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Old 28-02-2016, 01:55   #1
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Separate bow thruster batteries - Why don't I have them?

Hi there,

I have noticed that several other Beneteau Oceanis 40's have two additional batteries on either side of their bow thruster (for the bow thruster).

I guess mine runs off our service batteries or start batteries.

Whats the deal there? And should I add dedicated thruster batteries?
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Old 28-02-2016, 08:04   #2
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Re: Separate bow thruster batteries - Why don't I have them?

It doesn't really matter either way ....but I prefer to keep all the weight of the batteries further aft close to their charging and distribution components.... Bus bar etc.

By using the house bank...you just run suitable cable to the thruster up forward.
That is what I have.
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Old 29-02-2016, 17:21   #3
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Re: Separate bow thruster batteries - Why don't I have them?

The main reason for having batteries near the bow thruster is probably to cut down on the length and size of the conductors that provide power-thrusters draw a lot of current, relatively speaking. If you already have batteries installed aft and, presumably, correctly sized conductors running to the bow then there is nothing to be gained by adding a dedicated battery forward unless you find yourself drawing down the house or starting bank to an unacceptable degree because of how much you use the thruster. Even then, it makes more sense to simply add an additional battery aft where it can be charged by the alternator/genset/charger along with the others.
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Old 29-02-2016, 18:01   #4
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Re: Separate bow thruster batteries - Why don't I have them?

Having a 24-volt system, may have its advantages. ... Both starting and house batteries are about twelve feet from the thruster. Not sure which bank operates the thruster. Whatever, no lack of thruster power. (I'd lay my bet on the house batteries.) Good question for the builder.
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Old 29-02-2016, 18:20   #5
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Re: Separate bow thruster batteries - Why don't I have them?

As long as you have the correct size properly fused cable it shouldn't be a problem. I personally like only having 2 banks. A house bank and an emergency starting bank. Essentially everything runs off the house bank including starting. The back up bank gets charged by an echo charger and is only used should the house get drained too far down (which should never happen) or if there is a short in the house and or some other problem. This keeps it very simple to charge, maintain, and trouble shoot.

What gauge cable do you currently have going to your thrusters and how Long of a round trip run is it?
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Old 01-03-2016, 00:15   #6
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Re: Separate bow thruster batteries - Why don't I have them?

I don't know what cable or how long it is. When I get back to the the boat I will check. I hope it is going to the house batteries.

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Old 01-03-2016, 01:41   #7
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Re: Separate bow thruster batteries - Why don't I have them?

The other reason is to use start batteries instead of deep cycle batts. They produce short term heavy currents better than deep cycle batts.

But I agree with Pelagic -- it's not good for weight distribution to have batts up in the bow. With 24v and enough battery capacity it's not the end of the world, either to use your regular house batts, or somewhat longer cable runs.

The very large power draw of the thruster will cause voltage sag -- a combination of voltage sag from cables and from the batts themselves. If you have large battery capacity there will be less voltage drop there, so the cables don't hurt you as much.

Adequate cables also help. Obviously it's much harder to move a lot of power over any distance, at 12v. 36v or 48v would be still better. In my opinion 12v is really not suitable for this job.
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Old 01-03-2016, 02:00   #8
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Re: Separate bow thruster batteries - Why don't I have them?

Quote:
Originally Posted by ontherocks83 View Post
As long as you have the correct size properly fused cable it shouldn't be a problem. I personally like only having 2 banks. A house bank and an emergency starting bank. Essentially everything runs off the house bank including starting. The back up bank gets charged by an echo charger and is only used should the house get drained too far down (which should never happen) or if there is a short in the house and or some other problem. This keeps it very simple to charge, maintain, and trouble shoot. . . .
That looks like a good system. Even simpler would be a completely separate start bank with its own alternator and charger. Then your emergency starting is from the house bank, rather than vice versa, but an engine start batt which is separately charged and never used for anything else is very unlikely to ever need any backup.

My boat is somewhat more complex because house is 24v and engine and generator start 12v. Generator and engine each have their own completely separate start banks and charging systems. The start batts are in the same box and back up each other.


In seven years with this boat, I've only needed backup starting once. For some reason which I have never figured out, every battery on the boat ran down while I was off the boat for about a month. I arrived on my mooring (no shore power) one very cold evening and couldn't get either main or generator to start, and the house batts were dead (otherwise I could have pulled one of those out). I was fairly desperate. Finally I took the last good battery -- out of the dinghy. A tiny start battery for the 25hp outboard, but it was just enough. Added it to the run down main engine battery, and that started. After that, I was able to bring everything back to life. and the incident never repeated. Weird.
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