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Old 28-09-2015, 11:59   #16
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Re: Engine Tachometer shows lower RPMs than expected

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Originally Posted by sailalibi View Post
So it's OK to Rev up the engine in neutral?
I would make sure it's warmed up, and pay attention to the RPM as you advance the throttle, but yes unless it's broken ALL Diesels have governors that prevent over revving.
There is a one in a million chance your governor is malfunctioning though.
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Old 28-09-2015, 13:14   #17
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Re: Engine Tachometer shows lower RPMs than expected

Many years ago I bought my first sailboat a Morgan 32 with a Yanmar engine. Had the boat surveyed and the surveyor compared engine rpm on my tachometer versus what he measured. Very close.

Taking the boat to her new home took forever. Even though the engine rpm was at max cruise.

I later checked and found the tachometer was accurate at low rpm but above a certain rpm it was showing high rpm that was twice what the engine was turning at. No wonder it took forever. I thought I was maxed out when I was actually at half power. A new tach took care of that.


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Old 28-09-2015, 14:11   #18
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Re: Engine Tachometer shows lower RPMs than expected

I guess we are still waiting for what the RPMs are in neutral. That should eliminate the over propping or a tach. or governor problem. The latter seems unlikely. Depending on the results. Everything else at this point is speculation.
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Old 28-09-2015, 14:43   #19
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Re: Engine Tachometer shows lower RPMs than expected

I would check to make sure I wasn't over propped
by checking what is specced and no black smoke then I would live with the tach the way it is. It doesn't matter.
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Old 28-09-2015, 16:28   #20
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Re: Engine Tachometer shows lower RPMs than expected

get a photo tach and check the rpms at the frond pulley on the crank. if there is a difference then change the tach if not reprop. check the rpms first...ie...confirming the accuracy of the tach in the control panel. getting a hand held photo tach is the cheapest way to start your trouble shooting and after wards you have an other tool that can pay for it's self in the future
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Old 28-09-2015, 16:42   #21
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Re: Engine Tachometer shows lower RPMs than expected

Is the tach off the engine or the alternator?
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Old 28-09-2015, 18:00   #22
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Re: Engine Tachometer shows lower RPMs than expected

[QUOTE=Pizzazz;1924312]I have a Hunter 31 with the Yanmar 2GM20F fresh water cooled engine. The engine is specified as a 13 HP engine at 3400 RPM, max power 16 HP at 3600 RPM and the boat hull speed is just short of 7 knots (26.25 LWL).

In reality, I get 6.5 knots at 2200 RPM (as shown on the tachometer) and full throttle is 2800 RPMs, 7.1 knots boat speed. It seems as if the tachometer is reading low. The Yanmar manual says the tach is not adjustable.

What should I do? Is there a way to adjust the tach to measure real RPMs or to check the RPMs independently of the tach?

Thank you,

Vassil[/QUOT

I agree with the remote tach, and I would suggest that you check your fuel delivery. You may not be giving your engine the correct amount of fuel under load to achieve max RPM'S at WOT. The other problem may be loading, if you have lots of stuff you may be displacing more than one he designed weight. This will cause the engine to labor and act like you are "over propped."
A friend (excellent marine diesel mechanic) told me that when you over prop an marine engine it is like dragging sea anchor behind your boat, the engine has to work harder at any RPM not just MAX RPM'S. But it is best to check the cheap stuff first and move to the more expensive.Good luck.
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Old 28-09-2015, 18:01   #23
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Re: Engine Tachometer shows lower RPMs than expected

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Originally Posted by Stu Jackson View Post
Is the tach off the engine or the alternator?

All the yanmar gm series are off the engine.

I too suspect over propping. We have the 3gm30, max throttle under way is about 3,100. Our surveyor suggested getting closer to 3300 or 3400 by adjusting the max prop. We have not bothered and it has been fine.


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Old 28-09-2015, 19:39   #24
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Re: Engine Tachometer shows lower RPMs than expected

Thank you all for the comments. I appreciate it. Checked the engine speed at neutral, it is 2,900 RPM with the lever pushing against the engine block so it is not a cabling issue. It does not look like an overprop either.

I am ordering a laser tachometer from Amazon to check the actual RPMs. I like the lowest cost suggestion (to cruise at 2200 RPM and live with it, especially since I am hitting hull speed). I am a little worried though that the alternator may not be pulling its weight if for some reason the actual RPMs are really that low.

I will let you know once I get the laser tach and check.

