Diesel RPM?

[dannc]

Originally Posted by malbert73

...

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. ...

Agree about Dashew's books, though they can be had for free, https://setsail.com/free-books/. They did a great service by releasing the books for free. I managed to buy the physical books before they sold out a few years ago. Glad to have the physical and digital books.


Morgan's Cloud is well worth the small amount it costs to access, as you say, a treasure trove of information.

[kenbo]

I may be missing the argument on this post but please don't equate RPM with fuel economy. As an example think of a constant-speed generator engine, that always maintains 1500 or 1800 rpm. As the load varies on the generator head, the governor adds or reduces fuel to maintain power to match the load.

Same with a variable speed engine in a boat. Given a load, the engine will have to have a supply of adequate fuel to match the load. The RPM of the engine is almost irrelevant. So no matter what size prop you are running, the engine will burn roughly the same amount of fuel to produce the wattage to meet the load. The "rolling coal" scenario occurs when the fuel rack delivery exceeds the current RPM delivery of air. This is the situation marine engine manufacturers want to avoid in an over-propped (read overloaded) drivetrain.

Monitoring EGT will keep you out of trouble but Lee Jerry's point of properly spec'ing your drive train is still valid. Why buy HP you can never use.

[Adelie]

Originally Posted by Lee Jerry

And all that has absolutely nothing to do with the question I asked.

What do you think the benefit(s) is (are) of over-propping? If you could give a direct answer to that question, I can (most likely) explain why that is not the case and also give a handful of reasons for why it is bad (including the one you already ignored).

Over-propping will give you better fuel economy at less than max RPM.

[donradcliffe]

Right. When you are idling the engine at 800 rpm in neutral the fuel economy is zero. If you move up to a fast idle at 1100 rpm, you burn more fuel with zero fuel economy.

Consider the diesel as rotating machinery with low efficiency. Most of the fuel energy is turned into waste heat. Run that engine at twice the rpm and you have more than twice the losses from rotating bearings, sliding pistons, moving gases, and backpressure. You also have more noise, and if you have a British engine, more than twice the oil leakage.

Your choice. When I repowered, I went from a Perkins 4108 to a Beta 50. A slower revving, smoother and quieter engine. I had increased the pitch on the maxprop twice on the 4108, and started out with some intentional overprop on the Beta.

Your biggest care may be making progress to weather in strong winds. I had a sailboat that sailed to weather like a train. My biggest care was making it through 1000 miles of doldrums with no fuel stops.

[Adelie]

Originally Posted by Lee Jerry

...

As for fuel consumption, there are several ways to address and I'm not sure which is best. How about: at a given boat speed, the over- will operate at a lower RPM, but (essentially) the same power and therefore (essentially) the same fuel consumption. Again, the details could make either prop slightly better. Basically, the resistance is the same and the propulsive coefficient is essentially the same, therefore the FC is the same.
....

The specific efficiency of an engine is load AND rpm dependent.

If you look at a fuel map you can see this. An example of a random engine is shown below.

The BLACK line shows the ideal prop which can make max HP.
The WHITE line shows an overprop. It can't make max HP but it generally operates at a greater efficiancy.

This is why Controllable Pitch Props (CPPs) are popular in Europe. Significant fuel savings are achievable whenever the vessel is being run at less than full throttle. Where fuel is more expensive the added cost and maintenance of CPPs is less than the cost of increase fuel usage. In the US where fuel has long been very cheap compared to the rest of the world, Fixed Pitch Props are the default choice.

[Captain Bill]

I am with the over proping is more economical crowd. I have my 3jh3e proped for a maximum of 3200 rpm instead of the rated 3800 rpm. My high speed cruise is now 2400 rpm for 7 knots instead of the previous 2875-2900 which Yanmar recommends. I still believe there is no free lunch so fuel burn is not much less than before. the slight improvement is attributable to lower internal losses in the engines. What has changed significantly is oil consumption. I used to burn a quart every 15-18 hours in each engine, now it's less than a quart every 50. I'm guessing the rings are working better at the increased loading.

[Lee Jerry]

Originally Posted by malbert73

So many opinions with little engineering backing.

Riiiight...

Forum opinions are great, but when experts and engineers come to a conclusion backed by science, that's where I'll sit.

