Briefly, I have an old (1967) 45 HP Chrysler engine. It has the old
"points & condensor" type ignition and it runs just fine.

I installed a Volt meter in the dash, and I run the boat off a deep
cycle battery, only because I carry a trolling motor in the event of
engine failure, not for fishing.

Problem : After running for a while, the Voltage across the battery
rises to about 17 Volts, damaging the battery and possibly damaging
any other equipment I might run off the system (lights, fish finder,
GPS etc.) I intend to install a toggle switch to isolate the (Purple)
Positive charging wire, and flip it on as required.

Is there a possibility that this will cook the diodes? The charging
system has no regulator, just a bridge rectifier. Is there a safer
place to "break" the circuit? The ignition system takes it's power
from the battery, no magneto.

I'd rather this "wasted" energy of boiling the battery be removed 'cos
this old motor needs all the help she can get.

Note : The 17 Volts is across the battery, not the open charging
system. Also, do deep cycle batteries have a lower internal resistance
that's causing this high voltage? All connections and battery are in
excellent condition.

Thanks,
Derek.