Onboard tachometers, How accurate??

[lrice01]

Greetings board members,

Just wondering if there is a general consensus as to the accuracy of onboard analog tachometers? I purchased a used boat and need to confirm its propped correctly and need to rely on my tach to make the correct adjustments. The tach has a maximum of 6000 rpm but the needle goes beyond that point at WOT.

Thanks..................Lee

 

[crb478]

Generally speaking if its is not acting erratically, they are very accurate. If its bouncing around then something is wrong. If you can borrow another tach see if they indicate the same readings. Simple to hook up temporarily.

 

[lrice01]

crb478,

Thanks for the response. Its steady, doesn't bounce around. I guess I can assume that if it's reading 6000 rpm, that's probably accurate.

Lee

 

[JASinIL2006]

I compared my onboard tach to a TinyTach, and the onboard was spot on. I mean, it reads exactly the same.

 

[lrice01]

JASinIL2006

Thanks for the response. I'm now confident I can rely on my tach for accurate rpm readings.

Cheers..................Lee

 

[Silvertip]

No you can't be certain it is accurate just because it reads steady. If the calibration dial on the back is not set correctly it will obviously read steady but provide inaccurate results.

 

[lrice01]

Silvertip,

Thanks, so would your suggestion be to have it checked at a service center to confirm accuracy?

 

[JASinIL2006]

Yep, compare your tach to known-to-be-accurate method and then you'll know for sure.

 

[undone]

Some information on what motor it is will help, there are motors that can easily turn 6000 RPMs and there are others that will never reach it. So knowing what motor it is will help people give you more accurate opinions as to whether it may be working correctly or not.

 

[lrice01]

I will compare to a Known to be accurate tach. The outboard is a 2005 60HP Mercury 4stroke. Maximum rpms is 6000

 

[Silvertip]

You don't have to spend any money or take the boat to a shop for verification -- although that's one way to do it. The charging system stator on your engine probably has 12 poles. The tach signal is derived from those pulses, not the ignition system. Therefore tachometers have a dial on the back that can be set to correspond to the number of pulses from the stator. A 12 pole stator produces 6 pules (a complete sine wave) for every engine revolution. The simple task is to stick your head under the console and look at the back of the tach. If the dial is set to 6P there is a good chance your tach is accurate. If it is set to any other position the reading will not be correct for a 12 pole tach.

 

[lrice01]

Thanks, I will check the setting.

 

[Tail_Gunner]

They ly like rug's and need constant calibration...Just a opinion.

 

[dingbat]

Most are accurate enough to prop selection, not so much for setting idle, etc. A couple of years back I couldn't get the motor to idle at the proper rpm. A tube up and a six carb rebuild and still wouldn't idle correctly. In frustration I connected the tach on my engine analyzer. The boat tach was off by roughly 100 rpm at idle. Wasn't always that way. Used it to set idler since new. Burned a voltage regulator one afternoon. Tach was flakey. New voltage regulator flakeyness was gone, but 100 rpm off