Latest on this I had a Mercury expert come run the proper diagnostics. The stator AC section is shorted to ground. He could test that statically but will need a good stator to test the rectifier/regulator but reckons it's likely gone too. Stator on the way so this rev counter issue is going to be a very expensive fix.
Interestingly he did find what probably caused the carnage - the postive battery terminal crimp had never crimped the conductor and the heavy cable could just pop clean out.
It's certain death for the charging electronics of any modern engine in any vehicle if the battery disconnects while the engine is running.
It all took him about 15minutes so good value there
except for a few hours travel time
Another interesting nugget/question. The engine had recently had a full service and checkover by a main dealer. IMHO he really should have spotted a basic issue like a poor battery connection. Thoughts?
So two lessons for us here:
1) when routinely checking battery terminals for security,corrosion etc, loss of contact can be very, very expensive and way beyond a failure to start at the quayside.
2) a dead tachometer on at least a Mercury/Mariner is a a real danger sign that you may not be charging your battery and may be unable to key start your engine after shutting down or accidentally kill-cording it
That said, many of us have fishfinders or GPSs at least some of which allow you set minimum voltage alarms. You should set them!
Also as a last resort many outboards have a manual pull-cord start facility. Make sure you know your's and have the cord and the tools to get at the flywheel.