WOT question

I have heard you guys post about the need to have your engines be ble to hit WOT. I obtained past survey data (for a boat I am considering) from 3 years ago. WOT for the CAT 3116 -300HP is 2800rpm. The survey says it hit 2700 for one engine and 2710 for the other engine. Fuel tanks were full and 7 people on board. This is a miss of about 4% on the rpms. Is this too far off and something to be concerned about or within the margin of error. It obviously didn't worry the current owner as he bought the boat. But what if it comes up the same on my survey? Thanks again.

Certainly sounds close enough to me especially considering the load you were carrying

I'd also plan to check that the tachs are accurate before worrying about it.

Check your tachs.
Then if still overpropped cut some off your props to raise the rpm (not rpms). Find a good prop man to determine where and how much to cut. There are guys bang’in on props and there are good prop men.
And if you’re overpropped wanting to know if it’s too much think would it be too much if it was underpropped? In this case of 100rpm I’d rather be 100rpm underpropped than 100 overpropped. I think all the variables would, over time, cause a lack of WOT rpm .. not an excess.
If the prop man thinks he can bring the rpm up to 2875 by removing 1”of pitch I’d go that route. 2900 would be fine too as it’s unlikely you’ll ever gain WOT rpm. But you will loose rpm.

But I am no engineer. And most here would rather be 100rpm over than 100rpm underpropped. And your rated rpm is not a redline at all. It’s just the rpm that your engine develops it’s maximum power. Most engines can operate 3-400rpm over rated rpm, especially at lower loads. Talk to a good prop man or (best) an engineer.

Also if you’re going to run your engine at 30% load or less all the time disregard my post and just run it like it is.

David

1. Use a photo tachometer. Don’t trust the gauges
2. Make sure that the engines hit rated WOT with no load (in neutral) before doing anything. (If they were tuned down it wouldn’t be the 1st time I have run across this)
Then proceed with the above good advise

OK, I see that the Hi rpm got to 3010 and 2980. So doesn't look like these were tuned down.

No load RPM has it's own spec. Any surveyor worth their salt would be using his own RPM measuring tool, not the boat's gauges. Did the survey say what the cleanliness of the props and bottom were? Had the boat been weighed during the haul out prior to sea trial? I would rely much on a 3 year old survey, other than as a reference for the surveyor you will contract with. If the boat now doesn't reach WOT rpm with a clean bottom and props, a little pitch reduction will get you there if it is still a 100 rpm short.

The governor should limit the max rpm at 3000 to 3200. It varies but it’s usually about 300rpm over rated rpm.
And ideally the WOT rpm will be about the max power rpm. So for this engine 2850 would be at least excellent and quite likely ideal, as it’s better to be a bit higher than lower. 2900 would be fine too. But although 2800 sounds perfect it will not be 2800 as time goes on. I wouldn’t recommend that one should not accept 2750 .. for awhile keeping tabs on any change. But 2850 would Be considerably better.

Changing pitch is the preferred way to change WOT rpm. Because only a certain amount of rpm change results from a an inch of pitch change and only one inch increments are possible the way prop men do their work. On my smaller boat it’s 200rpm. It’s different for different engines or perhaps more correctly for different props. So if I want to lower my WOT rpm 100rpm I couldn’t do it w a 1” pitch change. I could only change to 1” too little or 1” too much. I’d need to cut off some of the edge of the blades, cup the TE or get a different prop.
But if you grind off some blade and find you need to change your prop load again 100rpm you may wish you had left the grinder on the bench. But boating isn’t easy.

I wouldn't worry about it. If you're planning on operating the engines in that range regularly, they will have other issues. If you limit operation to 10 to 20% below WOT, except in emergencies, they will live a longer happier life and the 100 RPM will be meaningless.

Ted

A. I question the wisdom of going WOT in neutral. WOT throttle has nothing to to with WOT with a load on the prop and shaft.
B. Why is everyone worried about getting up to the ‘Book Value’ of WOT. If you can reach the upper designed speed, IMO you are ‘home’.
C. Before chopping up the prop, I would verify the tach.
D. Got 2 engines? You can synchronize them manually or get an auto synchronizer.
When you get 2 different RPM and your boat is going straight, suspect the tachs. Get them calibrated, if it bothers you. Of course you could go the expense of verify the props on the next haul out. My bro had a boat 2 engines, with a synchronizer and the RPM read differently. When I pointed it out to him he said, he just ignored it. He apparently had greater faith in the synchronizer than the the RPM read out. The difference was with in 150 RPM. His boat, his decision.

OldDan1943 said:

A. I question the wisdom of going WOT in neutral. WOT throttle has nothing to to with WOT with a load on the prop and shaft.
B. Why is everyone worried about getting up to the ‘Book Value’ of WOT. If you can reach the upper designed speed, IMO you are ‘home’.
C. Before chopping up the prop, I would verify the tach.

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A: If the max governed RPM is adjustable, it's a one-time check to confirm that nothing has been screwed with (such as the engine being artificially RPM limited). Without knowing that for sure, it's hard to know what you're really dealing with when you don't reach expected WOT RPM. If there's no adjustment and full throttle lever travel is confirmed, this check is pointless. Also pointless on electronically controlled engines.

