Well, as usual, the length of this note got out of control. I'm down in fog with 100' visibility. Can't travel, so... Sorry, Bob... Scott, Be aware ahead of time, I think you may be signing up for lots of labor hours. I think in all likelihood you have one or more electrical connections that are in some state of internal corrosion. This kind of corrosion disproportionally affects high current circuits, like a thruster motor circuit. This commonly occurs after the passage of some time (aging) and when connections were not originally crimped with proper crimp tools or proper crimp prep and/or technique. For a thruster circuit that uses AWG 2/0 or 4/0 wire (BIG STUFF), the crimper that's used ***must be*** the tool that looks like a chain cutter. It's a professional tool with 3' handles, and the knock offs - while they can work - require substantial skill to use correctly. Also, the die in the tool must match the terminal, and the terminal must match the wire size, so with AWG wire sizes, must use AWG-sized terminal and die; with SAE wire (strongly discouraged), must use SAE-sized terminal and die, because SAE wire is slightly smaller in diameter than equivalent numbered AWG wire, and SAE uses type II strands vs. type III strands used in boat cable rated AWG conductors (Type BC5W2, THHN, THN, etc). SOLID COPPER WIRE IS NEVER, EVER USED ON A BOAT!!!!!!!! To locate and isolate the problem is going to take a couple of technicians an indeterminate amount of time. Indeterminate because it'll somewhat depend on luck. If the bad connection happens to be near where they start, then less time; but as we all know, the problems is always in the last place one looks, yes?!?! And that can be a long elapsed time into the process. The good news is, you can do this yourself. Start with a visual inspection of ***every*** connector in the circuit. Your looking for signs of over-heating and melting of insulation or discoloration of metal components. Also, wiggle the connection points. Normal connections will feel tight and substantial. Loose connections will feel sloppy. If none of this reveals anything suspicious, what you'll need to do is attach a voltmeter, sequentially, to both ends of each of the individual wires in the entire thruster circuit. DO THIS FOR BOTH THE GROUND RETURN (black or yellow) AND B+ SUPPLY (Red) WIRES. I'd suggest you start at the thruster motor terminals to see what's actually happening there. What you'll see if I'm right is the voltage at that motor will quickly fall off, from in the neighborhood of 10.5/11.0 VDC to something much less. IN ANY CASE, I suspect you will see the voltage at the motor fall because the corrosion or incomplete crimp will drop a lot of the voltage delivered by the battery. A BAD CRIMP OR FAILING WIRE WILL ALSO GET VERY, VERY HOT. So, another test is to simply feel of the connections as you have someone else run the thruster. BUT IF YOU DO THIS, DO QUICK AND TENTATIVE JABS - NOT TIGHT GRABS - OF THE CONNECTORS, TO AVOID BURNING YOURSELF! It will get hot enough that if you grab it, you will leave flesh behind... Don't ask how I know this... A moment of technical mumbo-jumbo follows: at the motor, in a normally operating circuit, you will see less than 12.8VDC, the battery terminal voltage, when the motor spins up under load. Remember, you are running a motor that - like a started motor - draws many hundreds of amps. It doesn't use many Amp Hours of stored energy capacity, but stresses the Reserve Capacity and Cold Cranking Capacity specs of the batteries. That's also why automotive start-service batteries are better for this application than deep-cycle commercial batteries are. The internal resistance of the batteries will rise quickly under these high current loads, and that will drop the battery terminal voltage to as low as 10.5 VDC, maybe less, in just a short time of "cranking." (Just how low this goes will depend a lot on how much battery capacity you have, but the point is, expect some. It is not an indicator of a problem; it's normal under these high loads. It is not the loss you're looking for.) In the old days, when there were TV repair shops, the technician would do a "smoke test" in a case like this. If the TV blew it's fuse, he would simply, put on his safety glasses, short the fuse terminals with a screw driver or pliers, plug the unit in, and then watch for smoke (or for a cap to blow). At that point, he had something to fix. You as the owner were none-the-wiser, as the unit came back to you in working order. Messy, but it worked. You *could* do a smoke test here, too. BUT IF YOU DO, HAVE PEOPLE STANDING BY WITH FIRE EXTINGUISHERS! But at least, you'll have something to fix in the end. I wish you had tested the batteries before changing them out. Sorry to say, that was a boat unit wasted. Old batteries in that kind of service are not necessarily sulfated if they have been properly cared for. And, they can last an amazingly long time... or not... That's why they should be tested before they are changed. Hope this helps. Jim Peg and Jim Healy aboard Sanctuary Currently lying Eggemoggin Reach, Brooklin, ME (at the Wooden Boat School) Monk 36 Hull #132 MMSI #367042570 AGLCA #3767 MTOA #3436