Bow thruster wiring

[jonlea]

I have suffered many starting problems on my motorboat and after many mechanical engineer visits have now isolated it to the bow thruster wiring. Put simply the bow thruster has its own battery located in the bow but is linked to the mains charger/alternator in the stern via small wires. However, even with all three principal batteries switched off (ie 2 engine and 1 domestic), there is still voltage coming through to the bow thruster battery leads. When manoeuvring under power this causes the engine starter batteries to drain down and then prevent the engine from starting again after mooring in tight spaces. The only way of starting the engines then is by finding a mains connexion and recharging the engine batteries again.

How exactly should a bow thruster be wired in, using its own battery up front vis a vis the mains charger/alternator/other batteries?

 

[pvb]

This is really too complex a question to be adequately answered via a forum.

However, a first observation would be that whilst manoeuvring under power, the engine alternators should more than keep up with any current drain imposed by the bowthruster. If they don't then you need the system checking thoroughly by a marine electrician.

If the engine alternators feed via a blocking diode to the start and domestic batteries, then the charge circuit to the thruster battery should also be via the blocking diode - that would prevent the start batteries discharging into the thruster battery (and of course you'd need 3-way diodes).

But I think your priority is to get a proper electrician on board to sort out what's really happening.

 

[Boathook]

I have an ardvec or is it adverc? 12/12 battery charger for my bow thruster battery. It is wired to the main batteries and takes excess charge to the bow thruster battery when the main ones are fully charged. It is a 3 stage charge bit of kit but I don't check it or the battery, only accessing the battery to turn the supply key off when not on board.

 

[pampas]

Common problem I1m afraid, your bow battery appears to be connected in permanent parallel to your other batterys, install heavy wiring from aft to thruster, capable of carrying total load required, or fit a blocking diode in the exhisting line but remember it will take a while to recharge the thruster battery, the best and safest way is my 1st suggestion.I hope that this clarifies the suitation.

 

[INKABOAT]

I had a Vetus 55kgf 4HP 24v thruster installed on my boat some two years ago. I heeded the advice of the Dutch engineer who fitted it and took a 5m heavy lead (12mm dia) directly from the domestic 440ah battery bank to the thruster and earthed it (the thruster) to the steel hull. I have sinced cruised some 2000 miles in the French canals without a moment's problem! The engine is always charging the main bank and the few second's worth of amperage used is quickly replaced. Some people will say there is a potential problem (forgive the pun) but it has yet to manifest itself! Hope this helps, but it worked for me.....

 

[jonlea]

Thanks for all your comments - certainly food for thought. Adjacent to the thruster battery is a black metal object to which the wires are connected. It has what looks cooling fins on it. The marine engineer said it was splitter charger - any idea what that does?

 

[PCUK]

It's probably a blocking diode that has been wired in wrongly. It should be a simple job to work out how the system has been wired and to rectify the problem. Sounds like your "engineer" isn't up to much!

 

[sailinglegend420]

Just to add a few thoughts. We had our split diodes shorted out by the previous owner of the boat when they failed, so our House and Starter batteries were always permanently connected. You may have something similiar.

As already suggested part of the problem is the "small wires" connected back to the stern charger/alternator. A friend with a yacht had a major problem with his bow thruster burning out the contacts because these "small wires" were no way large enough to fully charge the bow thruster battery, causing contact bounce because of the low voltage. The approved installation engineer had failed to connect the shore power, so the engine alternator could never deliver enough current to charge the Bow Thruster battery for the short time the engine was running. You really do need to follow the recommendations for the size of the cables based on the length and current being sent down these cables. You need no more than a 3% voltage drop down the full length of the cable. So a battery 30ft from the alternator needs a cable sized for a 60ft run (that includes the return length to the charger), capable of carrying a current somewhere near the capacity of the alternator. For a 43 ft boat that needs 2/0 cable which cost a fortune!

If anyone has had similar problems with wrongly sized charging cable I would appreciate a PM as my friend is currently about to take the installation engineer to court.

 

[jonlea]

Thanks for all your help and comments

 

[AlanPound]

Another simple possibility is that your bowthruster battery has a duff cell - that would cause it to be eternally pulling current from any parallel-connected main batteries, as they try to charge it...