Swan 41 earthing plate
Dear Forum, I visited Scotch Bonnet in NZ last week where the deck is nearly finished and starting to look good, we are now working through a list of other jobs. This includes servicing the seacocks and question was raised whether they should be bonded to earth. Should this be done? I've read that the older Swans were built without this so as all looks good at the moment unless the servicing reveals anything I thought to leave this job, at least for now? On a similar subject of lightning protection and earthing, is there a stainless plate on the bottom of the keel and should it be left unpainted? We identified the outline of what might be a plate but painted over, the yard intended to clean it off with a wire brush to investigate but if it is there I don't know if it should be left uncovered? I was also interested that the yard didn't seem to recognise the subject so maybe I am way off mark and there isn't an earthing plate but if there isn't how does the earth to the keel bolt get to ground and if it is through a stainless plate why doesn't this corrode? So many questions when it comes to electrics. Thanks for your help, Andrew 41/039

Andrew, as Blakes seacocks are usually all one metal and not attached to anything metalic, they are not in my experence bonded to any earth.

You need to check how your engine is wired now, eg my Perkins M65 is twin pole insulated return, the P Bracket and stern tube are bonded to the engine.

 My yacht also had a Faraday cage fitted which caused me and the yard some confusion interms of what all the earthing wires did. The forestay and shrouds are bonded to the keel. The backstay is used as a SSB ariel so not part of the Faraday cage. 

Thanks John, I am at "a little bit of knowledge can be dangerous" stage so not sure of implications around the engine earth, I hope as the original Perkins it will be ok but will ask the yard to check. Do you have any idea about a stainless plate on the bottom of the keel? Cheers, Andrew 41/039

Andrew.

The "stainless plate" at the bottom of the keel is a grounding shoe.

Grounding, as in "going aground" not electrical grounding. The bottom of the keel and the thin trailing edge is strengthened by stainless steel plates to protect the soft lead of the keel, from damage. All the other keels I have seen have been painted.

regards

Paul 411 042.

Dear Andrew
If you allow me to comment:
There is no reason to bond the seacocks into the electrical system, as John says they can take care of themselves.
The stainless plate at the bottom of the keel is the docking shoe, it is V-shaped, not intended for other purposes, and should be painted as the rest of the keel. Stainless is slightly nobler than lead according to he galvanic series, and the lead keel works as a sacrificial anode, but a very big one compared to the shoe, and therefore corrosion is negligible, particularly if painted.
The lightning protection system is separated from the electrical system, and forms a Faraday’s cage as John points out. The cables are connected to keel bolts, and this conveys a possible strike into the lead keel, from where the power is dissipated into the water. The lightning will find its way into the water also through layers of paint on the keel. It is important that the cables have enough cross section, and no sharp bends. If there is not an easy path for the lightning it may take the straightest route to the water, maybe through the bottom laminate, and then possibly also blow a hole along its path.
The aluminium mast is a much better conductor than the shrouds, and has a big cross section, therefore most of the strike will come down through the mast. It is important that the bottom of the mast has a good connection to the keel.
Kind regards
Lars

Dear All, thank you very much for your advice which much appreciated and understood, a couple less things to worry about that I can now cross off my list which is growing shorter!
Cheers, Andrew 41/039