Me too, but you haven't presented any (conclusions or science, just some name dropping).

Do you know my credentials? Do you need to?

Again, what you are ignoring, is that if you size a prop for full RPM, by definition you are ALWAYS underpropped at lower RPMs.

Sorry, no, I'm not ignoring anything. You are conflating two terms - prop matching and engine loading. Engine load comes from the operating speed/resistance. Perhaps best if I just quote you: "Running a diesel underloaded (nothing to do with 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.

Again sorry, but you can make your own case; I'm not in the mood for a homework assignment.

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.

I don't know the specifics of those engines, but you get my point.

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.

I've highlighted the two phrases to emphasize a point that some, not all, do not seem to understand - there isn't just "one" prop curve. See how the latter is described as the "calm water curve?" As operating conditions become more demanding, a "new" prop curve is made above/to the left of the calm water curve. I could describe it better, but I'm ~a week out from access to a printer/scanner, so maybe when I get back, or it already appears elsewhere on this forum in other threads (by me and others).

However, let me try to summarize my point one more time: it is (or might be) possible to get a modest improvement in fuel efficiency by "over-propping" and shifting the operating point in the engine map, but you are doing this to make a modest gain in fuel consumption at a time when consumption is already low (or its lowest, i.e. cruise speed in calm water) and you are giving up "performance" in many other operating conditions. That trade-off is not worth it to me. YMMV.

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.

No, you've missed the point. You are talking about with your existing prop, I was talking about a lower pitched prop.

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.

Again, the "safe" or "over" prop doesn't matter. The engine is loaded by boat speed.

[Lee Jerry]

Originally Posted by Adelie

The specific efficiency of an engine is load AND rpm dependent.

Yes.

If you look at a fuel map you can see this. An example of a random engine is shown below.

The BLACK line shows the ideal prop which can make max HP.
The WHITE line shows an overprop. It can't make max HP but it generally operates at a greater efficiancy.

That is a gasoline engine. So while the trends are similar, I believe the possible efficiency gains are much smaller (~half) with a diesel. Your lines show ~6.7% improvement, generally. Even so, while "generally" true, there are points on that map where the over prop is the same or slightly worse consumption. You need to have the map and know all the operating points to ensure you are improving consumption at the intended speed by overpropping.

As I said:

Originally Posted by Lee Jerry

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.

Finally, note that the "ideal" prop cannot make rated power at WOT in calm water. It should be to the right of the "nominal prop curve" shown by manufacturers.

This is why Controllable Pitch Props (CPPs) are popular in Europe. Significant fuel savings are achievable whenever the vessel is being run at less than full throttle. Where fuel is more expensive the added cost and maintenance of CPPs is less than the cost of increase fuel usage. In the US where fuel has long been very cheap compared to the rest of the world, Fixed Pitch Props are the default choice.

Yeah, OK, but not really germane to the conversation.

[Cheechako]

I am an engineer, (I dont think that means much really!) albeit not a marine architect and have helped to spec diesel engines for smaller commercial and military craft, when I worked in the metal boat industry.
When you go to the commercial shows and conferences you will find many diesel engines on display by the makers and will also find that the SAME ENGINE is specified by the manufacturers at different RPM depending on intended use. If you wish to do the same thing for your world traveler craft with a smaller engine, you can do that. (If you do it right)

For instance, a large engine for Recreational Use may be spec'd at 3600 RPM. The intention is for a boat that's used on weekends or occasional use.
The same engine for commercial Intermittent Duty may be spec'd at 2800 RPM. And one for Continuous Duty, like say a passenger ferry which runs non stop 24/7, will be spec'd at 2400 RPM.
The makers know engines last longer generally at lower RPM. The more speed, the more wear. It makes perfect sense. The more metal wears on metal, the more it wears.
How many times have I said this?

[malbert73]

Originally Posted by Lee Jerry

Riiiight...



Me too, but you haven't presented any (conclusions or science, just some name dropping).

Do you know my credentials? Do you need to?



Sorry, no, I'm not ignoring anything. You are conflating two terms - prop matching and engine loading. Engine load comes from the operating speed/resistance. Perhaps best if I just quote you: "Running a diesel underloaded (nothing to do with RPMs)..."



Again sorry, but you can make your own case; I'm not in the mood for a homework assignment.



