Over propped. Is it really a problem?

[rslifkin]

Again, this only applies to planing boats. In a displacement hull with fixed prop the mid-rpm torque capabilities of a diesel don't matter, because the full available power in that range is never needed on a continuous basis.

That's entirely the point. Propped correctly, you never use that power range outside of very short bursts if you throttle up quickly. But when overpropped, you're applying more load at any given RPM the engine can reach. So you can end up operating continuously in that range where the designer didn't expect you to.

 

[Jeff F]

That's entirely the point. Propped correctly, you never use that power range outside of very short bursts if you throttle up quickly. But when overpropped, you're applying more load at any given RPM the engine can reach. So you can end up operating continuously in that range where the designer didn't expect you to.

One more time: this only applies to planing boats. It is completely irrelevant to the OP's concerns and any discussions on propping full displacement boats cruising at 1.2SL.

I thought we'd resolved this earlier...

https://www.trawlerforum.com/forums/s6/3208t-320hp-overpropped-66644.html#post1148054

 

[rslifkin]

One more time: this only applies to planing boats. It is completely irrelevant to the OP's concerns and any discussions on propping full displacement boats cruising at 1.2SL.

I thought we'd resolved this earlier...

https://www.trawlerforum.com/forums/s6/3208t-320hp-overpropped-66644.html#post1148054

Get the hull type out of your head. It's irrelevant to the point I'm making. Nick F's graph illustrates very clearly how the load curve changes on an engine that's overpropped. Some engines have a fairly narrow difference between max power and max continuous power and are fairly forgiving of being overpropped. Some engines have a lot of difference between those curves and are installed in a boat where they're cruising close to max continuous. In that situation, overpropping is likely to load the engine beyond what the manufacturer would consider acceptable for continuous operation at cruising RPM and will result in a shortened engine life.

Without an EGT gauge and adequate information from the engine manufacturer it's hard to determine if you're overloading an engine at a given RPM.

Just because an engine can produce 100hp at 1500 RPM doesn't mean it can do anywhere near that continuously. If propped to spec it may only be loaded to 40hp at 1500 RPM. If the boat needs 70hp at cruise you'd be cruising at a higher RPM to get that 70hp. But when overpropped, you'll be demanding more than 40hp at 1500 RPM. If overpropped enough that you now reach cruising speed at 1500 RPM, the engine will have to make 70hp at that RPM. That means it's burning more fuel at the same RPM and EGTs will go up (possibly beyond what can be tolerated for long periods). The cooling system may also struggle to keep up with the increase in heat production without additional water flow from higher RPM.

If the boat is significantly overpowered, you're likely to have some margin for overpropping without issue as you're never going to run the engine anywhere near full power.

To further the point, some engine manufacturers are adamant that an engine must be propped so it always reaches at least rated RPM. Cummins and Yanmar both require that on a recreational boat prop selection must allow you to reach at least rated RPM (or higher) when the boat is fully loaded.

 

[Jeff F]

Without an EGT gauge and adequate information from the engine manufacturer it's hard to determine if you're overloading an engine at a given RPM.

That's great advice for sport fisherman. But in the trawler world it's malarkey. I invite you to take a prop curve for a displacement hull - any boat, any diesel engine, any prop - and show me where the engine is overloaded at less than 200 rpm under maximum achievable rpm.

Load demand will always drop off faster than engine capability as you reduce throttle from max rpm. That's not a universal truth, but it's true for the boat type being discussed.

 

[Brooksie]

You will get a lot of opinions on this so I'll add mine:
Due to the wide difference between the way engines produce power and the way propellers absorb power, mild over propping is harmful only at WOT. Of course manufacturers will say you are voiding your warranty (or any other excuse for that matter). You will have trouble explaining this to your potential buyers and assuring them you never ran at WOT for sure.

 

[Delfin]

FWIW, when I bought Delfin she had a new Cat 3306. To activate the warranty, a Cat tech had to be able to get up to max rated RPM. Does that tell anyone what Cat thinks about the issue?