Thanks,
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Old 29-09-2015, 07:14   #25
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Re: Engine Tachometer shows lower RPMs than expected

I have a friend with your exact engine. His would not rev up correctly either. Turns out that the kill lever had bent, preventing it from fully disengaging, hence robbing the engine of air, which then limited its max rpm level.


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Old 29-09-2015, 07:39   #26
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Re: Engine Tachometer shows lower RPMs than expected

If your not hitting your rated RPM in neutral, something is wrong, maybe it is the tach after all, next step is to measure actual RPM to see if it's making RPM or not.
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Old 30-09-2015, 10:37   #27
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Re: Engine Tachometer shows lower RPMs than expected

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Originally Posted by Pizzazz View Post
Thank you all for the comments. I appreciate it. Checked the engine speed at neutral, it is 2,900 RPM with the lever pushing against the engine block so it is not a cabling issue. It does not look like an overprop either.

I am ordering a laser tachometer from Amazon to check the actual RPMs. I like the lowest cost suggestion (to cruise at 2200 RPM and live with it, especially since I am hitting hull speed). I am a little worried though that the alternator may not be pulling its weight if for some reason the actual RPMs are really that low.

I will let you know once I get the laser tach and check.

Thanks,
-vs
Pizzazz,

If you are concerned about not achieving maximum charging at a lower RPM you can increase the size of your alternator pulley to compensate.

Most alternators do not start charging until they reach 1,000 RPM alternator shaft speed. 5,000 RPM alternator shaft speed is normally required to reach maximum output.

If you're not sure what the alternator shaft speed is, you can determine this with the pulley ratio. Measure the diameter of the crank shaft or drive pulley and the alternator pulley. Divide the crank shaft pulley diameter by the alternator pulley diameter.

This figure would be the engine-to-alternator RPM ratio. A normal ratio would be 2.5 to 1. For example, let's say we have a 7 inch diameter crank shaft pulley and a 2.75 inch alternator pulley. We would divide 7 inches by 2.75 which equals 2.54 to 1. If the engine was turning 1,000 RPM we would multiply 1,000 by 2.54 which would give us 2,540 alternator RPM.

Jeff
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Old 30-09-2015, 10:56   #28
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Re: Engine Tachometer shows lower RPMs than expected

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Pizzazz,

If you are concerned about not achieving maximum charging at a lower RPM you can increase the size of your alternator pulley to compensate.

Most alternators do not start charging until they reach 1,000 RPM alternator shaft speed. 5,000 RPM alternator shaft speed is normally required to reach maximum output.

If you're not sure what the alternator shaft speed is, you can determine this with the pulley ratio. Measure the diameter of the crank shaft or drive pulley and the alternator pulley. Divide the crank shaft pulley diameter by the alternator pulley diameter.

This figure would be the engine-to-alternator RPM ratio. A normal ratio would be 2.5 to 1. For example, let's say we have a 7 inch diameter crank shaft pulley and a 2.75 inch alternator pulley. We would divide 7 inches by 2.75 which equals 2.54 to 1. If the engine was turning 1,000 RPM we would multiply 1,000 by 2.54 which would give us 2,540 alternator RPM.

Jeff
Correction........decrease the size of the alternator pulley to increase the alternator RPM's
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Old 03-10-2015, 12:54   #29
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Re: Engine Tachometer shows lower RPMs than expected

Bringing closure to this topic. I checked the RPMs with a laser tach ($14.99 from Amazon) and it is all according to the manual. Idle is at 900, WOT at 3,600 in neutral.

The Yanmar tach measures 500 at idle and 2,900 at WOT.

One way to go from here is to start replacing the sending unit, the dampening system of the tach or the entire tach. Another way is to consider upgrading the sender unit with some more modern device that can send NMEA/Wifi output.

For now, I just put a never exceed duct tape marker on the tach glass at 2,700 RPMs

Thank you.
Vassil
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Old 03-10-2015, 13:07   #30
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Re: Engine Tachometer shows lower RPMs than expected

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Originally Posted by Pizzazz View Post
Bringing closure to this topic. I checked the RPMs with a laser tach ($14.99 from Amazon) and it is all according to the manual. Idle is at 900, WOT at 3,600 in neutral.

The Yanmar tach measures 500 at idle and 2,900 at WOT.

One way to go from here is to start replacing the sending unit, the dampening system of the tach or the entire tach. Another way is to consider upgrading the sender unit with some more modern device that can send NMEA/Wifi output.

For now, I just put a never exceed duct tape marker on the tach glass at 2,700 RPMs

Thank you.
Vassil
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