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Old 06-05-2020, 11:22 AM   #1
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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.
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Old 06-05-2020, 11:36 AM   #2
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Certainly sounds close enough to me especially considering the load you were carrying
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Old 06-05-2020, 11:39 AM   #3
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I'd also plan to check that the tachs are accurate before worrying about it.
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Old 06-05-2020, 11:58 AM   #4
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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.
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Old 06-05-2020, 01:52 PM   #5
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Old 06-05-2020, 02:31 PM   #6
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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
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Old 06-05-2020, 02:48 PM   #7
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OK, I see that the Hi rpm got to 3010 and 2980. So doesn't look like these were tuned down.
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Old 06-05-2020, 03:29 PM   #8
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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.
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Old 06-05-2020, 07:50 PM   #9
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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.
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Old 06-06-2020, 06:42 AM   #10
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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.

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Old 06-06-2020, 08:31 AM   #11
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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.
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Old 06-06-2020, 08:36 AM   #12
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Quote:
Originally Posted by OldDan1943 View Post
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.

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.
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Old 06-06-2020, 08:39 AM   #13
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Agree with Ted above 110%. Unless you are regularly going to try and push the boat past hull speed you, are fine.
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Old 06-06-2020, 08:42 AM   #14
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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.

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Old 06-06-2020, 09:16 AM   #15
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Remember, making a beer run to the islands with 120 cases of beer onboard just might effect the speed of the boat.
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Old 06-06-2020, 09:35 AM   #16
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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.
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Old 06-06-2020, 09:44 AM   #17
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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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Old 06-06-2020, 02:58 PM   #18
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Quote:
Originally Posted by Nomad Willy View Post
Good posts all .....
But I disagree w Ted. Iím thinking if youíre running 15% Below WOT you shouldnít be overpropped at all. And Ted dosnít say if itís 15% rpm, load or what. If one is overpropped 50% of load itís 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 itís 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 (Iím at 50%) and there 100rpm below rated shouldn't result in overloading. Perhaps close to it though. But thatís 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 wonít be overproped. Within reasonable numbers of course.
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
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Old 06-06-2020, 03:15 PM   #19
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OK Ted, Good clarification.

One function way more linear that the other.
One usually dosn’t see the prop hp curve.
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Old 06-06-2020, 04:40 PM   #20
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Was there any change exhaust smoke?
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