Negative cable

[snigsnig]

Hi

 

This will be a daft question to some, but its new to me so please bear with me - my BSS exam report advises that I move the main negative cable from my battery bank, currently bolted directly to the hull, to the engine block. My question is, is there a particular/designated place it should be bolted to?? I have a late 90's Beta 1903.

 

And could someone please give me a brief explanation of the science behind bolting to the engine instead of the hull? My examiner did explain it to me but at the time my head was spinning over the money i need to spend on other stuff from the report, so it went in one ear and immediately back out the other!

 

Cheers!

Neil

[nicknorman]

There may be studs on your engine for the purpose - on our Beta 43 they are mid way along the engine, on the mounting frame near the bottom. Not sure for an older model. If in doubt there may be a -ve connection for the starter , or find a bolt near the starter that is into the main body of the engine, clean it up and use that. Better into steel than aluminium though.

 

The reason is to avoid using the hull to transfer current. Passing current through the hull creates small voltage differences between different bits of the hull which can cause or accelerate electrolytic corrosion. Also if you think about it, the current from eg the starter motor or alternator has to get from the engine to the hull somehow, in order to get back to battery -ve. If you have rubber engine mounts, the other paths are through the prop shaft bearing or maybe through the engine control cables (throttle and gear), none of which are designed to carry large currents.

 

The correct way is to have the main thick cable running from battery -ve to the engine. Then run a thinner cable from eg battery -ve to hull, so that the hull is connected to -ve and any +ve wires etc shorting to the hull will blow the fuse rather than raising the hull voltage a bit.

Edited May 1, 2013 by nicknorman

[Paul C]

Its to do with the flow of electricity. When its bolted to the hull, it will flow from the hull bonding point, through the hull, to the engine mounts, and up into the engine block. This 'path' of flowing electricity can potentially cause corrosion, because corrosion is a similar thing to electricity at atomic level, with electrons flowing.

 

In addition to the potential for corrosion, bolting it directly to the engine should be a lower resistance connection, which is good for things like starting (especially when cold).

 

You still need a hull bonding from negative, but this is for safety reasons/fault conditions, and under normal circumstances there won't be any current flowing through the hull.

[David Mack]

Hi

 

This will be a daft question to some, but its new to me so please bear with me - my BSS exam report advises that I move the main negative cable from my battery bank, currently bolted directly to the hull, to the engine block. My question is, is there a particular/designated place it should be bolted to?? I have a late 90's Beta 1903.

 

And could someone please give me a brief explanation of the science behind bolting to the engine instead of the hull? My examiner did explain it to me but at the time my head was spinning over the money i need to spend on other stuff from the report, so it went in one ear and immediately back out the other!

 

Cheers!

Neil

 

Presumably your starter motor and alternator are earthed to the block, so connecting the negative cable there gives the shortest current path. With a hull connection (and no separate earth strap) you would be relying on connection via the engine mounts/propshaft/control cables or whatever, which doesn't sound too clever.

[snigsnig]

Great, thanks for the help guys.

 

So I should just move what's there (the main neg cable) onto the engine block, and then add a thinner cable as a replacement connecting to the hull. And how thin should that be please?

 

Cheers!

Neil

[Paul C]

It should be able to take the largest potential fault current without itself failing, eg around the same thickness as the cable for the largest power device on the boat,. Mine is 25mm2 I think (and 1m long)

Edited May 1, 2013 by Paul C

[bottle]

Some may disagree but the easiest solution is leave existing and put a large cable between the stud and the engine block

[ditchcrawler]

So is there no return from the engine, which is probably sitting on rubber mounts to the batteries or is it relying on the exhaust pipe for the starter motor current to get back to the batteries?

[snigsnig]

No return from the engine. It interesting to note that the two most corroded areas of the engine bay are the exhaust and where the prop shaft meets the hull. Not in a way to be too concerned about (yet), but noticeably the worst bits. I now know why! It looks like the batteries have always been setup this way, as the bolt holding the cable to the hull is clearly pretty old. And interesting that its only on this latest BSS exam that its a fail (3.7.1) (my first while owning this boat), but its always been okay in the past. Is is a new requirement??

 

Neil

[nicknorman]

No return from the engine. It interesting to note that the two most corroded areas of the engine bay are the exhaust and where the prop shaft meets the hull. Not in a way to be too concerned about (yet), but noticeably the worst bits. I now know why! It looks like the batteries have always been setup this way, as the bolt holding the cable to the hull is clearly pretty old. And interesting that its only on this latest BSS exam that its a fail (3.7.1) (my first while owning this boat), but its always been okay in the past. Is is a new requirement??

 

Neil

 

I have to say that, whilst the configuration is not optimal by any means, its hard to see why it should be a BSS fail point. I don't see where the hazard arises.

[snigsnig]

I think it creates a more likely fire risk in dodgy cabling because of the increased current in the hull.

 

Neil

Edited May 2, 2013 by snigsnig

[snigsnig]

I've just noticed that the BSS guidelines actually specify that any electrical current should be completely isolated from the hull, for personal safety.

 

Neil

Edited May 2, 2013 by snigsnig

[smileypete]

Another way to do it is have a hull earth common bonding point that's near the batt bank but towards the engine. Then take thick cables from the domestic bank negative common point, starter batt negative, and engine ground negative, plus all other negative return cables, all to the hull earth common bonding point.

 

I think it's much better and neater than having various return cables festooned on the batts, and makes it much easier to add ammeter shunt to the domestic bank later on, or isolate the batts for replacement without damaging something.

 

cheers, Pete.

~smpt~