Don't bother with the copper wire, trying to short out the voltage. Use new stainless cable. Throw out any cable that has rust on it!!! using a voltmeter on a 3 volt scale, you can measure your wire voltage, btw wire and batt neg., when trolling at 2-4 mph. should be about 0.6 volts.
If you run the gear way back at 40' back, you should be behind the voltage field, the theory goes.
also, if using trim tabs, they should have zinc on them, and a light wire brush from a stainless brush is a good idea. The changing of your zincs on the 2 motors is a great idea.