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Why does the tachometer read high?

2015 November 24

Why does the tachometer read high?

Dear Boat Doctor,

I just bought a used Catalina 36 MkII. I’m thrilled with the boat but I have an issue that came up during the survey that I’d like to resolve. The tachometer appears to read high. The boat idles at the proper speed, but the tachometer reads more than 1,000 rpm. A similar problem occurs at cruising speed as well. I discussed it with the surveyor and he suggested it was related to the alternator. What’s the issue here and how do I fix it?


Denise Cornell 

Sheboygan Falls, Wisconsin

Dear Denise,

Congratulations on the new boat. It sounds like you found a nice boat if this is your main concern from the survey. I have a pretty good idea what the issue is.

The tachometer gets input from the alternator, which then puts out a pulse that varies with the engine speed. As the engine turns faster, the pulses speed up. I am guessing that your boat has a replacement high-output alternator. Balmar (www.balmar.net, 360-435-6100) is a popular supplier of these. Some high-output alternators supply a faster pulse signal at a given engine speed than alternators that may have originally come with the boat. I think this is what is happening on your boat. 

The good news is that it is relatively easy to fix. Most tachometers allow for this with a small adjustment screw on the back. The trick is that you need to know how fast the engine is turning to know how much to adjust the tachometer. Some people would just tweak the tachometer down to a reasonable number, but you can better than that.

Photo tachometers, which optically read the speed of a rotating object, are available online, at auto parts stores or some hardware stores for about $20. To get them to work well you need to place a small piece of reflective tape, which usually comes with the device, on what you want to read. I’d recommend taking the reading from the crankshaft pulley—the big slower moving pulley at the bottom of the front of the engine. Start the engine and run it at idle speed, then point the photo tachometer at the pulley and read the speed. Leaving the engine running at idle, turn the little screw on the back of the tachometer until the reading matches the speed you read with the photo tachometer.

It’s a good idea to fix this as gauging proper cruising rpm for your boat is important to the long-term health of your engine.