Bow thruster charging

[Gordon Lane]

Hi
40ft planing launch - not a trawler but we do a lot of fishing !
This subject has been around a while but my experience is a little different.
Twin engine / alternators. Port alternator charges house bank. Starboard alternator charges starboard start battery then through DVSR charges port start battery then through second DVSR charges thruster battery in bow.
Alternators 50A. Thruster charge wire sized for 70A protected at both ends by fuse and 65A CB.
Big beautiful new (all American) thruster battery - oversized for thruster.
Everything worked perfectly until a longish burst on the thruster caused the thruster charging circuit CB to trip.
Measured current 90A in charge circuit when thruster running ( < 25A when thruster not running).
Thruster dragging current preferentially from alternator and start batteries because they were around 14V and thruster battery was obviously sitting static on 12.8.
Solution, hope it makes sense? Take a 12V signal from Thruster ON control circuit and use it to operate a relay that opens (disconnects) the 12V supply to Thruster DVSR electronic control circuit.
Seems to work ok anyone foresee any problem with this solution.

 

[ColonyCove]

Bowthrusters can be a problem. Here is my sad story; break out your handkerchiefs. I had an intermittent power problem with bowthruster in a 40 footer. checked all within my limited ability, then gave up and hired two marine electrical experts. Spent most of the day watching their tails sticking out of the bowthruster area beneath the mattress location in the bow cabin. They checked, pulled wires, tested, cursed, and grew increasingly frustrated. Finally went to the other end of the boat where batteries were located at bulkhead between engine compartment and generator & steering compartment. Turns out it was a loose nut where wires were attached to a fuse. $400 in 2010 dollars, but they tightened the nut and problem disappeared. While I'm on a rant here, I am not a big fan of long cable runs for heavy duty 12v marine applications. Most expensive nut I've ever had tightened, but that's boating.

 

[Comodave]

I have a stern thruster instead of a bow thruster but I added a dedicated thruster battery right next to the thruster. Works great, used 4/0 cable even though it is only a couple of feet. Put a separate battery charger on that battery. Also works great.

 

[Ski in NC]

I did as the OP is proposing. Thruster batt right at thruster in bow. Ran a light (12gauge) pair of leads to a 15A breaker in the house DC panel. Near the panel positive charge link lead goes through NC contacts on an automotive relay. Coil is fired by either P or S action of thruster through a pair of diodes to prevent cross feeding.

I chose light gauge and 15A breaker so lead resistance (cable run of about 25') would limit amps when relay closes after thruster use. Works well for my use. After about an hour from thruster use thruster batt is back up to like 0.1V of house bank (13.8). Charge link could be made heavier gauge is you use the thruster a lot.

15A breaker only tripped once during a boat parade, had a buddy at the helm. During the parade he basically steered with the thruster and finally ran the batt low enough to lose thrust. Did that for at least an hour. I came up to the helm and sure enough the charge link breaker was tripped. That was not a normal duty cycle for my thruster system!! After a couple resets, breaker held and next day batt was back up to normal.