WOT question

rslifkin said:

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.

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Absolutely understand that different manufacturers have different warranty requirements. That's why I didn't make a blanket statement. Guess JD is more realistic in this regard.

Ted

The engine's governor controls RPM rate and limits maximum RPM, assuming sufficient fuel and air flows are available.

High idle term

DavidM said:

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

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Sorry David not picking on you specifically but I’m seeing this constantly throughout the form and was using it as an example. Was wondering why do people keep using the term high idle to describe max no load rpm. High idle is either a term used to diagnose a problem with an engine when you cannot get the engine down to its rated idle speed or an option or setting on some diesels to run Small accessories off of a main engine while it’s idiling and not under load such as alternators, air compressors or hydraulic pumps just to name a few since most of them are designed to run most efficiently at a certain rpm which is usually higher then a regular idle but no where near a max no load rpm. Btw Im certified to work on and have gone to specialty schools for Cat, Cummins, Detroit diesel, yanmar and mercury outboards( eh it was free so why not much prefer diesels) . Also when it comes to loading of engines I’ll just use this as an example since The manual for a ford Leighman is the last manual I’ve read and know it off the top of my head. Without getting too deep into things there are generally different ratings for engines such as max no load rpm, max load rpm, and max continuous load rpm. And there may or may not be some sort of overlap between them. A ford Leighman 120 has a max no load rpm of 2650 . It’s max load rpm is 2500rpm at 120hp and it’s max continuous load is 2500 rpm at 114hp, so how you would read that is if you wanted to be able to use max rpm continuously you would need to have it slightly under propped so at 2500 rpm your propeller is only requiring 114hp from your engine. If you were trying to have it use all 120 hp at 2500rpm it should only be run at that load for a specific amount of time without any damage Or accelerated wear to the engine. That time is also generally specified in the manual, which it is not in the ford Leighman manual. Considering that it is not specified I personally would not use max load rpm for more than about 3 hours. That seems to be on the lower end for a time limit on those types of ratings but that’s just my oppinion on that, when it comes to a ford Leighman if an engineer told me less I’d defer to his judgment. Also while on the same thing the max continuous hp rating would be 114 so That would mean if you have the propeller set up to load the engine to 120hp at 2500rpm the max continuous rpm would be at the rpm curve for 114hp.

Your rated RPM should be hit while carrying your typical load. So saying 7 POB might reduce RPM is not correct . You should hit rated RPM with full fuel ,load and typical seas state.

Dont over think it, the engine needs to be able to hit whatever WOT is stamped on it. It should be able to obtain that with whatever max load you think you might be carrying around.

If you cant hit that number, then have some pitch taken out of the prop then try it again. Dont talk yourself out of this by over thinking it.

Missing WOT by 100 or so rpm's wouldn't stop me from buying a boat, but I would get it fixed as soon as possible so I can move on to stressing about some other stuff.

Pat T said:

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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Hitting WOT is done rarely. I wouldn’t worry about a 4% under rev, specially if full fuel, and full boat. Another consideration could be a clean bottom and clean props.
Those could make the difference.

Arc said:

Your rated RPM should be hit while carrying your typical load. So saying 7 POB might reduce RPM is not correct . You should hit rated RPM with full fuel ,load and typical seas state.

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That’s why Many say 50 to 100rpm over rated is ideal. Then when you do get the max load and bottom growth you may still not be overloaded w the lever “on the pin”. Loads and growth are given variables on all boats. And they increase 10 times (or more) as often as they decrease.

Actually for Cat 3116 engines with serial numbers beginning with the prefix 4KG, 2800 is the max rpm you should see, ever. The governor which is built into the injection control will limit the rpm. The "high idle" speed (full throttle with the gear in neutral) setting should be 3075 rpm, per the engine data plate.

It sounds to me like the governor setting might be a little low, which is fine. I'd be more concerned with the cooling system. Can the engines sustain full power without overheating? Fouled heat exchangers and failed seawater impeller are common.

You want to be able to count on getting full power to outrun weather, for example.

I also recommend getting Caterpillar S-O-S analysis of both lube oil and coolant. That is the gold standard in determining engine condition

Also, be aware there is a service bulletin on 3116 and 3126 engines directing the use of MONO-GRADE SAE 30 or 40 line oil exclusively. All oil should be API symbol CF or higher.
Do NOT use multi viscosity oils such as Shell Rotella 15W-40. Caterpillar sells a product called SAEO which is blended for 3116 and 3126 engines. My Cat dealer sells it at the same price I pay for Mobil Delvac.

Good Luck! Feel free to ask questions - I've been working on Cat engines a long time.

Interesting post so far, but I went back and read the original post - in summary on a heavily loaded boat, with unknown bottom and prop cleanliness, and engines of unknown age with tachs of unknown accuracy it was 100rpm short of its WOT. 4%?

I reckon those poor old engines need a bloody medal, not people suddenly ripping off their props and attacking them with a hack saw.

Yes I understand the importance of over and under propping but in this case I think its close enough for Government work (to quote a saying I learnt when living in the US)

I usually agree with Ted and this is no exception. Those 300 HP. 3116 are great engines, I had a boat for 20 years with twins and had very few problems, biggest problem was a tiny hole in a fuel line that JB Weld took care of. My boat would hit the top RPM as long as it wasn’t heavily loaded when heading to the Bahamas. Those few hundred RPMs never worried me.

Service the engines as needed and enjoy it.