And when purchased, she was an empty hull. Once I filled her up with cabinets, systems, etc. she was slightly over-propped. I took an inch out and the cruise rpm to produce 8 knots increased by 100 rpm, while the EGR remained the same. At WOT the EGT was 75 - 100 degrees cooler. Idle speed went from 4 knots to 3.

 

[klee wyck]

With a Mercedes OM 403 tier 1 in LIBRA I can get close to 2500 rpm in neutral and not over 1900 under load. I have run this same engine years ago in a farming application on an 8-row harvester and it was pretty much the same way. Both were designed specifically to run this way under load. I generally run around 1500 rpm while cruising.
I have not spent a minute of my life concerned about over propping.

 

[DDW]

Over propped is over propped through the full hp range. When over propped you are operating the engine outside of the manufactures specifications.

But when overpropped, you're applying more load at any given RPM the engine can reach.

This thinking is entirely incorrect. On any boat with a prop remotely in the ballpark, the torque the prop can absorb is FAR less than the rated torque of the engine at lower throttle settings (say less than 75%). You need to look at the torque and prop curves. When the rated torque curve and prop absorb curves get close, then you might have a problem. If they are close where you normally operate the engine, then you want to have the prop correctly pitched. If they are close at half throttle, then you don't have a prop that is remotely in the ballpark.

Thinking like this would mean that variable pitch props are invariably damaging to an engine.

An engine manufacturer will insist the prop allow full rpm because they don't know how you are going to operate it, and over propping allows the operator to abuse the engine by running at full throttle. That doesn't mean you are abusing it when running at half throttle.

This is different than a diesel truck or tractor. Those, you can overload, bog, and cause over fueling. You cannot do that with a prop anywhere even remotely close to right.

 

[Nick F]

You will get a lot of opinions on this so I'll add mine:
. . . mild over propping is harmful only at WOT. Of course manufacturers will say you are voiding your warranty (or any other excuse for that matter). You will have trouble explaining this to your potential buyers and assuring them you never ran at WOT for sure.

I agree with this and consider it to be in line with what my graph shows. If you do overprop, you will be able to take the engine into bad areas. You should set a new max RPM limit.

In fact, we already set ourselves the self-imposed RPM limit of, say, 200 RPM below max. Its just a question of changing the 200 value appropriately. And as far as the warranty goes, who can tell whether the owner was operating continuously at max RPM? (maybe the ECU in modern engines will snitch on you!)

 

[Nick F]

With the diversity of opinions here, maybe we should drift into the anchor choice question.

 

[Nomad Willy]

There is still a difference between max output at a given RPM and that output that's intended to be used continuously at that RPM. The additional output beyond the continuous point is only meant to be used for short periods such as while accelerating through that range.

Like this a lot rslifkin.

 

[O C Diver]

I repowered Slow Hand when I bought it with a John Deere 4045 132 HP. With the original 28" prop, it would do 7 knots at 1,200 RPM at 2 GPH (3.5 MPG). I then bought the flattest stock 28" prop made by Michigan. The net result was exactly the same (other than 1,450 RPM).

The problem was that the engine won't turn 1,900 RPM. I was concerned about the warranty because I couldn't turn 2,600 RPM. This turned out not to be a problem. There's a gauge on my instrument panel that measures percentage of load. It indicates the percentage of load based on that specific RPM. Instead of worrying about RPM, JD was more concerned about the percentage of time above 80% load and at 100% of load. So the ECM tracks the percentage of load all the time and stores it in the ECM.

The engine now has about 6,000 hours on it. It runs 95% of the time between 40 and 50% load.

Finally found a reason I like an electronically controlled engine.

Ted

 

[Nick F]

I repowered Slow Hand when I bought it with a John Deere 4045 132 HP. With the original 28" prop, it would do 7 knots at 1,200 RPM at 2 GPH (3.5 MPG). I then bought the flattest stock 28" prop made by Michigan. The net result was exactly the same (other than 1,450 RPM).