I don't know the specifics of those engines, but you get my point.



I've highlighted the two phrases to emphasize a point that some, not all, do not seem to understand - there isn't just "one" prop curve. See how the latter is described as the "calm water curve?" As operating conditions become more demanding, a "new" prop curve is made above/to the left of the calm water curve. I could describe it better, but I'm ~a week out from access to a printer/scanner, so maybe when I get back, or it already appears elsewhere on this forum in other threads (by me and others).

However, let me try to summarize my point one more time: it is (or might be) possible to get a modest improvement in fuel efficiency by "over-propping" and shifting the operating point in the engine map, but you are doing this to make a modest gain in fuel consumption at a time when consumption is already low (or its lowest, i.e. cruise speed in calm water) and you are giving up "performance" in many other operating conditions. That trade-off is not worth it to me. YMMV.



No, you've missed the point. You are talking about with your existing prop, I was talking about a lower pitched prop.



Again, the "safe" or "over" prop doesn't matter. The engine is loaded by boat speed.

Hi! Great boat by the way. Sorry this appears to have become contentious.



Let's leave it this way:
I agree with these points (I think) you are making
1. If engines are properly sized, they should be propped to reach full RPMs
2. Prop curves indeed vary a lot by sea state, payload, etc
3. Overpropping reduces available HP in an engine

My points, supported by marine and engineering experts:
1. Engines need to be loaded properly at cruising RPMs
2. Unless an engine is sized much smaller than the norm (and for my boat, proper size is 35-38hp), proper loading of the engine will occur "too high" on the power curve resulting in too much speed, lots of displacement wake, and too much fuel burn.


So, with a fixed prop and a 1 speed transmission, it all depends on what you want. I live in a light air area, and like to make miles with a quiet, trawler like experience with a slow revs engine- my engine is below cockpit sole and running at 1800-1900 RPM, compared with the 2600-2800 Yanmar it replaced, is far quieter and less intrusive.
And to gain that, I essentially de-rated my engine to a 45 hp engine. But, when I need the 45hp I get at 2300RPM, it's there.

Basically- you can't have it both ways. You can't have the "reserve power" enough to do what you imply- motor upwind in chop (which requires an oversized engine), AND also load the engine at moderate cruising speeds that don't make a huge wake and burn extra fuel. In other words, an engine that provides the extra powerplant HP most want, cannot be loaded at a high cruising speed which is very inefficient.

If you refute this last fact I cannot help explain it without assigning "homework" from experts who have proven this beyond a doubt. You would likely really enjoy reading the Morgans Cloud chapters I have linked which are explained by real experts, and I will not violate their website rules by sharing the text for free. I bet if you can maintain a Hylas, you can afford a subscription

My point is to challenge the "dogma" of propping to reach full RPM. Others may blindly follow this without realizing that it only applies to engines sized as auxiliaries, not as primary powerplants. The fallacy of that dogma is well supported by numerous marine experts and engineers I am quoting from. They aren't my opinion nor my expertise. So, I am just putting these well supported principles up against forum opinion.

[malbert73]

Originally Posted by Cheechako

I am an engineer, (I dont think that means much really!) albeit not a marine architect and have helped to spec diesel engines for smaller commercial and military craft, when I worked in the metal boat industry.
When you go to the commercial shows and conferences you will find many diesel engines on display by the makers and will also find that the SAME ENGINE is specified by the manufacturers at different RPM depending on intended use. If you wish to do the same thing for your world traveler craft with a smaller engine, you can do that. (If you do it right)

For instance, a large engine for Recreational Use may be spec'd at 3600 RPM. The intention is for a boat that's used on weekends or occasional use.
The same engine for commercial Intermittent Duty may be spec'd at 2800 RPM. And one for Continuous Duty, like say a passenger ferry which runs non stop 24/7, will be spec'd at 2400 RPM.
The makers know engines last longer generally at lower RPM. The more speed, the more wear. It makes perfect sense. The more metal wears on metal, the more it wears.
How many times have I said this?

Thank you. Made another point for me

[boatpoker]

I am confused. There is one here who often contradicts himself and appears to argue both sides of the coin with the goal of arguing rather than informing. I'll move on to the next thread.

[Lee Jerry]

Originally Posted by malbert73

Sorry this appears to have become contentious.