So thanks again all. I am learning more. I feel a little better about the possibilities of this boat but you understand the feeling about forking over big $$ - you want to be sure as can be. I will go back for another look at this boat soon.
I have inquired about getting engine surveyor and have talked with a supposedly certified CAT guy and they did mention oil & coolant analysis as well as blowby. And yes using photo tach for rpm.
BTW John, engine serial # prefix is 4KG on one engine and 1SK on other - I am hoping that the governor setting is a little low as you and others mention. You also mention high idle spec - If high idle is 3075 and these engines hit 3000 then they are missing by approx. 2.5% so maybe again a setting that can be adjusted? And this is what last survey had for coolant temp at 2700 rpm - 180 degrees port, 187 degrees stb.
Keep those comments coming!

I have 3126s and the load/rpm you’ve described is well within tolerance. While 2800rpm should be the target, there are many variables that would affect that number. In order...check/verify the tachs as being accurate(most are not). Next is the load...don’t know your fuel quantity at full but it’s probably in the 2500-2800lb range and with 7 adults...it’s a lot of weight. Next is the environmental conditions...wind? chop? Next is freeboard...do you have a big flybridge enclosure? Next is bottom paint condition...a few barnacles go a long way in destroying performance. Next is fuel filters...clean filters-better fuel flow- better performance. Bottom line...check those items before you ever start tweaking props.

The Cat 3116s have some interesting histories. Check boat diesel .com. Personally, being able to achieve rated RPM plus 25-50 is desirable IMHO. These engines do not tolerate an over heat well, which could arise from the vessel having a dirty bottom, too much weight or too "big" of props.

Cat engine surveys

Your engines probably came off the assembly line in Mossville IL and were run at wide open for at least an hour under load. A cat marine engine dealer can get the history if you give him the serial numbers. If you want a really good survey contact Tim Caruso in Stuart, FL who is the best in the world and ONLY does Cat marine engines. His company is Diesel Engine Analysts (771 285-6770). In most cases a good engine survey pays for itself and it is a wonderful learning opportunity. The high idle is on the engine tag and the engines should meet this number since the load and hull condition are not a factor, but the throttle control is a factor. If your surveyor did not use a photo tach you had the wrong guy.

Interesting stuff. Failing to hit a number could be indicative of a problem. Would need to be > 10% for me to be concerned. Good responses by many folks here. I'll suggest WOT performance is mostly about understanding if your boat (hull, prop or engine) has degraded performance (which'll cost you $ later.)
I specifically added 2" of pitch when I replaced my props and very happy about it. Yes, I did lose a little top speed but couldn't even tell you how much. Why? Because I would never run my engines at that RPM anyway. Across the rest of the operating range, I gained a knot. That's because the available power curve (look up any engine) is substantially linear, a ramp that flattens off at high RPM while the power demand curve (what you need for each speed) takes off exponentially; power demand is essentially cubic in speed & disregarding slip that means RPM. Maybe I should make a sketch but you're probably picturing it: below WOT, the engine is loafing. That can even be "bad" in that you don't run warm enough. This hit me when, running at 1900, I calculated 90HP (total) to move the boat (near hull speed) based on fuel consumption. I was way WAY under rated power: 2 Lehmans rated at 120HP/2500rpm. Of course: it was dialed to hit max rated power at 2500 at speeds where the boat drag would be way higher. At lower speed, power demand falls of exponentially, *way faster than available power.* For instance, 1650RPM instead of 2500RPM? 50% less RPM and boat speed but that's more than a 3x reduction in power. Engines loafing at 40hp each. I wanted more efficiency and therefore to operate closer to rated power in the heart of my speed range (1500-1800). Prop guy argued. Shop guy argued. "You'll never hit WOT." Did it anyway & very happy. A bit overpropped maybe but cruising faster at lower RPM. top of the power curve is flat. You won't hit WOT but very close to wide open Power, so you won't even lose much speed. Just like on the freeway, just like a long bike ride: if you're not running flat out, consider a taller gear.

Have never taken ours to WOT
Why would I?
WOT just burns a shittonne of fuel for no real speed benefit
She does 8 knots at a fast idle - mission accomplished.

CAT 3126 Engines

I have owned two trawlers with 3126 Cats and both ran flawlessly. I never cruised over 1300 RPMs except at least once a month I ran them up to WOT for 20 minutes to test the change in cooling temperatures and kept a very accurate record. I wanted to purposely stress the engines to see if the cooling system was in good condition which is critical on the 31xx series. In some ways these are fragile engines and they must never be allowed to overheat. Anyone buying a boat with these engines in my opinion should hire the best Cat mechanic he can find. CAT has specially trained and certified mechanics for marine engine surveys and they have to re-certify annually. The 32xx engines did not do well with high speed boats.

Simi 60 said:

Have never taken ours to WOT
Why would I?
WOT just burns a shittonne of fuel for no real speed benefit
She does 8 knots at a fast idle - mission accomplished.

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We never ran at WOT either.
But a short WOT test does give a baseline when you might want to help determine things like hull condition, and prop running gear condition.
Of course on boats that have faster speed WOT is more of a concern.

Why WOT?

Cat says you can run at WOT for one hour out of 10 - it is in the engine manual. By running at WOT you will stress (not damage) the engine to see if there is a weakness that is just waiting to happen. As an example one time when I did this it blew a water hose off the engine due to a broken hose clamp. Sometimes I get a slight increase in water temperature informing me the coolant system needed cleaning. I prefer to find the weaknesses under my control then when I am not 50 miles out in the Gulf.