Iggy
Guru
Measure voltage at and across each connection and see if there is a voltage drop. If there is a drop then the connection is suspect.
That I know ,but it also needs to be under load. If the boat is on the hard, it can not be done.
On the other hand, I would expect some amount of a voltage drop. It would be pulling about 325A. The question would be, how much is too much? Plus, given the wire is 21 years old I am thinking of replacing it anyways. The house bank and thruster are only 7 or 8 feet apart. Add some twist and turns its about 11 to a 12' run. With 4/0 cost for both wires are about $275.00