Bowthruster Electrics Supply

[101chugger]

I’m fitting a Vetus 25kgf bowthruster like this and would be grateful for help with two questions concerning electrics. The thruster will be powered by its own adjacent battery, which will be fed from the start battery on the boat through a VSR. Both batteries are new start batteries, 12v 75Ah 630 CCA, wet lead/acid sealed type. I’ll use 25mm battery cable between the thruster and the thruster battery.

The cable run distance from the start battery to the thruster battery is 4 metres: 3 metres to the VSR then a metre to the thruster battery. I’m conscious of voltage drop in the cables, and also the prospect of potential heat build up in the cables when operating the thruster. I’d like to use the thinnest cable that satisfies both criteria as thicker cable will cost more and will be more difficult to feed through the boat to the thruster battery.

• Should I use 6mm or 10mm cable between both batteries?
• Where best to earth the return cable and the thin black cable from the VSR? I’d rather keep additional wiring away from close to the engine, so would the negative terminal of the start battery be okay?

 

[ip485]

Out of interest why would you feed them from the starter battery? I am a believer that the starter battery should be totally isolated from all other systems for obvious reasons. Wouldnt you be better to wire whatever your charge source is direct to the thruster bank? This would also mean far less need to run heavy duty cables between the batteries. Just a thought.

There are those here with more experience than I of wire sizing and loads so I am sure someone like PaulR will be along with some excellent advice.

 

[Aurai]

Part 1: Choosing the Correct Wire Size for a DC Circuit - Blue Sea Systems

Hi, if you follow this chart, ie 200 A draw, anything from 0 to 20m is AWG 2.0 which is 35mm, by my reckoning. (10% allowance) If you support that reading, it becomes easy enough to dial in a 5A draw for your battery feed, that is a bit of a guess. To whatever distance you need. You could also consider an Echo Charge unit instead of a VSR. Good luck.

 

[pvb]

Do you really need an IP version of the thruster? It's about £120 extra.

I'd consider using a cheap trickle charge unit to keep the thruster battery topped up, which would only need thin cables.

 

[PaulRainbow]

I’m fitting a Vetus 25kgf bowthruster like this and would be grateful for help with two questions concerning electrics. The thruster will be powered by its own adjacent battery, which will be fed from the start battery on the boat through a VSR. Both batteries are new start batteries, 12v 75Ah 630 CCA, wet lead/acid sealed type. I’ll use 25mm battery cable between the thruster and the thruster battery.

25mm cable is not big enough, it needs to be 35mm.

The cable run distance from the start battery to the thruster battery is 4 metres: 3 metres to the VSR then a metre to the thruster battery. I’m conscious of voltage drop in the cables, and also the prospect of potential heat build up in the cables when operating the thruster. I’d like to use the thinnest cable that satisfies both criteria as thicker cable will cost more and will be more difficult to feed through the boat to the thruster battery.

• Should I use 6mm or 10mm cable between both batteries?
• Where best to earth the return cable and the thin black cable from the VSR? I’d rather keep additional wiring away from close to the engine, so would the negative terminal of the start battery be okay?

You'll need to re-think this. 6mm cable is rated at 53a and 10mm at 75a, the thruster has a current draw of 220a. Fitting a VSR is the same as wiring the batteries in parallel, so the thruster battery will draw at least some current from the battery that you choose to charge the thruster battery. You need to fuse the charging cable at both ends, to protect the cable, so even with 10mm cable you can only fit a 75a fuse, which could blow if the thruster tries to draw in excess of that from the charging source. You won't know it's blown until the thruster battery is flat.

To give you some better options, how do you charge the domestic and engine batteries ? Shore power, solar, alternator ? What split charge systems do each use ?

 

[ip485]

To give you some better options, how do you charge the domestic and engine batteries ? Shore power, solar, alternator ? What split charge systems do each use ?

As I mentioned out the outset I would be interested to know why you dont connect the bank direct to a charge source, unless your charge sources are limited?

 

[PaulRainbow]

As I mentioned out the outset I would be interested to know why you dont connect the bank direct to a charge source, unless your charge sources are limited?

You could connect directly to the charge source, but you'd need to use thicker cables to avoid current draw from the thruster blowing fuses.

 

[Dysan]

Mine was setup completely isolated with a battery in the fore peak. I’ve installed a 3 way Victron unit to keep all banks healthy on shore power and am busy pulling 16mm (for 30Amp charge) cable down to the thruster battery.

I’ve yet to work out how to deal with charging from alternator, but least some wires will be present.

 

[PaulRainbow]

Mine was setup completely isolated with a battery in the fore peak. I’ve installed a 3 way Victron unit to keep all banks healthy on shore power and am busy pulling 16mm (for 30Amp charge) cable down to the thruster battery.

I’ve yet to work out how to deal with charging from alternator, but least some wires will be present.

How do you currently charge the engine and domestic bank from the alternator ?

 

[Boathook]

My bowthruster battery is charged by an adverc battery to battery charger fed from the main bank. So it has been a fit and forget apart from the initial bt battery failing after 5 years. The current is restricted between the batteries by the charger to around 4 amps but you only use the bt for a few seconds normally.

 

[101chugger]

Thanks for all the replies. First, the cable size BT battery to BT will be 35mm not 25mm – sorry, my typo earlier. Second, I won’t need any bridge between both batteries as is employed with start to domestic batteries to help engine starting. The operating power for the BT should come only from its dedicated battery.

