Bowthruster

[jeb]

Hi All

I am in a bit of a quandry over the best way of powering a bowthruster. I am at the stage of build where I really must make a decision as to the best way so that I can make provision for any wiring.

 

The bowthruster in question is a Nobels 9hp and the length of the boat is 19 metres.

Battery capacity is recommended at 300amp.

 

Now what would be best.

 

1. Add the batteries to the domestic bank and run welding cable from the domestic bank direct to the thruster ? if so what size cable would be required ?

 

2. Fit a battery to battery charger and run ??size cable to 2 x 150amp batteries at the front of the boat.

 

3. Fit a 240v battery charger in a cupboard as close to the bowthruster as possible and wire to 2 x 150amp batteries.

 

What I think is clouding my judgement is the fact with option 2 & 3 there are 2 x 150amp batteries that are rarley going to be used and therefore not earning there keep.

With option 1 that is a hell of a lot of cable and how would you fit end terminals.

 

Or is there further options I don't know about yet ?

[Keeping Up]

I don't think you've included the option of adding extra batteries near the bow, then connecting them directly back to the stern in parallel with the domestic batteries, but using MUCH thinner cable than you would otherwise need (I'd guess at 50 amp cable). The bow batteries will handle the peak demands of the thruster, then will recharge themselves (at a lower rate) from the domestic batteries and their existing charging circuit. When you are not using the bowthruster, the extra batteries will actually help increase your domestic supply.

[jeb]

I don't think you've included the option of adding extra batteries near the bow, then connecting them directly back to the stern in parallel with the domestic batteries, but using MUCH thinner cable than you would otherwise need (I'd guess at 50 amp cable). The bow batteries will handle the peak demands of the thruster, then will recharge themselves (at a lower rate) from the domestic batteries and their existing charging circuit. When you are not using the bowthruster, the extra batteries will actually help increase your domestic supply.

 

Ahh now there's a thought and seems quite a good one to me. Does anybody know how to work out what exact size cable that would need to be used ?

[blackrose]

I am an electrical ignoramus so i can't tell you which is best in that respect, but just to give you an overview of my system:

 

My BT batteries are at the bow and are charged with the same alternator that charges the start battery at the stern. The charge is split with a BEP voltage sensitive relay which cuts in/out at 12.4 and 13.6v or thereabouts and switches the charge between the two banks. The domestic batteries are charged from an auxillary alternator.

 

I think the benefit of this system is that the charge for the thruster is ready at the bow and the cables from the alternator to the BT batteries don't have to be that thick - mine are only about 25mm2. Yes, there's obviously a voltage drop, but because the charge isn't directly used by the BT, the batteries have all the time that you're not pressing the button to charge.

 

Anyway, I think I'm limited to about 5mins/hour of BT use but that's plenty and with sparing BT use I've never run out of power.

 

The idea of adding to your domestic bank and powering your BT from this big bank at stern strikes me as odd. I might be wrong but not only would it require some mighty cables leading to the BT, but you might arrive at your destination with a less than fully charged domestic battery bank.

Edited January 15, 2008 by blackrose

[MikeV]

I think my bowthruster battery is wired in parallel with relatively thin cable. I haven't got wiring diagram for the boat so need to confirm this.

 

However, when I bought the boat last year the surveyor said he thought the bow thruster battery was performing poorly and might need replacing. I wasn't that concerned as I've never had a boat with a bowthruster before.

 

One weekend I thought I'd give the bow thruster battery a good charge using a mains battery charger and then see how it performed - not well! But what suprised me was that I had the isolator for the bow thruster battery open while I had the battery charger connected to it and back in the engine room I noticed that the ammeter on main domestic battery bank was showing they were also charging, suggesting they are connected together without even a splitter diode. I suppose there could be a split charge relay tucked away somewhere, but haven't seen anything so far.

 

Supplementary question therefore:

If this is the case, what sort of battery should I buy to replace the bow thruster battery?

