Is Tachometer Accurate?

Is Tachometer Accurate?
Richard Herbst

Date: November, 2006 Hull #: 93

I recently suspected that my helm’s tach was inaccurate. After investigating, I found as much as 1300 RPM error near the high end! The tachometer’s input signal comes from an upgraded alternator on my Westerbeke 42B engine, a Balmar model 91-100. So, if there’s an upgraded alternator in use, you could have a big tachometer error.

I found the error by using a $65.00 digital “Diesel Tiny Tach” from Design Technology Inc ( which claims an accuracy of +/- 10 RPM. My order included a longer cable [$10, + about $7 shipping], which allows the indicator to reach the helm. The Tiny Tach is powered by a 9-volt battery; its transducer clamps onto an injector’s fuel line, measuring the pulses of injected fuel to determine RPM. The unit can be set up permanently or connected when needed. I used

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