Several fellow mariners have reported their engine Tachs sometimes give sporadic readings with the needle bouncing around. I recently had a similar experience on my boat and did some research. Most high output alternators with smart regulators and many solar panel controllers charge batteries using a technology called Pulse Width Modulation (PWM). The Tach uses PWM to determine the alternator (and thus the engine) RPM. The two PWM sources may, at times, create an interference. Thus the sporadic readings on the Tach.
The solution to this is disconnect the solar panel when the alternator is running so the controller doesn't function. This can be easily done by putting a switch in the positive wire leading from the solar panel to the controller. Alternatively, you could put in relay (closed when no powered) on this wire and wire it so the relay is open when the engine is running and closed when it is not.
There is another very good reason to put a switch in the panel to controller wire. The smart regulator reads the battery charge condition and sets the output of the alternator accordingly. If the solar panel is producing power, the regulator will see the net of the battery charge plus the panel output and thus may prematurely put the alternator in float mode. this will result in getting limited performance out of that big expensive alternator.
Thomas Trimmer has been cruising with his Ericson 38 sailboat on the Great Lakes for over 20 years. He has pioneered the use of solar energy for wilderness cruising. He is continually designing and building equipment to simplify and enhance the cruising experience.