 

I would normally have thought the bow thruster battery should be a starter battery in order to deliver the high current for the bow thruster. However, if it is doing double duty and adding capacity to the domestic bank, then surely it needs to be a leisure battery, or one of these dual mode starter/deep cycle type.

 

 

Thoughts anyone?

[sideways]

Ahh now there's a thought and seems quite a good one to me. Does anybody know how to work out what exact size cable that would need to be used ?

Go to Lewmar.com. They have a graph or just ask the question and they will do the math for you

[bottle]

The isolator will be to isolate the bow thruster from the batteries, so that work can be carried out on the bow thruster.

 

If you were charging your domestics while charging the BT batteries then they were directly connected to each other and you are correct in the assumption that there is not a diode or split charge relay.

 

Bow thuster batteries = start type battery.

[steelaway]

Hi All

I am in a bit of a quandry over the best way of powering a bowthruster. I am at the stage of build where I really must make a decision as to the best way so that I can make provision for any wiring.

 

The bowthruster in question is a Nobels 9hp and the length of the boat is 19 metres.

Battery capacity is recommended at 300amp.

 

Now what would be best.

 

1. Add the batteries to the domestic bank and run welding cable from the domestic bank direct to the thruster ? if so what size cable would be required ?

 

2. Fit a battery to battery charger and run ??size cable to 2 x 150amp batteries at the front of the boat.

 

3. Fit a 240v battery charger in a cupboard as close to the bowthruster as possible and wire to 2 x 150amp batteries.

 

What I think is clouding my judgement is the fact with option 2 & 3 there are 2 x 150amp batteries that are rarley going to be used and therefore not earning there keep.

With option 1 that is a hell of a lot of cable and how would you fit end terminals.

 

Or is there further options I don't know about yet ?

 

Hi there

First of all you must find out what is the consumption in Amps for the BT when you operate it. I think you will be supprised

how high this is - maybe 150 amps. For this you will need at least 25mm cables from the BT battery's to the BT. I think you will be supprised how quickly you will flatten these battery's. If you are going to charge these batterys from the alternator you will need cables large enough for the maximum the alternator can produce - ie 110Amp alternator maybe - 16mm cable. If you are going to connect the domestic battery's together with BT battery's don't forget that you may have draw current from those battery's for the BT so then you need 25mm cables.

I also feel that because the BT has a very large current requirement (like the engine start) that the BT battery's should be

STARTER battery's type NOT leisure type battery's.

I think I would go with your sugestion 2. With the largest 240V battery charger I could afford.

 

regards Alex

[Keeping Up]

Hi there

First of all you must find out what is the consumption in Amps for the BT when you operate it. I think you will be supprised

how high this is - maybe 150 amps. For this you will need at least 25mm cables from the BT battery's to the BT. I think you will be supprised how quickly you will flatten these battery's. If you are going to charge these batterys from the alternator you will need cables large enough for the maximum the alternator can produce - ie 110Amp alternator maybe - 16mm cable. If you are going to connect the domestic battery's together with BT battery's don't forget that you may have draw current from those battery's for the BT so then you need 25mm cables.I also feel that because the BT has a very large current requirement (like the engine start) that the BT battery's should be

STARTER battery's type NOT leisure type battery's.

I think I would go with your sugestion 2. With the largest 240V battery charger I could afford.

 

regards Alex

I agree that a starter type battery would be better to supply the high current that the thruster needs, although with the option I'm suggesting in which the thruster batteries also help to increase your domestic capability you really need a battery which doesn't mind a relatively deep discharge - ie a dual purpose battery (I've seen them advertised as such but I don't know how good they are).

 

I don't agree with the part I've highlighted in red; because there is a long length of cable from the stern to the bows, there will be a significant resistance in that cable which will limit the current. You will never get the high currents that you describe.

 

In fact the cable size cancels out of the equation; a thicker cable can pass more current, and it does pass more current because it has a lower resistance, etc. I've just run some maths through a spreadsheet and I reckon that a 20-metre cable run between batteries (multiplying by 2, one positive cable and one negative cable) will only reach its rated current if the difference in battery voltages approaches 5 volts. I don't think the voltage difference will exceed that, which means you could literally use any size of cable that you like, the only consideration being that the faster you want your bowthruster batteries to recharge after each use, the thicker you need to make the cable. So you could go for some inexpensive 30 Amp cable for example, and I reckon it would work fine.

