspace

I was just reading on another forum about the reasons NOT to let a marine diesel idle for too long or run under minimal load. Some posters said they tried to run at 2500 RPM to make sure the engine was under load.

My current sailboat is fitted with a diesel/prop that pushes the boat at 5.7 knots at 1500 RPM. Yes, I can jack up to 2500, but at that RPM the boat is shaking and the engine burns a lot more fuel.

So.

Am I OK to run all day long at 1500 RPM? If not, what to do?

David
Seattle

What engine? For example, Yanmar manual states that the engine at full throttle, with a clean bottom, calm water, no wind , should spool up to about 95% of high idle. At that rate, you should safely be able to cruise at 75 to 80%of that.

[donradcliffe]

Originally Posted by 01kiwijohn

What engine? For example, Yanmar manual states that the engine at full throttle, with a clean bottom, calm water, no wind , should spool up to about 95% of high idle. At that rate, you should safely be able to cruise at 75 to 80%of that.

This makes absolutely no sense at all. By 95% of high idle do you mean to say 95% of WOT in neutral?

[captchetco]

On e persons approach. My extensive, but a bit dated experience with diesels of the 60s thru 80s is that the last the longest if sustained long hours running is at about 3/4 load. This is a complicated number for a lay person to come up with and dpends on engine, prop, boat design weight and loading. However, a simple approach is: ex: Perkins 4-108 60 hp, 3250 RPM. Multiply by 0.75, 2400 appx. Most economical cruise in my boat, run for days, at .9gph in 1-2' seas, 6 knots thru water. 2700 when rough bow seas and in a hurry. About 1.2 gph, 7 knots at very best, usually less. Not science, but works, and first rebuild was at about 10,000 hours, and not wear required.

[malbert73]

Originally Posted by Lee Jerry

If this engine is propped as you suggest (2500-2600 rpm @ WOT), then it will only produce ~30 hp, rather than the rated 32 hp. Why give up ~6% of your available power in the best case? It will only get worse as drag increases and the prop loads up more (weight, fouling, sea state, etc.) and the limit is reached at even lower rpm.

Hi Jerry Lee-
I'll quote your most reasonably stated post and skip the subsequent ones

There are only 2 reasons not to overprop:
1. the ability to extract max HP out of an engine.
2. A properly sized auxiliary for the boat- like 3--4 hp/ton


HOWEVER, many sailboats (including mine) are overpowered. We never need/use the top 10% of HP as it's so far beyond hull speed we are just digging a huge hole in the water and burning lots of fuel.
It takes a lot less power to push a boat in flat water. Running a diesel underloaded (nothing to do with RPMs) is in fact a recipe for glazing the cylinders.

Much smarter people than I have discussed this at length and suggest overpropping in this scenario is a good thing. Nigel Calder (I discussed this personally with him at the boat show 10 years ago), Steve Dashew, John Harries- all describe this well.
Key points:
1. In the absence of a multi-speed transmission, or a controllable pitch prop, sizing a prop to allow an engine to make full RPMs means for every lower RPM, the prop is undersized for the available power. This has to do with fuel mapping. This article series spells it out really well and I find it's well worth going behind the paywall
https://www.morganscloud.com/2016/04...r-your-engine/

https://www.morganscloud.com/2015/06...gine-fuel-map/

2. The "highway truck" analogy is wrong. Sure, they idle at truck stops (though that's generally being outlawed). But, going up hills, accelerating, pulling heavy loads- their engines load up a lot.

3. Load can never be truly estimated without a EGT. RPMs mean nothing unless you know how your engine is loaded

So why is overpropping (like I did with my Beta 50) good in many cases? Because you are now matching prop output closer to engine output at lower RPMs. This leads to quieter operation, and better fuel economy. You accept not using full RPMs (a near "never" event) for better running the other 99% of the time.
I installed an EGT on my Beta after deliberately overpropping by 250RPM. So I cannot get more than 2550 RPM at WOT. Still way more HP than I need.
I find even with overpropping, my engine only gets above 450F at 1850 RPMs making 7+ knots. This is my prime cruising speed. When I drop to 1650, making 6.5 knots, my engine is around 300-350F which is underloaded. So, if I do that all day, it's important to run it hard (say, coming in an inlet) for 30-60 min at 2000RPM.

SO yes, a Beta 43 may have been better sized, but having replaced a 3600RPM Yanmar 4JHE, one of my prime factors was the ability to run at low cruising RPMs which are quieter and more "relaxing" - since I cruise in a often light wind area so long motoring days are sometimes necessary.

[donradcliffe]

Originally Posted by donradcliffe

A couple of years ago, I delivered a boat back from Hawaii. A 75 hp turbo yamar 4jh4-te, and there was no wind so I had to motor for 200 hours at 2000 rpm. When I got close to land, I tried to run the motor up, but it wouldn't do over 2100 WOT. Wanna guess what the problem was?