The problem was that the engine won't turn 1,900 RPM. I was concerned about the warranty because I couldn't turn 2,600 RPM. This turned out not to be a problem. There's a gauge on my instrument panel that measures percentage of load. It indicates the percentage of load based on that specific RPM. Instead of worrying about RPM, JD was more concerned about the percentage of time above 80% load and at 100% of load. So the ECM tracks the percentage of load all the time and stores it in the ECM.

The engine now has about 6,000 hours on it. It runs 95% of the time between 40 and 50% load.

Finally found a reason I like an electronically controlled engine.

Ted

Good post! Informative and factual. (it also aligns with my opinion!)

 

[rslifkin]

I repowered Slow Hand when I bought it with a John Deere 4045 132 HP. With the original 28" prop, it would do 7 knots at 1,200 RPM at 2 GPH (3.5 MPG). I then bought the flattest stock 28" prop made by Michigan. The net result was exactly the same (other than 1,450 RPM).

The problem was that the engine won't turn 1,900 RPM. I was concerned about the warranty because I couldn't turn 2,600 RPM. This turned out not to be a problem. There's a gauge on my instrument panel that measures percentage of load. It indicates the percentage of load based on that specific RPM. Instead of worrying about RPM, JD was more concerned about the percentage of time above 80% load and at 100% of load. So the ECM tracks the percentage of load all the time and stores it in the ECM.

The engine now has about 6,000 hours on it. It runs 95% of the time between 40 and 50% load.

Finally found a reason I like an electronically controlled engine.

Ted

The key here is that you have an indication of what the actual load is (relative to max power at that RPM) and that you're within a range that the manufacturer has told you is fine. Some over-props will certainly fit into that category, but without explicitly asking the manufacturer (who may or may not give the appropriate info), many engines don't have enough information published to determine how much load at a given RPM is actually acceptable for long periods.

 

[sunchaser]

Again, this only applies to planing boats. In a displacement hull with fixed prop the mid-rpm torque capabilities of a diesel don't matter, because the full available power in that range is never needed on a continuous basis.

On a correctly sized hull to engine the planing example you cite applies to trawlers as well. I know of several new build trawlers where the engines were right sized and expected to operate near the continuous rating for days on end. Correct propping is a must in these builds.

There is no downside to correctly propping one’s boat no matter what the speed you choose to operate. Not to mention on a new build the engine manufacturer feels the same for warranty.

 

[Pascall]

I'll say it again, the only way to find out that an engine is being loaded correctly is to measure the exhaust temperature, in all commercial vessels it is determined that the engine is loaded correctly.
The required outlet temperature is specified by the manufacturer, a Dutch saying is to measure is to know, (meten is weten)
99% of these vessels also have an exhaust temperature gauge on board.
If you don't have an exhaust temperature on board, there is another way to test that your engine is not overloaded, although this way only says running at full power under load.
You set the engine to max rpm, prm, so not the regulator!
If you can turn the regulator even further without increasing the rpm, the engine is overloaded at full throttle.
You should check whether the full throttle position on the regulator corresponds to the full throttle position on the fuel pump.

Greeting

Pascal

 

[Jeff F]

On a correctly sized hull to engine the planing example you cite applies to trawlers as well.

Yes, but the consequences of overpropping on high powered planing boats can be dire in ways that don't apply to non-planing boats. That's why I'm trying to make the distinction. It's far less critical in a trawler.

I know of several new build trawlers where the engines were right sized and expected to operate near the continuous rating for days on end. Correct propping is a must in these builds.

Agree 100%. I'd pay a lot of attention to getting it right if it were my boat.

There is no downside to correctly propping one’s boat no matter what the speed you choose to operate.

Here I'm going to disagree. For older boats that are going to be cruised at < 50% capacity for the rest of their lives overpropping can be beneficial. Instead of a cruise speed where the engine is running at 10-20% capacity why not change the prop load to get it up to 30-40% capacity? And as an added benefit you drop cruising rpm.

For those concerned about overloading, just never run continuously at more than 300 rpm below max. It's as simple as that.