[MikeV]

I think you will be supprised how quickly you will flatten these battery's.

You shouldn't flatten the batteries unless you make a lot of use of the BT. As I mentioned in my previous post I haven't had a boat with a BT before, but I would expect to only use it limited circumstances with 5-10 sec bursts. Even at 200 amps, 20 seconds of use is only 1.1AH.

 

As I understand batteries will build up internal resistance when subjected to high current which will cause a temporary drop in performance, but will recover quite quickly when rested.

 

Incidentally, I was once showed over a boat by a lady who also had to move the boat we were looking over it. The manoeuvre across the marina was extremely smooth with no use of the BT. As she explained the boat's facilities she referred to the BT controls as the "girlie button". Clearly she thought it was an unecessary extra!!

[ChrisPy]

9bhp = about 7KW = about 600A current, in theory.

 

I have similar bow thruster and have wired in two parallel welding cables for the positive feed from the engine start battery, each 60sq.mm. Fuse is next to the battery +ve terminal. Again about 600A in theory, although the fuse supplied with the unit was 375A.

 

Contrary to all the advice given in the manuals, I have used the hull as the negative return. The reason why this is incorrect is because it sets up corrosion currents in the hull, however the b.t. is used for a few seconds occasionally, so this is not a significant issue.

Edited January 16, 2008 by chris polley

[tomsk]

You shouldn't flatten the batteries unless you make a lot of use of the BT. As I mentioned in my previous post I haven't had a boat with a BT before, but I would expect to only use it limited circumstances with 5-10 sec bursts. Even at 200 amps, 20 seconds of use is only 1.1AH.

 

As I understand batteries will build up internal resistance when subjected to high current which will cause a temporary drop in performance, but will recover quite quickly when rested.

 

Incidentally, I was once showed over a boat by a lady who also had to move the boat we were looking over it. The manoeuvre across the marina was extremely smooth with no use of the BT. As she explained the boat's facilities she referred to the BT controls as the "girlie button". Clearly she thought it was an unecessary extra!!

 

 

I strongly concur with the highlighted section, less is undoubtedly more... (retires abaft for a well deserved Condor Moment)

[jeb]

I strongly concur with the highlighted section, less is undoubtedly more... (retires abaft for a well deserved Condor Moment)

 

Quite agree and the button will be clearly marked. After all its only girlies that scream when they go over a wier sideways.

 

To all who have answered above ,many thanks and I will be back this evening to go through it properly.

[barge sara]

Or is there further options I don't know about yet ?

 

Forget the bow thruster and spend a much smaller amount on a boat handling course and a decent boat pole?

 

John

[steelaway]

I agree that a starter type battery would be better to supply the high current that the thruster needs, although with the option I'm suggesting in which the thruster batteries also help to increase your domestic capability you really need a battery which doesn't mind a relatively deep discharge - ie a dual purpose battery (I've seen them advertised as such but I don't know how good they are).

 

I don't agree with the part I've highlighted in red; because there is a long length of cable from the stern to the bows, there will be a significant resistance in that cable which will limit the current. You will never get the high currents that you describe.

 

In fact the cable size cancels out of the equation; a thicker cable can pass more current, and it does pass more current because it has a lower resistance, etc. I've just run some maths through a spreadsheet and I reckon that a 20-metre cable run between batteries (multiplying by 2, one positive cable and one negative cable) will only reach its rated current if the difference in battery voltages approaches 5 volts. I don't think the voltage difference will exceed that, which means you could literally use any size of cable that you like, the only consideration being that the faster you want your bowthruster batteries to recharge after each use, the thicker you need to make the cable. So you could go for some inexpensive 30 Amp cable for example, and I reckon it would work fine.

