Anytime you have that far of a run on a boat you can NOT use 8gauge wire. It's too small and you WILL lose a ton of voltage. To be honest, 6 is really too small but it is what is commonly used in boats cause 4 is tough to get a regular supply of. 8 is cheap, that's why they used it.

Remember this too, just like your batteries, just cause it's new doesn't mean it's right. For that matter, you could have had 6 gauge wire and still had a problem because someone made a bad connection at either end.

When you moved that battery up front it made a difference because you shortened the distance, less volts lost. But obviously a bad battery was your trump card. Glad you got it fixed.

You other guys with small TM wires, rip em out and go back heavy. Your batteries and most important, your TROLLING MOTOR will last years longer. They are always under a strain.