You won't like the answers...
Design Technologies. Hand-Held economical tachometers. Wireless tachometers.
In the second one look at tech tach TT-700
Basically you need an optical or mechanical tach. Or a tach that can pick up it's signal from the injection system.
Mechanical tach - If the engine has a mechanical port you will add the pickup, the wiring
and the tach head
. Usually picks up a one per rev from a gear
tooth on the flywheel or something.
Optical Tach - basically you attach a marker dot to the flywheel. The head
unit is a strobe as Pbalis describes but you "tune" the strobe to freeze the dot in the strobe light. The coresponding frequency converts to RPM
Pulse tach - Basically the injection pulses disturb a magnetic field in the handheld tach. The frequency of the interuptions converts to RPM considering a 4-stroke cycle.
Probably the tach on your boat is sensed off the alternator
. In this case the frequency of the DC power wave is measured against the known pulley ratio and RPM is determined. Changing pulley ratios, slipping belts, incorrect frequency (6 pole vs. 7 pole alternator) can all impact the accuracy read at the tach head.
Have you changed pulley ratios as well or something else along with the tach head swap?