After 200 hrs at 17 % of its power rating, most of you would think the problem was carbon buildup. It actually was the fuel filter clogging up. Changed the filter and it ran like new.

[Bowdrie]

Originally Posted by malbert73

HOWEVER, many sailboats (including mine) are overpowered. We never need/use the top 10% of HP as it's so far beyond hull speed we are just digging a huge hole in the water and burning lots of fuel.

Much smarter people than I have discussed this at length and suggest overpropping in this scenario is a good thing.

1. In the absence of a multi-speed transmission, or a controllable pitch prop, sizing a prop to allow an engine to make full RPMs means for every lower RPM, the prop is undersized for the available power.

3. Load can never be truly estimated without a EGT. RPMs mean nothing unless you know how your engine is loaded

So why is overpropping (like I did with my Beta 50) good in many cases? Because you are now matching prop output closer to engine output at lower RPMs. This leads to quieter operation, and better fuel economy. You accept not using full RPMs (a near "never" event) for better running the other 99% of the time.

Yes, a small amount of over-prop is just fine, as I've said in other posts, the 10 ton boat pushing against wind/wave/current, will never know the difference between 36Hp and 40Hp, (but it shows-up in the fuel bill).
Once you get past a S/L ratio of ~1 a greater proportion of energy, (and quickly rising,) is just wave-making.
However, a flatter pitch does give you quicker response when you need to "kick the boat around", like in a back-and-fill situation.
An aside to ponder over some fine brandy, (and perhaps a cigar,); there is no thrust without slippage, and max thrust is at 100% slip.

[hpeer]

Originally Posted by donradcliffe

A couple of years ago, I delivered a boat back from Hawaii. A 75 hp turbo yamar 4jh4-te, and there was no wind so I had to motor for 200 hours at 2000 rpm. When I got close to land, I tried to run the motor up, but it wouldn't do over 2100 WOT. Wanna guess what the problem was?

Same engine. I ran it at 1,600 for like 8 hours. Then it would not spin up. Forward, reverse, or neutral. Long story short the injection pump was "diety" and presumably jammed the governor or somehow limited it 1,600RPM.. IIRC I have read on someone else having the smell problem. That is how I got my day tank, so I can double filter fuel and catch problems early.

BTW I found a tachometer app for my iPhone so I could check my tachometer.

[Lee Jerry]

Originally Posted by malbert73

Hi Jerry Lee-
I'll quote your most reasonably stated post and skip the subsequent ones

There are only 2 reasons not to overprop:
1. the ability to extract max HP out of an engine.

This is too much of a simplification. It's more than that. It's: you do it (not overprop) for the health of the engine and to maximize the performance of the prop/engine system across the widest range of conditions.

2. A properly sized auxiliary for the boat- like 3--4 hp/ton

Several things wrong here: which "ton?"; there's a 33% difference between 3 and 4; both are too low.

HOWEVER, many sailboats (including mine) are overpowered.

I would say that more boats are underpowered. Especially when you consider age and the actual power that their old engines produce today. See more below.

We never need/use the top 10% of HP as it's so far beyond hull speed we are just digging a huge hole in the water and burning lots of fuel.
It takes a lot less power to push a boat in flat water.

Right, but you're not sizing for hull speed in calm water (even though that is where you "match"), you're sizing for the more demanding cases (fouling, wind and seas, etc.).

Running a diesel underloaded (nothing to do with RPMs) is in fact a recipe for glazing the cylinders.

YES! (The highlighted part.) Let's remember this.

Much smarter people than I have discussed this at length and suggest overpropping in this scenario is a good thing. Nigel Calder (I discussed this personally with him at the boat show 10 years ago), Steve Dashew, John Harries- all describe this well.

I'm more interested in what they say/said, not with who said it. Just because they're famous, does that mean they can't be wrong. See argument from authority fallacy.

Key points:
1. In the absence of a multi-speed transmission, or a controllable pitch prop, sizing a prop to allow an engine to make full RPMs means for every lower RPM, the prop is undersized for the available power. This has to do with fuel mapping. This article series spells it out really well and I find it's well worth going behind the paywall
https://www.morganscloud.com/2016/04...r-your-engine/

https://www.morganscloud.com/2015/06...gine-fuel-map/

While it is probably interesting (so I can/could respond), I'm not dealing with the paywall (at this time). Do you have any other, open-source references from any of them?

2. The "highway truck" analogy is wrong. Sure, they idle at truck stops (though that's generally being outlawed). But, going up hills, accelerating, pulling heavy loads- their engines load up a lot.

Agreed.

