Negative Battery Switch - is it necessary?

[NorthCoastJoe]

I have an aluminum boat. House and start batteries have positive and negative disconnect switches from the original builder in France.
I would not ever think about changing that.
Maybe in a plastic boat it might not matter much. My Pearson 30 has just positive disconnect switches, and never had a problem.

[SailingHarmonie]

Originally Posted by boatpoker

I've never seen this arrangement in a boat. Could you please note one or two of the standards that require this.

Boatpoker,

I am not at all sure what you are thinking of here, because I KNOW you know better…. but a master battery switch for both positive AND negative sides is very, very common on European boats built in the last 30 years. If you are standing in a marina anywhere there are sailboats, I’ll bet you a beer there is a boat wired just this way within 100 yards of where you are standing.

I do not entirely understand WHY this appears to be a standard approach in Europe, and most of the posts in this thread are little more than extremely uninformed speculation that make no sense, but that does not change the fact that is is “a thing”.

[Shaneesprit]

Originally Posted by noelex 77

It is quite common on many European boats to have a battery switch to turn off the negative supply. I am surprised you have not encountered this.

Below is the first photo thrown up by Google when searching for “Jeanneau battery switches”. The black battery switch isolates the negative supply.

One example of a standard that requires this is the Australian standard for charter yachts.

Yeap - I find it strange to have a positive switch, Negative switch is normal, at least in the machinery world.

[team karst]

Originally Posted by Shaneesprit

Yeap - I find it strange to have a positive switch, Negative switch is normal, at least in the machinery world.

Really?

Some basics: To isolate a grounded system you want to open the ungrounded conductor. Could be NEG or POS polarity.
Lets say the job at hand is to replace the engine starter motor. Lets say its a common NEG ground system. Further, its a French boat (like mine) and there are both POS and NEG disconnects. You open the black/NEG switch. At some point, a wrench contacts the large red starter wire and the engine block at the same time. No spark, and you are glad you opened the NEG switch. You then continue to remove the red cable and it hits the aluminum fuel tank. A large arc ensues, and you wonder what happened[emoji15]. You messed up and did not isolate the UNGROUNDED (pos) switch.

[Nicholson58]

Originally Posted by noelex 77

It depends on how the rest of the boat is wired, but isolating the negative supply can reduce the chance of stray current corrosion problems and fire. I would not change the manufacturer’s installation.

It is not required by the ABYC, but is is compulsory for some other marine standards.

Adding the neutral isolation has doubled the life of my galvanized chain. Chain out, set, breaker and neutral off.

[SailingHarmonie]

Originally Posted by team karst

Really?

Some basics: To isolate a grounded system you want to open the ungrounded conductor. Could be NEG or POS polarity.
Lets say the job at hand is to replace the engine starter motor. Lets say its a common NEG ground system. Further, its a French boat (like mine) and there are both POS and NEG disconnects. You open the black/NEG switch. At some point, a wrench contacts the large red starter wire and the engine block at the same time. No spark, and you are glad you opened the NEG switch. You then continue to remove the red cable and it hits the aluminum fuel tank. A large arc ensues, and you wonder what happened[emoji15]. You messed up and did not isolate the UNGROUNDED (pos) switch.

This makes no sense to me, but I might be missing something you are thinking of.

If you have disconnected the battery negative from everything with a master disconnect switch, how is the electricity getting from the red starter cabe back to the battery through the fuel tank? There should be no path for a circuit to be created.

(There are good reasons for a disconnect in the positive side, and those are not what I am debating here. I am just pointing out if there is a negative disconnect on the battery, it is, well, disconnected! There is no return route for the electrons.)

[team karst]

Metal fuel tanks are bonded to ships earth.
In my boat both NEG switches only disconnect their respective engines, not the battery itself. There is also a ships NEG disconnect, but i havent dived in to see exactly where it breaks the circuit. It kills bilge pump circuits so it stays closed.

[fourlyons]

Originally Posted by wyb2

For stray current corrosion though the current needs to be passed through the component in question to cause corrosion, correct?

So it needs to be an insulation failure on the bilge pump positive before the automatic switch, and it needs to be sitting in bilge water that is electrically grounded to exterior seawater (plausible, especially with a bolt on keel), so that the current can then use the sail drive or prop shaft as a path back into the boat to battery negative. Is this right?

The reason I ask is the first scenario I imagined was water trapped in a conduit. That could create stray current (or a short), but I don’t think would result in any corrosion issues.

Can a knowledgeable person please address the statement/question posed above?

This is the situation on my boat, I have a small bilge sump completely isolated from all metal except the bilge pump sitting in it. But I experience what I think is fast loss of my propeller and shaft zincs (conventional shaft drive). I have no other metal in the water except a bobstay fitting (dyneema bobstay) and rudder pintle/gudgeon fittings (transom hung rudder), none of which are bonded. Also this is a wood/epoxy hull. My boat is moored alone, no other boats, and no shore power.