My initial understanding was by using a VSR between the start and BT batteries, the VSR would protect the start battery from too great a discharge while the BT was in use. Hence I thought that this would allow a lower amperage recharge to the BT battery, which could be fed by a smaller wire gauge. The BT would operate only for 1 – 2 minutes max; it would then be recharged at a comparatively low rate by the start battery (which in turn is being charged by the alternator and a solar feed).
I felt I didn’t need a B2B charger since they were more suited to the more modern “smart” alternators, which mine is not, and so a VSR would do the job effectively.
Bottom line, do I really need to use 35mm cabling start battery to BT battery, or to allow the use of smaller gauge cabling would a different type of split charge resistor be required? Thanks again all - really informative posts.

 

[Boathook]

My B2B works ok on from a battery bank charged by solar panels and an outboard. The B2B doesn't work until it senses around 14 volts at the main bank, so the main bank is always charged first.

 

[RupertW]

As I don’t have a bowthruster I’ve never really considered a long DC run to another battery - although with a windlass perhaps I should. But from Paul’s reply on a VSR just paralleling the batteries therefore allowing a very high current to go between main battery bank and thruster battery I started wondering how you could limit the current so that a quick discharge of the bow battery could be followed by a slow recharge until next needed. Then saw the B2B charger mentioned which was a new device to me. I wonder also whether you would still get less loss by using a mains cable from an invertor to a mains battery charger at the bow?

 

[PaulRainbow]

Thanks for all the replies. First, the cable size BT battery to BT will be 35mm not 25mm – sorry, my typo earlier. Second, I won’t need any bridge between both batteries as is employed with start to domestic batteries to help engine starting. The operating power for the BT should come only from its dedicated battery.

The VSR is the bridge, as soon as you start the engine the VSR closes and the engine and thruster batteries are in parallel.

My initial understanding was by using a VSR between the start and BT batteries, the VSR would protect the start battery from too great a discharge while the BT was in use. Hence I thought that this would allow a lower amperage recharge to the BT battery, which could be fed by a smaller wire gauge. The BT would operate only for 1 – 2 minutes max; it would then be recharged at a comparatively low rate by the start battery (which in turn is being charged by the alternator and a solar feed).

The VSR will protect the engine battery from being discharged by the thruster because it will open if the voltage drops below a set voltage. But as i sadi earlier, using thin cable means it needs small fuses, trying to draw large loads through a small fuse will obviously blow it.

I felt I didn’t need a B2B charger since they were more suited to the more modern “smart” alternators, which mine is not, and so a VSR would do the job effectively.

B2B chargers serve various purposes, not just for smart alternators, you've been reading Sterling bullshit marketing.

Bottom line, do I really need to use 35mm cabling start battery to BT battery, or to allow the use of smaller gauge cabling would a different type of split charge resistor be required? Thanks again all - really informative posts.

It's not entirely clear what would best suit, you've still not said what your current charging systems are. How does the domestic bank and the engine battery currently get charged ? What split charging methods do you have for the alternator, what for mains charging. what solar charging ?

 

[BabaYaga]

Bottom line, do I really need to use 35mm cabling start battery to BT battery, or to allow the use of smaller gauge cabling would a different type of split charge resistor be required?

Is there some kind of main switch to the BT, that must be closed before the R/L buttons can be operated? If so, perhaps you might use a NC normal relay controlled by that main switch? That would make sure that the connection between starter battery and truster battery was held open during the period when the BT might be in use. Then the wiring between the two batteries could be sized for charging only.
Just a thought, I have no experience of BTs.

 

[PaulRainbow]

Is there some kind of main switch to the BT, that must be closed before the R/L buttons can be operated? If so, perhaps you might use a NC normal relay controlled by that main switch? That would make sure that the connection between starter battery and truster battery was held open during the period when the BT might be in use. Then the wiring between the two batteries could be sized for charging only.
Just a thought, I have no experience of BTs.

Interesting thought. Rather than a relay, a double pole on-on switch could be fitted, one pole interrupting the negative to the VSR and the other interrupting a wire from the controller.

 

[howardclark]

My Venus bowthruster was fitted 20years ago - I did the wiring and would make some comments
I too have a 3 battery system- house, engine starter and bow thruster/windlass.
after my engine start battery has done about 5years, it is relegated to be a bow thruster battery which is less critical and means I always have a relatively new starter battery.
over the years you may make changes to your engine/alternator system, shore charger etc. I would recommend you fit the largest reasonable charging cables to the bow thruster battery. For instance my original alternator was 40amp- when I fitted a new engine it became 180 amp! When the engine is going and the bowthruster is working, I think all 180/amps are going to the thruster.
After fitting them you will never fret that your cables are too large!

 

[halcyon]

Is there some kind of main switch to the BT, that must be closed before the R/L buttons can be operated? If so, perhaps you might use a NC normal relay controlled by that main switch? That would make sure that the connection between starter battery and truster battery was held open during the period when the BT might be in use. Then the wiring between the two batteries could be sized for charging only.
Just a thought, I have no experience of BTs.

Run a relay coil via diodes from either thruster motor terminal, when thruster runs, either way, relay opens, isolating VSR neg.

Brian

 

[pvb]

Is there some kind of main switch to the BT, that must be closed before the R/L buttons can be operated? If so, perhaps you might use a NC normal relay controlled by that main switch? That would make sure that the connection between starter battery and truster battery was held open during the period when the BT might be in use. Then the wiring between the two batteries could be sized for charging only.
Just a thought, I have no experience of BTs.

I'm not sure that's a good idea. Having to go in the boat and switch the thruster on every time you want to use it (and off again afterwards) would be a chore. My thruster's main switch is on permanently.

 

[BabaYaga]

I'm not sure that's a good idea. Having to go in the boat and switch the thruster on every time you want to use it (and off again afterwards) would be a chore. My thruster's main switch is on permanently.

I envisaged some kind of switch in the cockpit. Perhaps not a main switch, but something that would stop you from inadvertently running the thruster.
But as I said, I don't know how BT's operate.