"going up hills" = against wind and waves
"accelerating" = accelerating (positive and negative), especially while maneuvering in port
"pulling heavy loads" = towing another vessel, like maybe a dinghy or your buddies broken down boat

Might your boat want/need to do any of those things?

3. Load can never be truly estimated without a EGT. RPMs mean nothing unless you know how your engine is loaded

Not exactly true, but that might depend on your definition of "truly." Other methods can get you close (enough). Also, I would think that EGT is as or more useful for detecting engine problems.

So why is overpropping (like I did with my Beta 50) good in many cases?

It's not.

Because you are now matching prop output closer to engine output at lower RPMs.

This sentence makes no sense - engine output always matches prop demand. (Certainly in a static or steady state sense; perhaps you could argue a little lag during acceleration.)

This leads to quieter operation, ...

This is an interesting claim. Can you explain why you think this?

But I will preemptively give my argument against. You are basing this claim on the fact that we all know that as we throttle up our engines they get louder. But that is because they are using the same prop, so when you throttle up you are producing more power. If, however, you actually compared / measured the sound from the engine with two different props going the same boat speed (i.e. the same load on the engine), I expect the noise would be nearly identical because they are producing the same amount of power, and it's the power (i.e. the explosions in the cylinders) that is making (most of) the noise not the revolutions - the proper prop makes more, smaller explosions while the overprop makes fewer, larger explosions.

...and better fuel economy.

Perhaps. As I said, it is possible that you move to a slightly more efficient spot on the engine map for a small range in operation. It's also possible that you are making it worse (at your target speed); it would depend on the engine and system. But even if you hit it you're probably looking at only 3-5% except in rare occasions.

You accept not using full RPMs (a near "never" event) for better running the other 99% of the time.

Not sure what type of sailing you do, but for the general forum member: do you carry a life raft? How often do you use it? An EPIRB? In a car, do you wear your seatbelt? How often do you "use it?"

I installed an EGT on my Beta after deliberately overpropping by 250RPM. So I cannot get more than 2550 RPM at WOT. Still way more HP than I need.
I find even with overpropping, my engine only gets above 450F at 1850 RPMs making 7+ knots. This is my prime cruising speed. When I drop to 1650, making 6.5 knots, my engine is around 300-350F which is underloaded.

It's great that you monitor EGT, pretty rare on a sailboat (as far as I know). A little more common on power boats. But, THAT HAS NOTHING TO DO WITH PROP MATCHING.

If you had a "properly" matched prop, you'd run at 2100 rpm making 7+ kt and get above 450F EGT; and drop to 1900 rpm and make 6.5 kt with around 300-350F EGT - because the load (boat speed) is the same and it has "nothing to do with RPMs."

So, if I do that all day, it's important to run it hard (say, coming in an inlet) for 30-60 min at 2000RPM.

Yup, said the same up-thread.

SO yes, a Beta 43 may have been better sized, but having replaced a 3600RPM Yanmar 4JHE, one of my prime factors was the ability to run at low cruising RPMs which are quieter and more "relaxing" - since I cruise in a often light wind area so long motoring days are sometimes necessary.

The Beta 43, with proper prop matching, would be:
- cheaper to purchase and install
- lighter (improving speed under both sail and power)
- smaller, giving more space in the engine compartment
- give the same performance, since you are voluntarily "handcuffing" your 50 anyway

[skipperpete]

That’s a good post Lee Jerry, well thought out. I had the opportunity to drive a boat with a well matched Gori overdrive prop and could compare “ slightly overpropped” to “properly pitched “ prop on a delivery and it was interesting to experiment with the variations…. There are times when overdrive works perfectly as advertised( motor sailing or calm wind and sea) and other times ya just need to be correctly pitched (or even underpitched) ….. the overdrive loads up the engine too much.
My old Gardner diesel was about 60 hp at 1200 rpm swinging a 36”Hundested propeller through a 5:1 reduction and as conditions changed I had to vary the pitch frequently to maintain 1200.

[malbert73]

Originally Posted by Lee Jerry

This is too much of a simplification. It's more than that. It's: you do it (not overprop) for the health of the engine and to maximize the performance of the prop/engine system across the widest range of conditions.


Several things wrong here: which "ton?"; there's a 33% difference between 3 and 4; both are too low.



I would say that more boats are underpowered. Especially when you consider age and the actual power that their old engines produce today. See more below.



Right, but you're not sizing for hull speed in calm water (even though that is where you "match"), you're sizing for the more demanding cases (fouling, wind and seas, etc.).



YES! (The highlighted part.) Let's remember this.



I'm more interested in what they say/said, not with who said it. Just because they're famous, does that mean they can't be wrong. See argument from authority fallacy.