I posted on this issue before and a knowledgeable forum member suggested adding a negative side disconnect switch to address the zinc loss. This is particularly relevant right now because I am rewiring the boat.

Thanks

[Bowdrie]

Originally Posted by fourlyons

But I experience what I think is fast loss of my propeller and shaft zincs

Install a "Drivesaver", let the shaft/prop zincs do what they were originally intended to do, i.e. reduce the interaction between a stainless shaft and a bronze prop, not to protect a boat from loose electrons.
If you need/want an electrical connection to the water, (if AC is aboard there are additional things to think about,) use a keel bolt or an external "ground plate"
Oh, and don't use shaft packing containing graphite.
Hypothetical situation: Positive bilge pump wire leaks into bilge water that touches keel bolt.
Keel bolt is connected back to the negative buss.
Perhaps a few electrons flow, but they just go back to the source without passing thru other metals, the prop/shaft no longer forming a pathway in or out for stray electrons.
However, a good search for the source of the loose electrons that are attacking your zincs should be first on the list.

[Pete7]

surely that doesn't solve the problem, just masks the effects. It would be better to find the problem and solve that.

This might be worth watching:

https://www.youtube.com/watch?v=Fth8...HarborsUnknown

[fourlyons]

Originally Posted by Bowdrie

Install a "Drivesaver", let the shaft/prop zincs do what they were originally intended to do, i.e. reduce the interaction between a stainless shaft and a bronze prop, not to protect a boat from loose electrons.
If you need/want an electrical connection to the water, (if AC is aboard there are additional things to think about,) use a keel bolt or an external "ground plate"
Oh, and don't use shaft packing containing graphite.
Hypothetical situation: Positive bilge pump wire leaks into bilge water that touches keel bolt.
Keel bolt is connected back to the negative buss.
Perhaps a few electrons flow, but they just go back to the source without passing thru other metals, the prop/shaft no longer forming a pathway in or out for stray electrons.
However, a good search for the source of the loose electrons that are attacking your zincs should be first on the list.

Thanks, I have considered a Drivesaver, but I have a vee drive. If I slide the shaft forward enough to fit a Drivesaver then I won't have room for a shaft zinc. But if I do that, maybe I can get away with just a prop zinc

I don't use graphite packing.

My keel bolts terminate above the sole. And anyway the main bilge is always bone dry, dust and cat hairs are the problem there. The sump I'm taking about is behind a water tight bulkhead aft and below the cockpit. The sump is there to collect the drip from the shaft seal and any leaks from the cockpit locker lids, if the pump is leaking current into that sump I don't see how it can get out.

[fourlyons]

Originally Posted by Pete7

surely that doesn't solve the problem, just masks the effects. It would be better to find the problem and solve that.

This might be worth watching:

Thanks, I did watch it, interesting. But none of that relates directly to my boat, cold molded wood hull, freestanding wood masts, dyneema lifelines etc. But I get the overarching point, diagnose core problem, don't mask the symptom.

[Bowdrie]

Originally Posted by fourlyons

Thanks, I have considered a Drivesaver, but I have a vee drive. If I slide the shaft forward enough to fit a Drivesaver then I won't have room for a shaft zinc. But if I do that, maybe I can get away with just a prop zinc
the main bilge is always bone dry, dust and cat hairs are the problem there.

When you fit a Drivesaver you slide the shaft aft, you open-up a space between the flanges of ~1" to fit the Drivesaver.
If anything, you have ~1" more shaft sticking out between the Cutless and the prop, of course you need to make sure that having the prop 1" further aft won't interfere with the rudder.
Jeepers, I have the same problem, dust and cat hair in the bilge.

[Fuss]

Originally Posted by Nicholson58

Adding the neutral isolation has doubled the life of my galvanized chain. Chain out, set, breaker and neutral off.

That's a similar problem to me (alu boat). I switched/isolated the negative for my bowthruster.This did show up as not fully floating the negative before so I knew that this needed to be done as the anodes were being eaten.

However this is different from a negative battery switch which I would not use to solve anode problems on my boat.

[fourlyons]

Originally Posted by Bowdrie

When you fit a Drivesaver you slide the shaft aft, you open-up a space between the flanges of ~1" to fit the Drivesaver.
If anything, you have ~1" more shaft sticking out between the Cutless and the prop, of course you need to make sure that having the prop 1" further aft won't interfere with the rudder.
Jeepers, I have the same problem, dust and cat hair in the bilge.

No, unfortunately not on my boat, I have a ZF15 MIV vee drive transmission and a one piece shaft that comes through the output flange which means I have to slide the shaft forward to gain space between the output flange and the shaft coupler.