Bow Thruster Power Sources

[Byekurman]

Bow thruster question. 2003, Mainship, 390. Yesterday I removed my bow thruster. I also replaced the battery located at the thruster. During the disconnection of the battery from the thruster I discovered a second set of very large cables, 1 red 1 yellow, also attached to the thruster. With the battery fully disconnected from the thruster, these additional cables had 13.5 vdc on them. I turned off my battery chargers thinking that this may be supplying charging voltage to the battery, but the voltage on these conductors remained at 13.5 vdc. It appears there are 2 sources of 12 V power being supplied to my bow thruster.

1. Does anyone know where this second source of this power may be coming from?

2. Does anyone know where I can cut off this source of power when I reconnect my thruster?

 

[Byekurman]

Even when I turned my Battery 1, Battery 2, OFF switch to OFF, power remained on these cables.

 

[Bmarler]

It can take a little time for voltage to drop after turning off the charger, but you should see it in a reasonable amount of time.
Hard to say where the other cables come from, but I would guess they are hooked to the start battery, or whatever battery receives the alternator feed.

 

[Jeff F]

My guess is that you'll find the other end connected directly to one of your battery banks, maybe with a shutoff switch.

How is the thruster battery charged?

I'll go out on a limb here, and say that originally the thruster was powered from the house bank, and somewhere along the line someone stuck a battery in the bow to make the thruster work better.

 

[Comodave]

Very large cables, how large? There should be markings on them.

 

[Bacchus]

I'm not familiar with the MS 390 but my 2008 34HT has a separate 8D batty for (2) thrusters with their own batty switches.
Battys will show higher V, due to charging, for a period of time until the surface charge on plates dissipates. If you pulse the thruster while checking V it will drop significantly during the pulse but won't return to the same (higher) V after the pulse, if the charging is no longer connected. If it comes back to the same V (or higher) there must be another charging source.
AFAIK MS did not install batteries at the thruster so I'm guessing the other cables are MS factory install and a PO added the thruster batty and tied it to the factory source. MS also was using diode isolators to provide (alternator & shore) charging to multiple batty banks. You might look for a diode isolator to see if it is wired correctly.

 

[Bacchus]

If you don't have the MS 390 Manual this thread has some useful files included
https://www.trawlerforum.com/f...s-manual-2000-390-a-63909.html

 

[Cold Duck]

My thruster battery also powers the windlass. Are you sure those cables aren't going to the windlass? As why still shows 13v I don't know.

 

[Comodave]

Look at any other batteries on the boat and see if they have that same size cables running forward, also check to see if there are any on a battery switch that run forward.

 

[Long Island Boater]

The bow thruster on our 2004 MS390 is wired to the house bank. There’s a dedicated fuse behind the panel containing the master battery switch.

 

[Arc]

It is probably a house battery connected to the thruster battery. That is an ok set up but there should be a disconnect switch . I have seen thruster contactors weld closed. That keep the thruster on until something bad happens. You will either burn up the thruster or crash into something. It’s not enough to have a switch at the helm. That only shuts off the voltage to the coil of the relay. You need to be able to cut the complete power source in an emergency.