B: The effects are somewhat engine dependent, but some engines are sensitive to being run at a higher load everywhere in the RPM range (like what happens when overpropped) and will suffer lifespan-wise for it. Mind you, if you only run at very light load (such as a boat powered to do 20+ kts cruising at 7 - 8), it's unlikely to matter and a slight overprop might save a tiny bit of fuel. But if the engines are ever run close to max continuous power, then it matters more.

C: Absolutely. And confirm nothing else could be slowing the boat down, adding load, or reducing engine power before tweaking props as well.

Agree with Ted above 110%. Unless you are regularly going to try and push the boat past hull speed you, are fine.

Checking to see that the engine revs up to its "high idle" speed (wot, no load, in neutral) is to confirm that nothing else is limiting rpms, like not hitting the throttle stop, governor malfunction, etc.

Then with that confirmed you can check the wot rpm under load with a phototach or a phototach calibrated dash tach. If the engine is not reaching the rated wot rpm then the engine is under too much load and it should be reduced by cleaning the bottom or a prop adjustment if it is clean.

David

Remember, making a beer run to the islands with 120 cases of beer onboard just might effect the speed of the boat.

The concern is if you cannot reach WOT under load, you are overloading your engine if you cruise regularly at the standard 20% off the factory WOT RPMs. In your case, rather than cruising at 2350-2400 rpms you'd better cruise at 2200 rpms.

Good posts all .....
But I disagree w Ted. I’m thinking if you’re running 15% Below WOT you shouldn’t be overpropped at all. And Ted dosn’t say if it’s 15% rpm, load or what. If one is overpropped 50% of load it’s about all one can run without overloading the engine. Of course overpropped 150 rpm is way different than 50rpm.

Ted are you referring to rpm or load? And if it’s rpm is it % of what is attained or What is listed as rated rpm? But IMO 15% of any of that would be overloading. Probably 80% of trawler skippers are running below 60% load (I’m at 50%) and there 100rpm below rated shouldn't result in overloading. Perhaps close to it though. But that’s just my opinion judging by the conversation on the forum.

And there is a point that FF has brought up many times. No mater how much one overprops If you reduce rpm enough you won’t be overproped. Within reasonable numbers of course.

Nomad Willy said:

Good posts all .....
But I disagree w Ted. I’m thinking if you’re running 15% Below WOT you shouldn’t be overpropped at all. And Ted dosn’t say if it’s 15% rpm, load or what. If one is overpropped 50% of load it’s about all one can run without overloading the engine. Of course overpropped 150 rpm is way different than 50rpm.

Ted are you referring to rpm or load? And if it’s rpm is it % of what is attained or What is listed as rated rpm? But IMO 15% of any of that would be overloading. Probably 80% of trawler skippers are running below 60% load (I’m at 50%) and there 100rpm below rated shouldn't result in overloading. Perhaps close to it though. But that’s just my opinion judging by the conversation on the forum.

And there is a point that FF has brought up many times. No mater how much one overprops If you reduce rpm enough you won’t be overproped. Within reasonable numbers of course.

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I was referring to RPM. In the OP's example, if the WOT is 2,800 RPM the normal limit should be between maybe 2,300 and 2,500 RPM. The key is to look at the manufacturer's graph of engine HP and prop HP absorption. In most cases, when you drop from WOT, while the engine HP declines, the expected prop HP absorption fall away more dramatically.

Ted

OK Ted, Good clarification.

One function way more linear that the other.
One usually dosn’t see the prop hp curve.

Was there any change exhaust smoke?

My N46 with a Lugger, dry stack, when we brought it down from Long Island and outside, we ran it WOT 12 to 24 hours a day and night. The only problem was, it would approach ‘over temp’ so we’d cut back a bit on the throttle until the temp returned to normal. This started about mid FL. Everything was boiled out (keel cooler and engine).... so I attribute to the temp of the FL water. I did like the Lugger. I was your basic engine, nothing fancy or electronic, it worked or didn’t work and it always worked.

Checking your engines at WOT is an important diagnostic tool. It starts with doing so the first time to make sure the engines and boat are performing as they should. You should exceed the rated RPM at WOT if you're running with very light load, typically 50-75 RPM. It's also a means of checking the two engines against each other if you have twins. If you can't hit that max RPM, something is wrong. It can be many things and the props are just one. However, if you're over-propped, you're over-propped and that doesn't just impact you at WOT. It may be you run low enough RPM that it doesn't significantly impact you, but if you run at 70-80% load then it could cause issues.

Now, once you have everything set, occasional checking at WOT is a quick and easy way to find out if something has changed. You may quickly identify what has changed, perhaps a dirty bottom, perhaps damage to props, perhaps engine just not performing. But this is a means of quantifying before and after and targeting work to regain what was lost. We consider no engine maintenance to be complete without a WOT check.

Perhaps this is more important to us because of the speeds we run, but I think it's more likely we'd just notice it more quickly. If you're losing 5% whether you're fast or slow, you still have an inefficiency.

