Governor/WOT?

Longtime lurker, first time poster. I have been following this forum and others as well as researching and studying the market for almost a year now towards the purchase of a used trawler. Came across one of interest to me recently and when I asked for add'l info on the Lehman 120 hp engine a 1975 34' CHB tri-cabin the responder also stated " gets close to governor speed at WOT (wide open throttle) under load". Perhaps someone could explain to me what this means. Many thanks

Welcome Rio - It's always hard to interpret what other folks mean but here's my shot.
The engine has a governor that restricts the maximum rpm of the engine. If you rev up the engine w/o a load it will run up to but not exceed that rpm.
If the engine is weak or the boat is not prop'd properly it will not get close to reaching this governor rpm.
I believe your 'responder' is trying to tell you the engine is strong, prop'd correctly and everything working as it should.

Next...

Anode, I read it differently. The responder said the CHB is possibly over propped, or the engine is shot, or the fuel system is not properly set up, or filters are plugged or the linkages are out of whack thus will not achieve full RPM.

Achieving full RPM is a vital check box when surveying and evaluating vessels. If full RPM cannot be achieved, buyer beware until certainty established as to why not. Herein lies the other side of the coin when trawler owners intentionally over prop - time to sell arrives and a potential buyer is rightfully curious as to why full rated RPM cannot be achieved. Taking the seller's word for it is simply not enough.

Sunchaser - You may be right. Depends on what 'gets close' means. I took it to be 25 or 30 rpm. If it's it's 500 or 600 rpm then it's a different story.
Guess Rio's next question to the seller should be - 'what exactly does gets close' mean?

I think the usual governor speed is 100 to 300rpm over the rated speed but it would seem to me that it must be far enough over so one can get an indication how much one is underpropped. This knowledge is useful when you get to the prop guy and need to tell him how much pitch to increase. And if you want to let the prop guy make that call you'll need to tell him what the governor cut-off rpm is.


Since a lot of people that shop for trawlers do'nt know about these things perhaps he's trying to give the impression he's a very knowledgable skipper. People often talk over other people's heads to make themselves appear smarter. However if this situation is as Tom suggests the intent is to cover up problems w the boat either on purpose of by accident. But if this situation is the way Anode states it is as I interpreted it to mean that the boat is propped properly and has not been overproped. Just say'in

Also, in my opinion every trawlerman should know what the rated rpm is, the WOT rpm is (under load), and what the governor speed is without load.

Should your boat be propped to hit the rev limiter? Should you run your motor at the rev limiter? I'd answer "No" to both questions.

I just talked to American Diesel this week. The Lehman 120 should turn 2,500 WOT under load and 2,750 WOT no-load.

Thanks

Thanks for the input, will definately mention this to surveyor if we get to that point as i haven't physically seen the boat yet.

My John Deere 4045 normally-aspirated, 80 h.p diesel has a rated maximum RPM of 2500 but the governor limits it to 2400. Max RPM (2400) can be achieved with full load resulting in exceeding hull speed by a tiny bit in my heavy displacement "trawler". Normally cruise between 1400 and 2000 RPM. ... This appears to be copacetic, or am I misunderstanding?

Mark,
If your rated rpm is 2500 you should be able to reach it at WOT and your governor limited engine speed should be about 200rpm higher. I wonder how your governor speed came to be less than your rated speed. I'd check your throttle linkage and your tach. Something is not copacetic.

I don't know why one would run an engine beyond its rating. Sounds like abuse to me. Do diesel engine manufacturers recommend operating higher than specified maximum RPM, even on a test basis?

Regardless, the JD engine has met my performance expectations: capable of moving the boat at maximum expected speed.