 

[Jeff F]

The problem was that the engine won't turn 1,900 RPM. I was concerned about the warranty because I couldn't turn 2,600 RPM.

So you're 700 rpm over propped?

 

[Nick F]

For older boats that are going to be cruised at < 50% capacity for the rest of their lives overpropping can be beneficial. Instead of a cruise speed where the engine is running at 10-20% capacity why not change the prop load to get it up to 30-40% capacity? And as an added benefit you drop cruising rpm.

For those concerned about overloading, just never run continuously at more than 300 rpm below max. It's as simple as that.

Yes! Agree 100%. I run at 1300rpm @ 7.2 knots with nice quiet boat.

 

[rslifkin]

Yes, but the consequences of overpropping on high powered planing boats can be dire in ways that don't apply to non-planing boats. That's why I'm trying to make the distinction. It's far less critical in a trawler.

Agree 100%. I'd pay a lot of attention to getting it right if it were my boat.


Here I'm going to disagree. For older boats that are going to be cruised at < 50% capacity for the rest of their lives overpropping can be beneficial. Instead of a cruise speed where the engine is running at 10-20% capacity why not change the prop load to get it up to 30-40% capacity? And as an added benefit you drop cruising rpm.

For those concerned about overloading, just never run continuously at more than 300 rpm below max. It's as simple as that.

It's not necessarily a matter of planing or not. It's more a matter of how much power the boat has vs how much you actually use at cruise. The more overpowered the boat is, the less likely overpropping is to be an issue. But if the boat is appropriately powered for your cruising speed and has a good bit of load on the engine, it's not necessarily any different than the planing boat in terms of engine load at cruise.

In the case of a very overpowered boat where the engine is chronically under-loaded, then yes, overpropping may actually have some benefits. Realistically though, I'd hesitate to do it without some kind of engine load measurement (either an ECU readout or EGT gauge) and if overpropping by any significant amount, I'd be inclined to consult the engine manufacturer like Ted did (as they'll have more information than is typically published to help determine what is an appropriate operating range).

 

[Pascall]

Yes! Agree 100%. I run at 1300rpm @ 7.2 knots with nice quiet boat.

Exactly!

 

[O C Diver]

So you're 700 rpm over propped?

Yes.

6 knots @ 1,200 RPM.
7 knots @ 1,450
8 knots @ 1,750 +/-

Ted

 

[DW1979OA]

Thanks for all the input. I think exploring the reasons for over heating will be topmost. The boat is from 1979 and who knows what has been getting slowly plugged up over the years in the area between the intake and the strainer. I've made sure the rest of the system is clean.
Thanks to LovetoBoat for the link Nice easy read.

 

[dpmcgarry]

This is also a good resource: https://mvdirona.com/TechnicalArticles/DieselEngineOverload/

You may want to inquire about this on the boatdiesel forums. There are a bunch of very knowledgeable folks who chime in there.

 

[Jeff F]

It's not necessarily a matter of planing or not. It's more a matter of how much power the boat has vs how much you actually use at cruise. The more overpowered the boat is, the less likely overpropping is to be an issue. But if the boat is appropriately powered for your cruising speed and has a good bit of load on the engine, it's not necessarily any different than the planing boat in terms of engine load at cruise.

I'll try to explain why the distinction matters. On a planing boat the prop demand is highly dynamic, and is not progressive through the operating range. The engines are often working their hardest getting the boat on to a plane.

On a non-planing hull the prop load is progressive, predictable, and exponential. So a curve like Nick produced earlier is a very good prediction of real-time load. To calibrate the curve just plug in max rpm.

If you look at that curve you'll see that the spread between required hp and available hp *always* gets greater as you decrease rpm. That's a function entirely of the prop demand curve. That was true well before engine manufacturers started goosing their engines to increase mid-range performance.

So with a non-planing boat once you know your max rpm that gives say 80% load by definition every rpm under that will be less than 80% load. That is not true with a planing boat. Further, in a non-planing boat you can see where that 80% threshold lies without instrumentation by referring to the prop curve.

