Do you connect through-hulls to an anode?

[VicS]

why Westerly chose it connect this through hull fitting

I cannot think of a reason unless it was because the fitting used (gate valve ??) was not of a corrosion resistant material and therefore needed to be bonded to the anode system.

If not bonded to anything it would just be sitting there, not part of any circuit and therefore if of a corrosion resistant material doing nothing.

Not sure I know what you mean by the engine becoming "live". Positive presumably, but positive in respect to what? The crafts earthing system and the hull anodes?
If that happened the stern gear and shaft anode would be at risk of damage by electrolysis but an unbonded skin fitting would still be sitting there and, because not part of any circuit, doing nothing. If it is bonded to the engine then because it is then also made positive then it will also be at risk of electrolytic attack.
If there is any risk of a fault making the engine positive with respect to the hull anodes, or any other under water fittings, bonding the inlet seacock to it can only make matters worse.

Be interested to know why Westerly did choose to bond the inlet seacock alone to the engine

 

[Vonasi]

I cannot think of a reason unless it was because the fitting used (gate valve ??) was not of a corrosion resistant material and therefore needed to be bonded to the anode system.

If not bonded to anything it would just be sitting there, not part of any circuit and therefore if of a corrosion resistant material doing nothing.

Not sure I know what you mean by the engine becoming "live". Positive presumably, but positive in respect to what? The crafts earthing system and the hull anodes?
If that happened the stern gear and shaft anode would be at risk of damage by electrolysis but an unbonded skin fitting would still be sitting there and, because not part of any circuit, doing nothing. If it is bonded to the engine then because it is then also made positive then it will also be at risk of electrolytic attack.
If there is any risk of a fault making the engine positive with respect to the hull anodes, or any other under water fittings, bonding the inlet seacock to it can only make matters worse.

Be interested to know why Westerly did choose to bond the inlet seacock alone to the engine

My feeling is that the anode, propshaft, prop and this hull fitting are all mounted within close proximity of each other. If there were to be some current leakage to earth on the engine from the alternator or starter then the anode, prop and shaft would all be at equal voltage and so no corrosion would occur on them, but the engine intake hull fitting would be located in a good position to become eaten away. If it's bonded then there can be no potential voltage difference and no corrosion.

I apologise if I'm talking rubbish but it's a long time since I've read up on this subject properly.

 

[boatmike]

the `urghh!´ is frightening
I just damaged the plastic speed/transducer of Autohelm st50 of my steel boat built 1976. Should I replace with bronze (plus a bonded anode)?

I didn't mean to frighten you! Actually most of us use plastic for transducer housings....no problem. Many use "marelon" or similar for other uses too. I don't like them only because if you do the nuts up too tight or have valves etc hanging off them internally without independant support they can break off. Not a problem if used properly, but I just don't like them!

 

[boatmike]

Be interested to know why Westerly did choose to bond the inlet seacock alone to the engine.

I think the assumption is that manufacturers always do the right thing. Having worked in the industry I can tell you it ain't necessarily so and many things are done because...well we always do that don't we?

The dubious argument I have heard for bonding inlet seacocks goes as follows.

When the engine is running, or even if it isn't if the waterpump is below the waterline, the seacock, waterpump and engine are directly connected through the column of water inside. Closing the seacock does not necessarily break this connection either as both sides of the valve are "live" still. Therefore the engine, propshaft and prop are connected to the same circuit as the seacock and it should be protected.
The nonsense is of course that if you mount the anode near to the prop (as you should) and the anode is miles away, bonding it will do now't anyway and if it is connected through the column of water the strap connecting it is redundant anyway!
It's just another one of those urban myths that are prevalant in the marine industry!

 

[Danny]

I've received conflicting advice on this, and will probably be replacing a few seacocks soon; if it's better to wire them up then I will. However one source suggested that wiring them up actually increased corrosion! Hence my confusion. What does the panel think?

I reckon it works something like this:
Assuming there's no other connection to it, a bronze seacock is a lump of corrosion-resistant alloy sitting in seawater. The seawater may be connecting it to other bits of metal such as the engine (through the column of water in the inlet pipe), the prop shaft, other seacocks, anodes, the keel, etc. but, if there's no other connection APART from the seawater then there's no circuit so no current flows out of or into this lump of metal. It's like dangling the seacock over the side on a bit of string.

 

[mitiempo]

Agreed and as long as the shaft zinc exists in good condition it is the least noble metal in the vicinity and will dissolve before anything else.

 

[Kelpie]

Thanks for all the replies, seems to be quite a consensus.
Leaving them unconnected seems to be the best course. The existing ones are unconnected and are probably original, according to my surveyor, which makes them nearly 40yrs old! Not too shabby, even if they are getting past it by now.

 

[Seanick]

In the mid eighties is was in vogue and was taught to bond everything.
That advice has been revised, even by anode suppliers, to bond as little as poss, and protect at source.