Hi there

I am a bit confused. Are you saying that when the BT battery's are flat and the BT wants to draw power from the other domestic bank that it cannot draw the 150 amps it wants down that 25mm cable.

Please help I dont understand the science behind it.

Thanks Alex

[Keeping Up]

Hi there

I am a bit confused. Are you saying that when the BT battery's are flat and the BT wants to draw power from the other domestic bank that it cannot draw the 150 amps it wants down that 25mm cable.

Please help I dont understand the science behind it.

Thanks Alex

If you have thick (eg 25mm) cables between the batteries, your BT could draw a high current from the domestic batteries. 150 Amps is certainly possible, but is not desirable because you don't really want to use your domestic batteries that way; and as Chris Polley points out the BT may actually need a lot more than 150 Amps.

 

No I am suggesting that you deliberately use a much thinner cable, whose own resistance will limit the current that the BT can take from your domestic batteries. The BT will then operate from its own batteries, taking something between 150 and 600 amps from them, and then recharge those batteries more slowly, at maybe 30 amps, from the domestic system afterwards. If you use the BT for 10 seconds it would take maybe 5 minutes to recharge (so don't switch off the engine for 5 minutes after mooring up).

 

A small side-issue, the cable should have a fuse at both ends.

 

PS I must point out that this is a theoretical solution, I don't have a bowthruster on my boat.

[tomsk]

If you have thick (eg 25mm) cables between the batteries, your BT could draw a high current from the domestic batteries. 150 Amps is certainly possible, but is not desirable because you don't really want to use your domestic batteries that way; and as Chris Polley points out the BT may actually need a lot more than 150 Amps.

 

No I am suggesting that you deliberately use a much thinner cable, whose own resistance will limit the current that the BT can take from your domestic batteries. The BT will then operate from its own batteries, taking something between 150 and 600 amps from them, and then recharge those batteries more slowly, at maybe 30 amps, from the domestic system afterwards. If you use the BT for 10 seconds it would take maybe 5 minutes to recharge (so don't switch off the engine for 5 minutes after mooring up).

 

A small side-issue, the cable should have a fuse at both ends.

 

PS I must point out that this is a theoretical solution, I don't have a bowthruster on my boat.

 

 

Good chap, glad to hear there are still some manly, hairy chested 'real' boaters about

[steelaway]

If you have thick (eg 25mm) cables between the batteries, your BT could draw a high current from the domestic batteries. 150 Amps is certainly possible, but is not desirable because you don't really want to use your domestic batteries that way; and as Chris Polley points out the BT may actually need a lot more than 150 Amps.

 

No I am suggesting that you deliberately use a much thinner cable, whose own resistance will limit the current that the BT can take from your domestic batteries. The BT will then operate from its own batteries, taking something between 150 and 600 amps from them, and then recharge those batteries more slowly, at maybe 30 amps, from the domestic system afterwards. If you use the BT for 10 seconds it would take maybe 5 minutes to recharge (so don't switch off the engine for 5 minutes after mooring up).

 

A small side-issue, the cable should have a fuse at both ends.

 

PS I must point out that this is a theoretical solution, I don't have a bowthruster on my boat.

 

Thanks Allan, it makes more sense now.

 

Alex

[DHutch]

We have a 3kva 24v vetus bowprop (now there 55kgf model i think) which has its own dedicated set of batterys in the bow.

- We dont have any start batterys, so it made sence to have what we had in the bow. Two 90amp starter batterys at current.

- There then charged though some thinish cables from one of the outputs of out mains charger, or from the (singal) alternator via a manual change over switch.

 

Its not ideal, becuase the voltage drop over the long thing cables means you get little charge from the alternator, or even the mains unit, which is only a bit closer.

- Not really an issue as we dont really use it, and leave it plugged in for 48hours when we get home for winter so that up for alteast some of the year.

- However i plan to get around to playing with our sterling alternator controler to make the setup battery sensing, which should allow them to charge better while on the move.

 

Not sure what i would do i had a diesal engine with a starter battery, but proberbly about the same, but just have more alternators rather than switching one about.

 

 

 

Daniel