Bond or Unbond?

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swanny

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New engine just fitted and engineer put a wire to bridge an electronic path over the rubber flexible coupling. I had a flexible coupling before but without the wire bridge. He told me he does with all his installations. I mentioned that my boat is 'unbonded' and he shrugged and repeated his first comment. Nothing is bonded on my glass fibre boat, AC is not connected in any with DC, even the earth/negative. I do have a galvanic isolator which simply sits on/between the AC earth wire. As far as I can see, there is no chance for AC to mix with DC - perhaps maybe through the shore power charger? Dunno. I have only one sacrificial anode which sits on the prop shaft unbonded to anything else. Should I keep the bridging wire or remove it???????
 
This is a very interesting question as most boats nowadays in the UK I think go for the bonded approach. I think I'm right in saying that surveyors would be critical of a non bonded boat.
From what I've read by Nigel Calders, the non-bonded approach can work provided that all internal wiring is done to a high standard, the insulated-return type wiring system is used and all the skin fittings are of high quality and electrically isolated.
So, it can work, but I think I'm right in saying it is not considered best practice in the UK.
Perhaps someone knows more and will tell us.
 
I think that the engineer isn't thinking through what he is suggesting (or doesn't understand). The acid test is whether you have suffered a problem with your underwater fittings up to now. If you have an anode on the propshaft to protect the prop, and everything else is ok, then take the wire off and ignore his iffy advice.

There are lots of people who do things because they've always done them that way - doesn't make it right though. If everything else truly is isolated then take the wire off and leave well alone.
 
I think that the reason for bridging the flexible coupling is to enable the propellor to be protected by a hull-mounted anode. Brushes rubbing on the prop-shaft can do a similar job. If, as you say, you have a shaft mounted anode, this will protect the prop and you don't need the bridge.
 
I am very confused by your description of the Galvanic Isolator if you don't need a connection mains earth to ships earth ie the water then you don't need a GI. It sounds like it might be expensive balast. olewill
 
My theoretical knowledge of this subject is zero. However my prop has had a bonding wire to connect it to the engine for a long time and there have been no problems. But again, my engine is connected to a hull mounted anode. Lacking this I would revert to your original set up and see what happens, nothing probably. My hull anode never wears much, and this is a good reminder to check continuity whilst I am out of the water - another job on the list..
 
Hmm, I can go either way then. The anode on the shaft does wear out during the season and is replaced every year. It is small though. Two years ago I was only in the water for three months and the anode was destroyed however, last year I was in the water for seven and a half months and there was still half the anode left. I guess it depends on where you sit also. both years was spent in different marinas. I bought the Galvanic Isolator three or four years ago on the advice of the sailing press. I have never read that it is only useful if AC earth is connected to DC earth. Doesn't mean it is wrong though. I don't know. To now bond everything is a major job too far this year and unless I hear anymore authorititive contributions I shall take of the wire and continue as normal. I am back to old marina this year so we shall see if the anode will last more than three months!
 
I think the reason the engineer put the bonding wire on is it is considered best practice to bond skin fittings and the engine. If he had not done so and there was a problem later he could be open to question.
If you don't have a bonded system I can't see what benifit a galvonic isolator will give you. As you may know all a galvonic isolator does is stop small voltages enter your boat from the shore supply on the earthing circuit. If your short supply relies completly on the earth from the shore supply then galvonic type voltages are isolated from your boat anyway. Perhaps you have a hybrid system where some things are bonded and some are not....only looking at your boats system in detail will tell us.
 
I cannot see that it makes any difference in your case whether the boding wire is there or not.

Your stern gear is protected by the shaft anode, if any protection is actually needed, but for it to be effective there must be a good electrical connection between it and the shaft. You would expect it to be corroded away fairly quickly being relatively small and fairly close to a big chunk of copper alloy (the propeller) A prop made of a true tin bronze should not in fact need protection but manganese bronze is a common alloy for props. That is actually a copper/ zinc alloy and therefore really a brass which can suffer from dezincification if not protected by an anode.

If your stern gear was protected by a hull anode bonded to the engine or gearbox then the bonding wire on the coupling would be essential to complete the electrical connection to the prop. The system of brushes mentioned making contact with the shaft immediately inboard of the stern gland is possibly a more reliable method. MG Duff market a brush system they call an "Electro eliminator".

I cannot see that your galvanic isolator is doing anything useful if the shore-power earth is not connected to any part of the boat. The current recommendations, however, are that the earth should be connected to the main metal structures on the boat which will almost certainly also then include the negative side of the 12volt system unless that is an isolated system. In your case there will be a path to earth from the engine etc via the stern gear and the water so you cannot claim to be totally isolated from the shore earth. IMO you should comply with current thinking and connect the shorepower earth to the engine block etc, but not skin fittings, via the GI
The purpose of the GI is then as already indicated to block currents from low voltage galvanic sources that can cause corrosion of under water fittings but at the same time allow currents from high voltage sources, ie the mains, to pass. This then gives the normal protection that an earth provides.
Even then the GI is only necessary if the boat is normally left connected to the shore-power supply.
 
Thanks for that guys, very interesting. It makes me wonder what the manufacturers thought about bonding back in 1979. The boat is an old but worthy Contest and I guess back then they either didn't consider it or did so and remained unbonded. I need to read up some more on the issue before I go further. One new point though is this year I have fitted a Kiwi Prop which as we all know has 'plastic' blades on a stainless steel boss. Does this mean the anode will be protecting nothing this year? I guess the shaft 'key' is made of brass or something! If I don't connect the AC earth to the engine then I shall probably take off the wire bridge and leave it at that. With the vastly broad range of opionions it is difficult to know which way to turn. I have the wrong hobby if I want definitive answers I guess. Anymore?
 
[ QUOTE ]
'plastic' blades on a stainless steel boss. Does this mean the anode will be protecting nothing this year

[/ QUOTE ] Yes I guess it does. I'd be rather disappointed if I found it had a brass key but it must be made of "something" If you fitted it did you not notice.

Stainless steel does not normally need the protection of an anode because it is protected from corrosion (apart from crevice corrosion) by an oxide film that forms on its surface. Once passivated by this film it occupies a similar position in the galvanic series to copper alloys (That's why you can use copper ferrules on stainless rigging or bronze props on stainless shafts for that matter.) Once cathodic protection is applied however the oxide film is lost and you end up protecting the stainless in the same way as you would if it were plain steel. Stainless steel can in fact be protected by anodic protection, in effect the opposite to cathodic protection
 
Yes I did notice it was made of something, I just don't know what that something is. I am not good at identifying metals but it looked like brass or bronze. Do I need to protect this if it is within a stainless steel boss? Also, I don't normally antifoul the prop shaft and do get some barnicles on it but now I think perhaps I should include some antifouling on the stainless boss of the new prop.??? I know. I know nothing but I have a boat!
 
[ QUOTE ]
like brass or bronze

[/ QUOTE ] More or less impossible to tell the two apart visually (but I bet the antique dealers can). From a purely engineering point of view its probably not brass and I would not have expected bronze either. I'd have expected it to be stainless steel.

However there is no problem with bronze or brass in contact with stainless and there's nothing you can do to protect it inside the prop hub anyway.

I'd only antifoul the shaft or prop hub if I found it necessary.

I dont know anything about the Kiwi prop. I tried Googling and got loads about New Zealand rugby players!
 
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