It's an inductive tachometer. Designed for use on any engine that produces an electromagnetic pulse
at regular intervals, through an easily accessible wire. It may require a filter to cut down on white noise
generated by other electrical components, but if it has a spark plug wire, then as long as you know the
number of times the wire is pulsed per rpm, then all the tach does is count the pulses. Contact the
folks at Tiny Tach direct, and ask which model is designed for use on your outboard,
and if there are any problems that need to be watched for.