The load on a marine engine is different than the load on an OTR truck engine. If you think about what happens in a truck, as you enter a hill the load increases to the point where the torque available is no longer sufficient to overcome the driveshaft resistance and the vehicle starts to slow down, eventually you pull a lower gear and start all over. That doesn't happen in a boat. The prop provides the resistance and that resistance doesn't change measureably. There's probably a theoretical change due to prop fouling over time but for our purposes the resistance from the prop is constant. So one of two things happens as you increase the throttle - either the engine torque is greater than the prop resistance and the RPMs keep increasing until you hit the governor OR the prop resistance is great enough to prevent you from reaching full governed (WOT) RPMs. Ideally the two things will happen at about the same time - you will hit the governor and at that point the engine won't have sufficient torque to turn the prop any faster so either way, the RPMs won't increase any more.

If you are overpropped you will never reach full governed RPMs and you will be constantly overfueling with attendant higher EGTs and possible engine damage (burnt valves/melted pistons). If you are underpropped then you will never make use of the full HP available from your engine, even at WOT.

bobofthenorth said:

If you are overpropped you will never reach full governed RPMs and you will be constantly overfueling with attendant higher EGTs and possible engine damage (burnt valves/melted pistons). If you are underpropped then you will never make use of the full HP available from your engine, even at WOT.

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If that's so, I'm properly propellered.

Mark,
You're not "properly propellered" until:

1. Your engine reaches it's rated rpm at full throttle (plus or minus about 100rpm).
2. When you have your boat in neutral and advance the throttle all the way (assumably w a warm engine) the engine will stop increasing rpm at an rpm of about 200 to 300rpm above rated rpm.

In your case 2500rpm at WOT in fwd gear and about 2700rpm in neutral. If the engine speed (rpm) dos'nt stop increasing when you get to 2800 rpm it should probably be adjusted to a lower rpm. Talking to a John Deere rep about the governor limiting speed FIRST would be the best thing to do as all engines are different.

Some people over-prop intentionally such that their engine will not reach rated rpm. This is a bit of an overdrive and since you're basically going up hill in a boat (engine load wise) it's not a good idea and I do'nt think any engine manufacturer will recommend it. The over-propping is done to decrease fuel consumption and engine noise.

Your engine stops increasing engine speed when it reaches 2400. It could be doing that:
1 Because the boat is a bit over-propped or:
2 because the governor is limiting the max engine speed.
My guess is that you are correctly propped. It's OK to be 100rpm over propped if one inch less pitch of the prop produces a 200rpm increase (or more) in rpm. If one inch pitch decrease would produce your 2500rpm goal that would be ideal but it would probably increase 200rpm or so. It's ideal to be at 2500 WOT w full load and half a dirty bottom. Fine tuning can be achieved by trimming a tad bit of blade area off the prop the next time you (during the course of regular maintenance) have the prop off the boat. A good prop man will know how much to take off. Then your boat will run perfectly.

Bob,
Very good point about ONLY being able to use/develop your engine's maximum rated power ONLY if propped to the engine's rated speed (rpm). Anything other than propped to the rated rpm will result in a loss of power.

Mark wrote:
"Do diesel engine manufacturers recommend operating higher than specified maximum RPM, even on a test basis?" YES......but only for a few moments to establish that things are set and adjusted correctly for best engine performance.

Oh and Mark,
Do'nt let a prop man talk you into a cupped prop. They are for higher speed boats only. Seems there are a lot of prop guys that are rather cup crazy.

manyboats said:

Bob,
Very good point about ONLY being able to use/develop your engine's maximum rated power ONLY if propped to the engine's rated speed (rpm). Anything other than propped to the rated rpm will result in a loss of power.

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I experienced that one first hand on my ski boat which was propped for about 4200 RPM when we bought it. Max HP is at 5400 if memory serves so when I changed to a 4 blade prop I also went to a slightly finer pitch. None of this really matters but when you want to catch an extra couple MPH for racing your buddy's boat then you want to use every bit of available HP.

manyboats said:

Mark,

2. When you have your boat in neutral and advance the throttle all the way (assumably w a warm engine) the engine will stop increasing rpm at an rpm of about 200 to 300rpm above rated rpm.