 

[rslifkin]

I'll try to explain why the distinction matters. On a planing boat the prop demand is highly dynamic, and is not progressive through the operating range. The engines are often working their hardest getting the boat on to a plane.

On a non-planing hull the prop load is progressive, predictable, and exponential. So a curve like Nick produced earlier is a very good prediction of real-time load. To calibrate the curve just plug in max rpm.

If you look at that curve you'll see that the spread between required hp and available hp *always* gets greater as you decrease rpm. That's a function entirely of the prop demand curve. That was true well before engine manufacturers started goosing their engines to increase mid-range performance.

So with a non-planing boat once you know your max rpm that gives say 80% load by definition every rpm under that will be less than 80% load. That is not true with a planing boat. Further, in a non-planing boat you can see where that 80% threshold lies without instrumentation by referring to the prop curve.

The climb onto plane is a transient condition though. Either you're overpropped so badly the boat can't climb onto plane or it climbs up just fine. You're generally not operating the boat in that range for any sustained period of time, so even if you're at 100% load for a few seconds it's unlikely to be an issue much like how running an engine up to WOT briefly for testing isn't going to hurt it.

The difference is that to get your required cruise power when overpropped you're running at lower RPM and higher load. That's not automatically a problem, but depending on the engine in question and how high the load actually is, it *may* be a problem. And many engine manufacturers don't publish enough information for us end users to determine for ourselves if we're still operating in a good range for a long lifespan or if we're pushing something hard enough to risk cooling issues, shortened lifespan of some parts, etc.

Now, if you have things adequately instrumented and have enough data (or a discussion with the engine manufacturer) you can easily determine whether you're at risk of hurting anything or not. Having some kind of indication of load vs RPM is a good idea on any engine, but particularly important when you start to stray from the parameters the manufacturer says are definitely acceptable (EGT, electronic load readout, or for a gas engine, a vacuum gauge).

Just blindly backing down 300 RPM from max or to 80% load isn't necessarily going to keep an engine happy and give a long life. Depending on the design, power rating, etc. some engines are fine to run at 100% load continuously within a certain RPM range, others may not want to see over 70% load for any sustained period and will hold up better cruising at higher RPM with lower load (compared to lower RPM and higher load). There also may be a difference in how the boat performs or loads the engine in rough seas where speed gets dragged down, as the engine running faster with less load will have more ability to absorb those load spikes before losing RPM.

 

[sunchaser]

This is also a good resource: https://mvdirona.com/TechnicalArticles/DieselEngineOverload/

You may want to inquire about this on the boatdiesel forums. There are a bunch of very knowledgeable folks who chime in there.

This link is worth a read and pretty much the gospel from those pros in the know.

 

[Jeff F]

Just blindly backing down 300 RPM from max or to 80% load isn't necessarily going to keep an engine happy and give a long life.

It is in a trawler. If for some reason you've got got an engine rated for planing applications you can cut from 80% to 70% as your upper threshold. Whatever. The story is still the same. Define a safe upper rpm and everything below it is safe.

 

[Delfin]

Here I'm going to disagree. For older boats that are going to be cruised at < 50% capacity for the rest of their lives overpropping can be beneficial. Instead of a cruise speed where the engine is running at 10-20% capacity why not change the prop load to get it up to 30-40% capacity? And as an added benefit you drop cruising rpm.

For those concerned about overloading, just never run continuously at more than 300 rpm below max. It's as simple as that.

If you could reference any prop shop or marine diesel professional (like, say, Tony Athens) that would agree with statement I'd love to hear from them.

Just wondering, but have you ever flattened the pitch on an over pitched boat and observed what actually happens? And how is dropping the cruise rpm a "benefit"? Finally, are you aware that prop demand curves published by manufacturers are based on the assumption that the prop is correctly pitched, and that if it isn't, the curve is wrong, and with it, any conclusions on hp used at a given rpm?

 

[freshalaska]

If this over propping is such a problem then why are some displacement boats set up with a controllable variable pith propeller. Seems these are specifically to load or unload at any given rpm to get optimal performance.