> My engine is a hundred miles away

Maybe next time you'll turn it off when you park the boat

ISTR that most if not all of the tachometers discussed here in the
past (they'll be in the archives, which unfortunately don't seem to be
working at the moment) use a pick-up on a spark plug lead. The
output from the pick-up can be low-voltage, high-impedance, which
is fairly easy to filter with RC and diodes

Even this crude tacho

http://members.tripod.com/xexorz/schematics/analogtach.html

doesn't have much in the way of filtering (LM2917 i/p max is 28V)

FWIW, here's a PIC tacho for a diesel using the alternator signal

http://www.vermontficks.org/dtachs.gif

via

http://www.vermontficks.org/pic.htm

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