Temporary Tach?

[Stephen Trapani]

I'm thinking I don't really need a permanently installed tachometer. All I need to do is adjust the throttle governor screw on the motor (a two cylinder, raw water cooled Yanmar 2QM15) so the rated max RPM from the service manual is the same as the max RPM I can get with the throttle. I have two questions.

First, is there anything wrong with this idea?

Second, I have a portable volt/RPM meter on the boat but it doesn't have a setting for two cylinders. I don't have this meter on me now, but I think it has a 4 cylinder setting. It definitely has an 8 cylinder setting. How do I extrapolate and where do I attach the leads?

Currently I have no clue what RPM I am running. I have been gauging where to run it off the engine sound and the color of exhaust til now, but it would be nice to have a frame of reference.

 

[Steve Dion]

If you want to determine your approx. RPM you can purchase a laser tach. It will measure the RPM's from the crank pulley.

You do not want to run your engine at 100% of max rated RPM, so how are you really going to know how fast you are cranking the engine without a tach? You really want to run your engine at about 85% for the max rated RPM.

 

[Don12364]

Sounds as if you might have a tach for a spark ignition engine.
No sparks on a diesel!

 

[Bill Roosa]

DIY tachs

Your voltmeter/tach is used for gas engines. Reads the number of spark plug coil impulses and divides by the appropriate number of cylinders. Since you don't have a coil you have to do some other method. Your volt meter/tach is just a frequency meter so it should respond to a variable frequency source. A little known fact is that one of those outputs (the "A" terminal) that your regulator plugs into (the one that is not used by the regulator BTW) is an AC voltage source. The frequency of the AC is directly proportional to the RPMs of the alternator shaft.

You can run the engine up to max and note the alternator RPM on the voltmeter/tach. It is not going to be correct as we are using it incorrectly. That would be 100%. Half of that RPM would be 50% "throttle and 25% would be 25% throttle. You could construct a card with key RPMs and voltmeter/tach readings so you can return to specific engine RPMs.

 

[RAD]

My two cents.......I didn't have a working tach for a few years that I owned my boat and I put a new three blade prop on the 2nd year I owned it and was pleased with the performance then a few years later I bought a new tach and whoa I notice that I cannot get over 1500 rpm and the boats doing 6 knots.....I know this is not good for the engine and now I know why the transom is always full of soot......so I pull the boat and put the original prop back on and now I see 2500 rpm @ 6.5 knots
BTW I found this universal tach on EBay for a song and a dance

 

[rardiH36]

formatting

... All I need to do is adjust the throttle governor screw on the motor (a two cylinder, raw water cooled Yanmar 2QM15) ....

Hi Stephen:

Aside from your question about the tachometer itself, from what I have read about sailboat diesel rpm's and engine loading, I would rethink about the idea of adjusting the governor screw. For two reasons:

1) From my Seloc repair/service manual that I have which covers the Yanmar 2QM series:

"On all engines, Yanmar stresses the importance of refraining from adjusting governor mechanisms. Many of the adjustment screws are sealed in place with lead-bound wire. These seals are placed on the adjustment mechanisms at the factory, where the injector pump was properly adjusted on a testing machine. The adjustments on the injector pump and governor mechanism are set for optimum performance and should not be changed."​

The idle rpm can be adjusted to your satisfaction, but at to the top end my guess is that your engine is still more-or-less calibrated.

2) Another item is that the engine's max rpm with the gear box in neutral will be limited by the governor. (So the engine won't self destruct.) But with the transmission engaged, the max rpm (below the the max allowed by the governor) will be determined by the propeller. Ideally, if the propeller (when clean and newly tuned) is perfectly matched to the engine and the boat, you will max out at or very near the rated rpm. But if the propeller isn't a perfect match and it won't allow the rated rpm (over-propped), then by adjusting the governor you might be forcing the engine into an even more overloaded condition.

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[Larry14133]

You do not adjust the governor for max RPM you change the prop. If the engine is running to fast you do not have enough prop and if its to slow you have to much. Either will shorten engine life. On the diesel the tach normally attaches to the tach sense of the Alternator.

Regards,

Larry

 

[Seattle Scott]

When my Yanmar tach failed, I bought one of these http://www.tinytach.com/tinytach/diesel.php

 

[kenn]

A couple of the marinas and yacht clubs here have flea markets a few times a year, and there's usually one or more vendors selling old instruments removed from scrapped or updated boats. I've seen tachs there in the range of $1 to $5. They usually have adjustments to tweak the reading. I've bought a couple to play with... they seem to work.

For calibration, some handheld meters can measure frequency on the sense lead, in Hz, whose readings you can convert to get RPM by multiplying by 60. For example a reading of 30 Hz on the frequency meter corresponds to 1800 RPM.