Bow thruster Battery

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dougg

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I need to replace our bow thruster battery.
The thruster is, i believe 3/4 hp. The old battery is an Optima red top, 50ah 815 cca.

I dont want to buy another Optima due to cost and unavailbility here.
When I look around I find that even the 60-70 ah batts have no where near the same cca.

Suggestions welcomed.
 
dougg said:
I need to replace our bow thruster battery.
The thruster is, i believe 3/4 hp. The old battery is an Optima red top, 50ah 815 cca.

I dont want to buy another Optima due to cost and unavailbility here.
When I look around I find that even the 60-70 ah batts have no where near the same cca.
Click to expand...

Your bow thruster will probably draw about 300A, so you don't really need an 815CCA battery. I'd go for a decent 100Ah engine starting battery.
 
Are the dimensions of the battery required to be no greater than the optima to suit the available space?
 
Space is not really an issue, as its under the bed in the front cabin.
 
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pvb, I'm not sure how you arrive at 300amps

"Your bow thruster will probably draw about 300A"

3/4 hp is about 560 watts, at a nominal 12 volts that's 47amps, if the battery voltage drops to 9volts as it might do, then it'll be about 63amps.

The instantaneous current when the motor is first switched on might be higher than this, depending on how the motor is controlled (i.e. if it's just a relay then the motor will look like a short circuit), but the high current will only last for a fraction of a second until the motor starts to spin and generates a back emf.

I think most lead acid batteries designed to start a car of a reasonable size will drive this bow thruster.

If you run the thruster for 15 seconds at 9 volts and 63 amps that's about .26 of an amp hour or about amp hour per minute of use, so a 50 amp hour battery should be fine.
 
Ian_Edwards said:
pvb, I'm not sure how you arrive at 300amps

"Your bow thruster will probably draw about 300A"

3/4 hp is about 560 watts, at a nominal 12 volts that's 47amps, if the battery voltage drops to 9volts as it might do, then it'll be about 63amps.

The instantaneous current when the motor is first switched on might be higher than this, depending on how the motor is controlled (i.e. if it's just a relay then the motor will look like a short circuit), but the high current will only last for a fraction of a second until the motor starts to spin and generates a back emf.

I think most lead acid batteries designed to start a car of a reasonable size will drive this bow thruster.

If you run the thruster for 15 seconds at 9 volts and 63 amps that's about .26 of an amp hour or about amp hour per minute of use, so a 50 amp hour battery should be fine.
Click to expand...

Simple, I read the OP's post as meaning that his thruster is 3-4hp, not 0.75hp as you've assumed. Have you ever heard of a 0.75hp bow thruster? The smallest one Vetus sell is 2hp. For their 4hp thrusters, Vetus recommend a minimum of a 108Ah battery.

Your assumptions on the power used ignore the Peukert effect, which is very important with such large discharge currents, and is probably why Vetus recommend 108Ah as a minimum, which is about what I suggested.
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pvb said:
Simple, I read the OP's post as meaning that his thruster is 3-4hp, not 0.75hp as you've assumed. Have you ever heard of a 0.75hp bow thruster? The smallest one Vetus sell is 2hp. For their 4hp thrusters, Vetus recommend a minimum of a 108Ah battery.
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I automatically read it as 3-4 HP but then I have the same model of boat so automatically filled in the correct size. So it isn't 0.75HP and current calculations sound about right for 4HP, though I don't have the manual to hand. Factory fitted bowthruster option is 4HP for my 2009 42DS.

Update: Just found tech. specs. my boat has a Max Power CT80 rated at 4.69HP with consumption of 446A at 10.75V 3.5min duty cycle. Also found a picture of the installation and label on side is slightly different CT80 U=12V I=360A Maxi S2 3 min. I guess the manual I downloaded was for a newer and more powerful version of same model.

Current draw is pretty significant and I found sluggish performance on mine was down to dirty pitted connections. Jeanneau had put dirty clamps over dirty posts and they looked fine externally. However, they were arcing like mad inside even though the posts and clamps looked clean and covered with vaseline. Dismantling, cleaning and putting on vaseline made a huge difference to performance.
 
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Yes, you're right I'd read 3/4 as .75 not 3 to 4, I'm not familiar with bow thrusters on yachts so didn't make the connection and the numbers quoted by pvb didn't ring true based on 3/4 hp
 
dougg said:
Sorry i didnt make that clear!! I knew it was 3 to 4 hp but now it seems its 4 and a bit!!!!
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Looking at my pics. and manual I think that the specs. quote Amps drawn and HP part refers to the output side.

That means 446A at 10.75V equates to 6.4HP but manufacturer claims 4.69HP
The plate on my bowthruster shows 360A and they might measure that at 12V shown or 10.75V as per manual.
So if there's no big change in efficiency, that puts the output at 3.8-4.2HP. I'm back to describing mine as a 4HP bowthruster, all very confusing.

Pretty safe to assume that yours won't draw more than 450A and more likely 360A.

Perhaps a 100Ah car start battery might be worth a try. Vibration will be a problem if you spend a lot of time pounding into a head sea. But car batteries tend to have high CCA. However, I imagine that the plates aren't designed to pull 350A for a minute or so. Might still be worth trying as it even if it only lasted a year. Car battery ~£75 Optima ~£150 (and you can't get them as easily locally).
 
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My book makes no mention of the HP, just the amps drawn. It also says go for cca over ah. min 800!
 
I'm going a different "route" with the installation of my Vetus 55 - supply diversity.

I already have 35mm2 cables going from the general house batteries (3x105Ah) to the anchor windlass, so will fit an 85Ah battery adjacent to the bow thruster. I will use the windlass cable to charge the thruster battery, and have the diversity of the smaller sized battery (slightly limited space) and the feed cable for the windlass to supply the thruster with 50mm2 cable from the battery/windlass cable supply, to the thruster - a matter of less than 1mtr. The fuse will be in-line from battery to thruster.

If that does not work, (and I see no reason that it should not, voltage-drop-wise), then I will have to fit 70mm2 cable from the main batteries to the thruster.

So, time will tell.
 
philip_stevens said:
I'm going a different "route" with the installation of my Vetus 55 - supply diversity.

I already have 35mm2 cables going from the general house batteries (3x105Ah) to the anchor windlass, so will fit an 85Ah battery adjacent to the bow thruster. I will use the windlass cable to charge the thruster battery, and have the diversity of the smaller sized battery (slightly limited space) and the feed cable for the windlass to supply the thruster with 50mm2 cable from the battery/windlass cable supply, to the thruster - a matter of less than 1mtr. The fuse will be in-line from battery to thruster.

If that does not work, (and I see no reason that it should not, voltage-drop-wise), then I will have to fit 70mm2 cable from the main batteries to the thruster.
Click to expand...

It should work. Note that you'll need to fuse the windlass end of the windlass cables too.
 
philip_stevens said:
The windlass is already "fused" at the main battery end - so should be high enough with diversity.
Click to expand...

No, you're adding a new battery at the other end of the cable, so you have unfused cable running the length of the boat, connected to a 12v battery. That's why you need a fuse at the windlass end.
 
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