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Eric - Push an engine in neutral with no load 200 to 300 rpm over its rated WOT rpm, even a diesel. I've not heard of that before... Isn't that a bit severe? Just wondering.

No Art it's standard procedure. Ask any diesel mechanic. Seems crazy but one needs to know at what rpm the governor is set for or if it's working at all. Like Mark. He dos'nt know if he is slightly over propped or if his governor is limiting his engine speed. If it's the latter he could be under propped. Or if he could be over propped and his governor is'nt working at all. One should check the throttle linkage to insure the throttle shaft is being rotated all the way before doing either the max load or the no load test. My Mitsu is right at 3000 (it's rated speed) in gear and 3200rpm in neutral. What's yours?

Huh? If the governor is set for a maximum of 2400 RPM, why/how would it have a higher RPM in neutral (no load)?

Eric

My twin screw 350 cid 255 hp gasoline engines would throw rods, pistons, and valves right through their blocks if I pushed WOT with no load... they'd simply, internally disintegrate! And, no, they do not have a governor. Don’t need one – I simply don’t push my engines too far for too long

When at running temps, each motor can easily rev to 6K rpm on 1 second minus throttle bursts, while in neutral. WOT in cruising/planing mode is 4,400 to 4,700 rpm depending on boat weight. Exactly as they are rated... props are correct! Our Tolly is doing over 22 knots at WOT. Briefly Sucking Petrol Like a Crazy Girl!

Reason I asked about your statement regarding pushing to 3000 or so rpm at WOT in neutral for diesels: Years ago when I was around Perkins (helped install a new one in 1966 and traveled on the boat for years) I clearly recall being warned to not rev too high with no load applied. That said... times change. But, I still can't get my head around WOT with no load. Seems to me the diesel motor's interior parts and parcels would just be a jittering and shivering into one another with no load at that high of rpm???

There is a RATED RPM which is the rpm level the engine mfgr. states you can pull full or maximum hp. That's what you see in your brochures,
X hp @ Y rpm. That point is also the point the propellor should be sized for and allow the engine to reach when the boat is FULLY LOADED with EVERYTHING and EVERYONE. Not that it is ok to run at that rpm for very long for pleasurecraft ratings. There are ratings that will allow that but then the final hp allowed and usually the final rpm is lower than that commonly offered for pleasurecraft. At that point the engine will be pulling full fuel and making full hp if the propellor is correct.

If not then you are either overpropped or underpropped. Overpropping can be a serious problem, causing engine damage, if not handled correctly and carefully.

There is also HIGH IDLE or WOT, NO LOAD [out of gear] This is the point the mfgr. sets the governor for to stop any further rpm increase. At this point very little fuel is actually used. This HIGH IDLE rpm is always somewhat higher than the RATED, often 200-300 rpm. This is tested for with a fully up to running temperature engine and then only for long enough for the governor to stabilize the rpms, get the reading and then back off, usually on the order of a few seconds. There is no damage done to the engine and this test is normal and for diesels necessary to check governor operation. All engine mfgrs publish this spec. although it may or may not be in the manuals you get unless you purchase beyond the usual owners manual. SOmetimes it is actually stamped right on the ID plate.

There is also DROOP. That is the difference between the HIGH IDLE and the RATED. When the governor is approaching the HIGH IDLE it actually starts to defuel the engine. If the two were set at the same point the engine would never reach max hp since the governor would need to be defueling before it actually reached RATED to be able to stop further rpm increase at RATED.

As Eric pointed out with out doing these two tests plus a few other checks you will usually not know if you are under or overpropped.


Art
you are correct about your gas engines but they have no governor. The closest you would come is a revlimiter as I used to know them in race cars. Those things would start cutting the spark when a predetermined rpm was reached to avoid blowing the engine. If a high power shift was missed , before those limiters, you almost guaranteed you would be redoing/replacing the engine. I've seen that happen and in boats too when someone panicked and put the gear in neutral when the throttle was set for cruise speed. Usually a response to seeing a log right in front of you resulting in an extra hole in the block, wrong lever.

