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Old 27-09-2015, 15:46   #1
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Engine Tachometer shows lower RPMs than expected

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
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Old 27-09-2015, 15:58   #2
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re: Engine Tachometer shows lower RPMs than expected

You could be over propped and your tach is correct..
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Old 27-09-2015, 16:25   #3
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re: Engine Tachometer shows lower RPMs than expected

Go to amazon and type in photo tachometer. The cheapo one will tell you your actual engine speed.


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Old 27-09-2015, 17:20   #4
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re: Engine Tachometer shows lower RPMs than expected

Spent & wasted money on repitching. Found my tachos were incorrect & did pull them down to recal. I would check the engine RPM with an optical tacho & make up a "correction chart" if the tacho is reading low.
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Old 27-09-2015, 17:46   #5
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Engine tachometer shows lower RPMs than expected

I'm going to bet your over propped, Truthfully I think most boats that have had the prop replaced are overpropped, I think most people like being overpropped as there is a belief that you burn less fuel and engine noise is less, and as long as you use your judgement and don't overload the engine it's not harmful. Overloading can be harmful though so you have to ensure you don't.
So in your case cruise at 2200, and don't exceed say 2500.
Most props you can read the diameter and pitch on the prop, so dive the boat and see what your prop is compared to what it's supposed to be.

An optical tach is cheap, I got mine off of Amazon, you have to stick a little piece of reflective tape on your harmonic balancer.

One way to get a good idea if the tach is off, is what will she turn in neutral? Should be a little more than 3600, but not much more, so if you get 3600 in neutral but not in gear, tach may be right, or close to anyway.


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Old 27-09-2015, 18:33   #6
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re: Engine Tachometer shows lower RPMs than expected

As A64 says. First question is "What's your max RPM in neutral?"
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Old 27-09-2015, 20:29   #7
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re: Engine Tachometer shows lower RPMs than expected

If you have an inverter you can use it and a light to check rpm. An inverter puts out 60 cycle so an engine at 1800, a multiple of 60, will cause something to stop, like a spot on the front pulley. Very very accurate.
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Old 27-09-2015, 21:28   #8
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re: Engine Tachometer shows lower RPMs than expected

Quote:
Originally Posted by Guy View Post
If you have an inverter you can use it and a light to check rpm. An inverter puts out 60 cycle so an engine at 1800, a multiple of 60, will cause something to stop, like a spot on the front pulley. Very very accurate.
Unless you're in one of the 80% of countries in the world which uses 50 hertz
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Old 27-09-2015, 23:07   #9
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re: Engine Tachometer shows lower RPMs than expected

Since the inexpensive "photo tachometers" have been mentioned here for checking propulsion engine speed, I'd like to add that they are invaluable for checking almost any motor, or motor-driven, equipment.
Most electric motors have their normal operating RPM(s) on the label.

Stuff like air conditioning fans, blowers, whatever. If you establish a baseline RPM for all your motors, it's easy to routinely check them to see if there has been a change. Marine growth on the prop, a bit of line, whatever.

The tach is also good to use around the house on home A/C units, particularly the outside condenser fans, which tend to weaken over the years.
I
f the condenser fan is down more than 50-100 RPM (many run at ~900RPM IIRC), it's time to replace the motor (or it's bearings, unless it's the kind that needs periodic lubrication and you forgot).
Those few RPM's down affect the whole system efficiency a lot.
I've replaced many of them, $60-140 each for the motor (used to be $25-40, better quality too).
You can sometimes upgrade the fan blades too.
A commercial A/C service would gig you $300-400+ for the same very easy job (if they even detect it, short of a total failure; been burned by those con artists too many times, they're like those comments on 'marine mechanics' I've seen here--better to do it yourself if you can).
Newer A/C systems may have a variable speed motor, but there is usually an 'override' to force a set speed that can be checked.
Same for marine A/C and reefer units.

If you suspect actual trouble is developing, an RPM check with the ol' photo tach might pick up a drop in RPM's, often an early sign of bearing problems. If it's the kind with a visible beam (acting like an old "timing light' for spark-fired gas engines), any 'wobble' seen may indicate slop in the bearings or shaft misalignments.

An all-around excellent tool to have.

(on a home A/C, using the tach, paired with an infrared temperature sensor tool to monitor delivery vent and condenser exhaust air temps, the effect of replacing a slowed down condenser fan is dramatic)


Moving on, How about that 'supermoon' total eclipse tonight?
https://en.wikipedia.org/wiki/Septem..._lunar_eclipse

(many of us will likely be gone before the next one like this, saros/metonic/tetradic cycles:
https://en.wikipedia.org/wiki/April_2032_lunar_eclipse)

Great sight to see, even with a partial cloud interference (high 'gauzy' type here, just enough to blur it).

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Old 28-09-2015, 09:53   #10
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re: Engine Tachometer shows lower RPMs than expected

Quote:
Originally Posted by Pizzazz View Post
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.
Im no mechanic so my thoughts are free. I recently repowered from an old Yanmar to a 14hp motor. I have a similar boat and set up to you. On calm days the boat easily achieves hull speed at about 2,100 rpm. In fact, just at idle the boat sits on 3 knots. But Ive several times been really happy that I have plenty of revs to spare for when were pushing in to a 30+ knot wind and big waves. She can still make way at 4-5 knots.
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Old 28-09-2015, 10:41   #11
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re: Engine Tachometer shows lower RPMs than expected

Quote:
Originally Posted by meridian28 View Post
You could be over propped and your tach is correct..



A shallower prop may let you get hull speed, I think you are, and rated RPM with less engine wear and better economy? Just a thought.
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Old 28-09-2015, 10:42   #12
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re: Engine Tachometer shows lower RPMs than expected

You also might double check the bottom and prop for growth which will certainly slow the prop and boat speed. You got good advice here, check the RPM's in neutral and order up an optical tach on Amazon for $15.00.

Please share your results.
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Old 28-09-2015, 10:55   #13
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re: Engine Tachometer shows lower RPMs than expected

So it's OK to Rev up the engine in neutral?
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Old 28-09-2015, 11:01   #14
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re: Engine Tachometer shows lower RPMs than expected

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So it's OK to Rev up the engine in neutral?
Excellent question, as to, if he is over propped or it is the tack or something else?
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Old 28-09-2015, 11:19   #15
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re: Engine Tachometer shows lower RPMs than expected

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So it's OK to Rev up the engine in neutral?
Yes, after it has warmed up.

Reality is that most boats come from the factory with too much prop. There are plenty of arguments that make sense on all sides as to whether running lower RPM's is bad for a diesel and don't want to get into a debate about that, but I currently have an over propped Yanmar that cruises way below the rated cruise RPM for that engine and has done so for close to 5000 hours without a single issue. The key is to get them to temp, and not run them too hard when over propped.

If your engine runs up to WOT at idle, then it is up to you whether you want to pull the prop and have it re-pitched or re-propped. But keep in mind that an extra knot of boat speed can equate to twice the fuel burn and you can change the harmonics of the engine.

Check the Hunter forums for others with the same boat and what they have done, if anything.
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