While it is probably interesting (so I can/could respond), I'm not dealing with the paywall (at this time). Do you have any other, open-source references from any of them?



Agreed.

"going up hills" = against wind and waves
"accelerating" = accelerating (positive and negative), especially while maneuvering in port
"pulling heavy loads" = towing another vessel, like maybe a dinghy or your buddies broken down boat

Might your boat want/need to do any of those things?



Not exactly true, but that might depend on your definition of "truly." Other methods can get you close (enough). Also, I would think that EGT is as or more useful for detecting engine problems.



It's not.



This sentence makes no sense - engine output always matches prop demand. (Certainly in a static or steady state sense; perhaps you could argue a little lag during acceleration.)



This is an interesting claim. Can you explain why you think this?

But I will preemptively give my argument against. You are basing this claim on the fact that we all know that as we throttle up our engines they get louder. But that is because they are using the same prop, so when you throttle up you are producing more power. If, however, you actually compared / measured the sound from the engine with two different props going the same boat speed (i.e. the same load on the engine), I expect the noise would be nearly identical because they are producing the same amount of power, and it's the power (i.e. the explosions in the cylinders) that is making (most of) the noise not the revolutions - the proper prop makes more, smaller explosions while the overprop makes fewer, larger explosions.



Perhaps. As I said, it is possible that you move to a slightly more efficient spot on the engine map for a small range in operation. It's also possible that you are making it worse (at your target speed); it would depend on the engine and system. But even if you hit it you're probably looking at only 3-5% except in rare occasions.



Not sure what type of sailing you do, but for the general forum member: do you carry a life raft? How often do you use it? An EPIRB? In a car, do you wear your seatbelt? How often do you "use it?"



It's great that you monitor EGT, pretty rare on a sailboat (as far as I know). A little more common on power boats. But, THAT HAS NOTHING TO DO WITH PROP MATCHING.

If you had a "properly" matched prop, you'd run at 2100 rpm making 7+ kt and get above 450F EGT; and drop to 1900 rpm and make 6.5 kt with around 300-350F EGT - because the load (boat speed) is the same and it has "nothing to do with RPMs."



Yup, said the same up-thread.



The Beta 43, with proper prop matching, would be:
- cheaper to purchase and install
- lighter (improving speed under both sail and power)
- smaller, giving more space in the engine compartment
- give the same performance, since you are voluntarily "handcuffing" your 50 anyway

So many opinions with little engineering backing. Forum opinions are great, but when experts and engineers come to a conclusion backed by science, that's where I'll sit.
Again, what you are ignoring, is that if you size a prop for full RPM, by definition you are ALWAYS underpropped at lower RPMs.

As far as open source, feel free to pick up Steve Dashew's book at the library, or pay the paltry amount for Morgan's cloud subscription which is a treasure trove of well supported information, not opinion.
The Beta 43 was only a few hundred cheaper. It's the same footprint/block as the 50, so not smaller. And, I prefer a 45 hp 2500RPM slow turning diesel.

As a teaser, I'll quote from the engineer's discussion which hopefully will help folks realize that a forum such as CF is great for opinions but sometimes a few dollars are well worth it to get real information:



"Furthermore, the engine’s longest lifespan and longest time-between-repairs is obtained in the optimal region of the fuel map. Run a diesel for a while at low BMEP, for example, and you risk glazing the cylinders; the working pressure is too low to properly load the piston rings, allowing byproducts to build up on the cylinder walls.

Low BMEP also implies low combustion temperature (and therefore low exhaust temperature)—ideal conditions for carbon deposits to form on the valves and other components.

Your car’s automatic transmission is programmed to shift gears to keep the engine as close to the ideal point as possible; a good driver with a stick-shift does the same intuitively.

Propellers, though, don’t behave that way. A fixed propeller’s RPM versus torque curve always follows one specific shape*, shown here in blue:

https://etugri87v3i.exactdn.com/wp-c...MjgyMi4wLjAuMA..

You’ve probably noticed the problem by now. There’s only one point—full throttle—where the calm-water propeller curve comes near the engine’s ideal operating point for the desired power."

BMEP - brake mean effective pressure, which can also be estimated using an EGT.


In your example above, if I run at 2100 RPM, I am producing way too much horsepower, and burning way too much fuel. Like I said, I cruise at 7+ knots at 1800-1900 RPM, and properly load the engine. If I were correctly propped, I would have to run at 2200-2300 RPM to load the engine, and I'd be going only 1/3 knot faster, making a huge wake, and burning nearly double the fuel. I can motor at 6.5 knots into pretty much anything (20-30 knots) at 2000-2100 RPM. Do not need more power.

And this is the case for most sailboats. 50hp in a 40 footer is pretty common out there these days, and if they all have the "safe prop" they almost never are loaded up.