A diesel governor won't allow that to happen even with a sudden unloading. Of course the real reason is to stop overevving, whether from accident or intention.

Going to "high idle" is a standard test on diesels of the size we use. Yes it is somewhat scary the first time you do it, at least it was for me.

I remember when I repowered my old Mainship in 2000 with a brandy new Cummins 6BTA , I had just finished getting power to the starter so I cranked it up and it was running maybe 5 minutes as I was checking for leaks, etc.
The sales/tecnician guy from the Cummins distributor shows up to see how my install is going. First thing he does is say "let's see how this thing checks out", and runs it up to high idle and checks the rpm with a hand held. "3050. Perfect" he says.
Kinda scary the first time.

C lectric's, Eric's and Jay's responses should be bookmarked for the next inevitable "no, no not full RPM in neutral!" or "should I prop for full rated RPM?" diesel engine discussion. Nice to have experienced hands on board.

My 120 lehman would reach only 2200 in neutral or in gear still running great after the 1500 hrs I put on it

C lectric,
I think the newer outboards have "rev limiters". Seems to me my e-tec Evinrude book says something about that. Do you know about that?

jleonard,
I thought it was just plain crazy (like Art) and was very nervous the first time I did it. The engine was new too.

Tom,
Thanks

motion30
Is 30 the length of your boat or your age? Must be your age as the boat looks longer. You could fix that problem you describe but some people (like FF) like to overprop. It's sorta OK if you NEVER run your engine anywhere near is rated power and rpm. I'm guessing you run at 1500-1600rpm. It would probably be bad to run that engine at 2000.

Motion 30
There are lots of vessels out there where the throttle stops limit RPM to say 85% of full rated RPM, Good thing too. Many of these vessels are over propped and if they were run at full rated RPM would overheat and or suffer from too high EGTs. Tony Athens had a response to this same question today on boatdiesel regarding re-powering an old Chris Craft.

Have never heard of this on a marine engine...if it's rated at xxxx rpm...that's what it is rated for if properly propped/loaded when you push the throttle to full (properly adjusted).
Now the question is..what rating...diff manufacturers call it diff..but if it's continuos duty (commercial) or intermittent (pleasure craft) etc..etc... the the "rated" rpm is usually lessened and governed so.
If that's what you mean..then I agree...

Interesting Discussion

My Detroit's are rated at 2800rpm. My high idle is set at 2300rpm digital tachs. My full rpm in gear is 2300rpm. I surveyed two 48's with Detroit's both were set identically. Hull speed for is reached before max rpm @1800rpm. I was told by my mechanical surveyor this was ok and probably set this way as full rated power was not needed. What say you?

Tom,
Thanks

motion30
Is 30 the length of your boat or your age? Must be your age as the boat looks longer. You could fix that problem you describe but some people (like FF) like to overprop. It's sorta OK if you NEVER run your engine anywhere near is rated power and rpm. I'm guessing you run at 1500-1600rpm. It would probably be bad to run that engine at 2000.[/QUOTE]

The boat with the lehman was a MT 34 , and yes cruised around 1650rpm she is sold now, new boat has 3208t would love to have twin 120s

You're very right psneeld and usually in your owners manual the continuos limitations are laid down. I think for my Mitsu it's up to an hour at WOT. Do'nt remember what the max continuos limitations are but I think it's 2800rpm. I'd run it at 2800 all day long but I'd either look at the book or just not run continuos above that. One needs to keep in mind that if you're 5% down in rpm you're prolly at least 20% down on load/power. My engine has an industrial rating of 37 hp at 3000. Vetus markets a marineized version at 42 hp and Westerbeke sells the exact same engine at 44 hp.

Scary,
I'd say it's not right as you do'nt know how you're propped. You may be quite overpropped and have no way to know. Without the governor you may top out at 2400.

Sometimes you can't reach WOT due to excess back pressure on the exhaust.
Nothing to do with the prop. Before re propping I would check that first.

SD