Engine will not rev above 2,100 RPM - SailNet Community

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  #1  
Old 06-02-2008
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Engine will not rev above 2,100 RPM

Been having the above problem now for quite some time. Inititially thought it was the maximum RPM governor. However adjusted that today and still could not get the engine to rev higher than 2,100RPM when the engine is under load.

Under no load the engine is happy to rev up to 3,500RPM where it hits the maximum RPM governor (which is actually just an adjustable bolt on the throttle). However as soon as the engine is in gear it will only go up to 2,100 RPM. You can advance the throttle further however it does not change anything.

I also do not think it is the wrong propeller as the previous owner measured the prop and took it to a specialist and the advice was that it was about the right size and pitch for the boat.

The engine is a 98HP Nissan SD33 diesel. Maximum RPM is 4,000RPM and maximum continuous RPM is 3,200.

The engine should be easily able to get the boat up to hull speed (7.5kts) as the recommended engine for this boat is 50-80HP. At 2,100RPM the boat is doing 6 kts. I reakon if I can get the RPM up to 3,000RPM the boat should be pretty close to hull speed.

Anyone have any ideas why the engine will not rev above 2,100RPM?

Ilenart
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Old 06-02-2008
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Not withstanding the measurements the previous owner made it really sounds like a wrongly pitched propeller. If that is the case you will be grossly overloading the engine so I would expect to see a lot of black smoke and it take a long time to rev up to the 2200. Are you seeing that?
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Old 06-02-2008
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Nope, no black smoke and the motor revs straight up to 2,100RPM and then no higher.

Today we took the boat out and rev'd slowly up to 2,100. Then put the engine into neutral and the engine reved up to 3,500RPM no worries. Then put the engine back in gear and was back upto 2,100RPM within about 45 seconds. I played with the throttle a bit and it easily goes to 2,100 RPM but no higher.
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Old 06-02-2008
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If you are sure it's not the prop, then I would look at a fuel starvation issue. Change the fuel filters and maybe get the feed pump checked for flow volume. Odds are you are either putting too much load on the engine [prop] or not getting enough fuel to it to allow full throttle.
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Old 06-02-2008
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If no load RPM is 3500 and loaded maxs RPM at 2100 it's the prop.

The ONLY difference is the load on the engine, ergo the cause is the load on the engine. Life really is that simple.

OTOH, if max rated RPM is 4000 you should be able to hit 4000 with no load. That means that fuel starvation or air starvation might also be issues. My bet is the PO upsized the engine without consideration for the supply side. Of course he might also have put a monster alternator on it which is overloading the engine but you haven't mentioned one.

Nothing in the world of sailing gets you in more trouble that believing the previous owner without question (e.g. "the rigging is fine, don't worry about the rust marks" is right up there with "I measured the prop and it's correct")
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Old 06-02-2008
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Quote:
Originally Posted by Ilenart View Post
I also do not think it is the wrong propeller as the previous owner measured the prop and took it to a specialist and the advice was that it was about the right size and pitch for the boat...Ilenart
Get a second opinion from a new expert. Clearly the P.O. was having a problem if he sought advice -- unfortunately is sounds like that advice was perhaps less than scientific. Prop pitch is almost a black art, and opinions vary. My guess is that the prop is over-pitched.

For your consultation, have the prop size and pitch in hand, as well as other important specs (engine hp, red line, displacement, etc). Also, sometimes props lose or gain pitch from the original spec and need to be re-tuned, so the specs may no longer be accurate.
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Old 06-02-2008
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I donít know if this will help but I have fitted several Max Prop propellers (and are very satisfied with them including several customers), when they came down from the manufacturers they had a setting that they should be set for that vessel/engine and I have normally set them up at that, however most of them we have had to re-set them slightly one way or another.
In one instance we pitched the prop more and the boat gained about a knot extra speed, but the engine revs were down from 3400 flat out to 2800, the friend was very happy as he could now keep up with us!!, but the engineer who looked after the boat said it was overloaded, however its run like that for over 10 years without any problems.
On our boat, which is a similar size, engine is a ford 90bhp and we only get 2000 rpm at best however itís a much lower revving engine (max cont revs 2500) and at 2000 rpm we are getting 8.5 Knots.
When we first fitted the prop, on the settings from the manufacture we were getting 2300 rpm but only 7.5 Knots, we pitched the prop a degree more and gained the extra speed and less revs, we hardly ever run the engine over 1800 rpm and for more fuel economy (while the price is high) we have been running it 1500 rpm giving the said 7.5 Knots.
It sounds like an incorrect prop size, what it the ratio of the gearbox and what size prop do you have?
Also could it be a problem in the gearbox and as suggested by others a fuel starvation? How clean is the bottom/prop? I knew someone who had a shortage of power under load and go the fuel pump re-calibrated and all of a sudden regained the lost revs (I think it was about 5-600) it could be this that causing your problem.
Go to the boat manufacturers and see what prop should be on this set-up, or a well established prop manufacture should be able to give you a guide.
I hope this helps, sorry to go on a bit.
Please let us know how you get on
Best wishes
Michael
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Old 06-02-2008
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You said the governed (max) rpm is supposed to be 4000, but you are only getting 3500, I think that is part of your problem. I'm no expert on this engine but I really doubt that the governor is an adjustable stop screw. The governor should be an internal mechanism in the fuel injection pump. I would get the manual or consult a dealer for the engine, but I don't think you are getting full throttle/HP. You should be able to get 4000 no load. By restricting the throttle with that stop screw I think you are limiting the inj. pump from delivering enough fuel.

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Old 06-02-2008
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You'll need to approach this step-by-step. It sounds like the prop to me, but it could be "all of the above" creating what looks to be a single-point problem, if you see what I mean.

Large alternators overtensioned and foul bottoms could easily steal a few hundred RPM. What is your RPM at idle, by the way?
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Being really simple, are you sure your throttle cables are running freely and well adjusted to WOT ? Then, check your prop system with a trusted mechanic. Then, check engine compression.
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