Thanks for all the help guys. I might add that the survey was done in late Sept. so maybe the bottom was a little dirty and reason for not hitting the 2800 rpm.

An engine is overloaded throughout it's operating range if over propped. That is, it is working harder to generate X amount of RPMs no matter the value of X. It's like lugging your car engine at various speeds. For some insight into this, here is one article from a respected source.

https://www.sbmar.com/articles/engine-life-vs-engine-loading/
https://www.sbmar.com/articles/engine-life/

caltexflanc said:

An engine is overloaded throughout it's operating range if over propped. That is, it is working harder to generate X amount of RPMs no matter the value of X. It's like lugging your car engine at various speeds. For some insight into this, here is one article from a respected source.

https://www.sbmar.com/articles/engine-life-vs-engine-loading/
https://www.sbmar.com/articles/engine-life/

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Sorry, that's just not true. Modern engines with electronic fuel injection control, often have a gauge that shows percentage of load for that specific RPM. My engine is way over propped (not all by choice). Yet, when cruising at 7 knots, the percentage of load for 1,500 RPM is between 45 and 50%. Further, the installation met John Deere's warranty requirements as the engine computer tracks time above 80% load. As the dealer explained it to me, you can prop the engine to reach WOT, but if you have an engine failure as a result of towing stuff, they won't warranty it. Their warranty considerations are partially based on your percentage of time above 80%, regardless of RPM. The computer will record loads in excess of 100%. They can look at the computer to see how you operate or abuse the engine.

Ted

O C Diver said:

Sorry, that's just not true. Modern engines with electronic fuel injection control, often have a gauge that shows percentage of load for that specific RPM. My engine is way over propped (not all by choice). Yet, when cruising at 7 knots, the percentage of load for 1,500 RPM is between 45 and 50%. Further, the installation met John Deere's warranty requirements as the engine computer tracks time above 80% load. As the dealer explained it to me, you can prop the engine to reach WOT, but if you have an engine failure as a result of towing stuff, they won't warranty it. Their warranty considerations are partially based on your percentage of time above 80%, regardless of RPM. The computer will record loads in excess of 100%. They can look at the computer to see how you operate or abuse the engine.

Ted

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Different manufacturers and engines have different requirements. IIRC, Cummins says that if it can't reach rated rpm under a few different conditions depending on the application, no warranty for you. And some engines are more sensitive to high loads at low rpm than others just by their nature.

The engine is designed to be run at wot, not in gear, right up until it hits the governor. This is called high idle. Then the engine is designed to be run at wot in gear and should hit the governor or the limits of the running gear. Under load.

As to how long the engine is certified to run at wot, depends upon the rating of the engine. My Cummins is rated M1 which means it can run at wot for 24 hours, which is probably what it would do if it was attached to a pump or a generator. Another model of my engine is rated wot for 1 hour every 24. So no blanket pronouncements over wot for all engines.

Wot tells you if everything is running as the factory intended. All of the engine parameters should be within the limits set in the manual while at wot.

One of the main problems with wot is that the average boat owner babies their engines and rarely runs them wot, thinking for some reason that wot will hurt their engine, which is not true. The other issue is that wot is incredibly noisy and sounds like the engine will literally leap off its mounts. It probably won’t but the sound is overwhelming so many will not complete a wot test.

If something blows at wot, it was weakened or damaged already and would have let go under worse circumstances. This way it was found under “test” conditions.

A Lehman was not a success as a truck engine but it excelled in combines and generators where it would run 30,000 hours plus at wot, never or rarely shutting down.

Nomad Willy said:

Check your tachs.
Then if still overpropped cut some off your props to raise the rpm (not rpms). Find a good prop man to determine where and how much to cut. There are guys bang’in on props and there are good prop men.
And if you’re overpropped wanting to know if it’s too much think would it be too much if it was underpropped? In this case of 100rpm I’d rather be 100rpm underpropped than 100 overpropped. I think all the variables would, over time, cause a lack of WOT rpm .. not an excess.
If the prop man thinks he can bring the rpm up to 2875 by removing 1”of pitch I’d go that route. 2900 would be fine too as it’s unlikely you’ll ever gain WOT rpm. But you will loose rpm.

But I am no engineer. And most here would rather be 100rpm over than 100rpm underpropped. And your rated rpm is not a redline at all. It’s just the rpm that your engine develops it’s maximum power. Most engines can operate 3-400rpm over rated rpm, especially at lower loads. Talk to a good prop man or (best) an engineer.

Also if you’re going to run your engine at 30% load or less all the time disregard my post and just run it like it is.

Click to expand...

Doesn't the idea behind getting 100 RPM over WOT spec apply more to a boat in relatively light load with the idea that the WOT under the inevitable later increased loading will sag somewhat?

If I read the post right he had a lot of load in that boat. I really don't see where hundred or 75 RPM deserves much attention

motion30 said:

If I read the post right he had a lot of load in that boat. I really don't see where hundred or 75 RPM deserves much attention

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Kinda where I was headed.....