Ditto.

Let's leave it this way:
I agree with these points (I think) you are making
1. If engines are properly sized, they should be propped to reach full RPMs
2. Prop curves indeed vary a lot by sea state, payload, etc
3. Overpropping reduces available HP in an engine

My points, supported by marine and engineering experts:
1. Engines need to be loaded properly at cruising RPMs
2. Unless an engine is sized much smaller than the norm (and for my boat, proper size is 35-38hp), proper loading of the engine will occur "too high" on the power curve resulting in too much speed, lots of displacement wake, and too much fuel burn.


So, with a fixed prop and a 1 speed transmission, it all depends on what you want. I live in a light air area, and like to make miles with a quiet, trawler like experience with a slow revs engine- my engine is below cockpit sole and running at 1800-1900 RPM, compared with the 2600-2800 Yanmar it replaced, is far quieter and less intrusive.
And to gain that, I essentially de-rated my engine to a 45 hp engine. But, when I need the 45hp I get at 2300RPM, it's there.

Basically- you can't have it both ways. You can't have the "reserve power" enough to do what you imply- motor upwind in chop (which requires an oversized engine), AND also load the engine at moderate cruising speeds that don't make a huge wake and burn extra fuel. In other words, an engine that provides the extra powerplant HP most want, cannot be loaded at a high cruising speed which is very inefficient.

If you refute this last fact I cannot help explain it without assigning "homework" from experts who have proven this beyond a doubt. You would likely really enjoy reading the Morgans Cloud chapters I have linked which are explained by real experts, and I will not violate their website rules by sharing the text for free. I bet if you can maintain a Hylas, you can afford a subscription

I can accept that we are now generally on the same page (or close enough that I don't want to repeat) with one big exception: you aren't changing the engine load by over-propping. The load is determined by the cruise speed. It is the same with both props at the same boat speed (with the usual "all things being equal" caveat).

My point is to challenge the "dogma" of propping to reach full RPM. Others may blindly follow this without realizing that it only applies to engines sized as auxiliaries, not as primary powerplants. The fallacy of that dogma is well supported by numerous marine experts and engineers I am quoting from. They aren't my opinion nor my expertise. So, I am just putting these well supported principles up against forum opinion.

I have to add that I don't understand the highlighted bit. But it's been a long day and I've had a couple adult beverages, so...

[Lee Jerry]

Originally Posted by Cheechako

I am an engineer, (I dont think that means much really!) albeit not a marine architect and have helped to spec diesel engines for smaller commercial and military craft, when I worked in the metal boat industry.
When you go to the commercial shows and conferences you will find many diesel engines on display by the makers and will also find that the SAME ENGINE is specified by the manufacturers at different RPM depending on intended use. If you wish to do the same thing for your world traveler craft with a smaller engine, you can do that. (If you do it right)

For instance, a large engine for Recreational Use may be spec'd at 3600 RPM. The intention is for a boat that's used on weekends or occasional use.
The same engine for commercial Intermittent Duty may be spec'd at 2800 RPM. And one for Continuous Duty, like say a passenger ferry which runs non stop 24/7, will be spec'd at 2400 RPM.
The makers know engines last longer generally at lower RPM. The more speed, the more wear. It makes perfect sense. The more metal wears on metal, the more it wears.
How many times have I said this?

This is close, but not quite right, IMHO. Or maybe incomplete is a better description.

In many/most cases, they're not exactly the "SAME ENGINE." The same block and basic engine, yes, but they will have programming and often parts changed.

Also, along with the differing engine speed specified there is usually also a power rating change with it. So they aren't just moving left on the engine map, but rather down and left. (There is often some overlap, where one speed rating has a couple power ratings, but not the same power across a huge speed change.)

Finally, I don't think longevity (or maintenance intervals) is based on RPM so much, but rather load factor and/or how much actual fuel has been burned (the latter might be how they determine the former, I'm not sure).

[Lee Jerry]

Here you go:

Propeller Match.gif Views: 19 Size: 41.6 KB ID: 287895" style="margin: 2px" />

Note design point G1 on the calm water prop curve (curve 1, dashed) at 100% rated engine speed and 90% MCR on power. If you start over-propped on curve 2 (solid black) you "quickly" run out of available power and torque (curve 2', red).


YMMV