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S&S Swan General - Lighting Protection
14 November 2015 - 21:46
#1
Join Date: 01 February 2007
Posts: 234

Lighting Protection
I have just been on a tour of Discovery Yachts factory in the UK and the subject of conducting a lightning strike came up. Discovery say that aluminium is a better conductor than copper, so they wire the mast directly to the keel bolts in two places. Is this an improvement on the Swan approach of fitting a faraday cage?

John B
Swan411 010

15 November 2015 - 01:20
#2
Join Date: 31 July 2007
Posts: 88

A friend of mine got hit by lightning one day out of a Westcoast port. He had a 28 ft boat. It destroyed all his electronics, except the handheld VHF he had put into the oven. I guess the stove was grounded. He returned to port for repairs. My 040/012 has copper cables from the chainplates to the keel bolts. I have been in lightning storms and it is spectacular. The lightning hit the wave tops around me but never the boat. I read somewhere that the purpose of a lightning rod is, to discharge the electric static charge to ground, without the visible flash.

15 November 2015 - 16:59
#3
Join Date: 30 January 2007
Posts: 461

I have just been on a tour of Discovery Yachts factory in the UK and the subject of conducting a lightning strike came up. Discovery say that aluminium is a better conductor than copper, so they wire the mast directly to the keel bolts in two places. Is this an improvement on the Swan approach of fitting a faraday cage? John B Swan411 010

The matter of lightning on sailboats is highly controversial but, in particular, stating that the aluminum is a better conductor than copper does not make sense. The specific resistance of aluminum - the quantity commonly used to define electrical characteristics - is nearly the double than the one of copper and only for applications where weight is of high concern, Al may be a better choice due to its low specific weight. Notice: weight (or, better, mass), not volume which, indeed, must increase to obtain the same characteristics!
Regarding the Swan approach, I do not see any difference with the case you describe. In both cases the mast is connected to the keel although the use of aluminum requires a higher cross-section link (or a double cable) according to what I just explained above.
Daniel, 411/004

16 November 2015 - 12:10
#4
Join Date: 02 February 2007
Posts: 202

Hello John, Peter, Daniel,
This is a rather frightening subject, as no-one (as far as I can understand) can ever be sure where the strike will go after a lightning strike on a mast.
On our dear Swans, one must be very careful about mast bottom corrosion (aluminium versus the stainless steel mast shoe), and to avoid it I have been inserting a 2 mm tufnol sheet in the shoe (under a 10mm aluminium plate) whose thickness is probably not sufficient to stop a lighting strike.
So I still wonder if it would be of any use to add a copper wire to make a contact with the rest of the grounding of the boat. As long as it stays dry of course.
I would be interested to have your views.
Thank you, Kind regards.
Philippe 41/022

16 November 2015 - 12:57
#5
Join Date: 30 January 2007
Posts: 461

Hi Philippe,
you are right, it is a frightening subject also because, as far as I know, there is no definitive answer on which is the best approach.
I found a rather interesting 1995 booklet on the subject "Lightning and boats" by M.V.Huck but, I am afraid, it is not at all comforting and since then no important advancements were made.
Notwithstanding I do believe that there should be a good path for directing the lightning charge to the water and this implies a good electrical connection (i.e. with low resistance and able to sustain the stress) between the mast and the keel.
There is not a unique way to achieve this and the choice is a matter of personal taste, I think.
Daniel, 411/004

16 November 2015 - 18:23
#6
Join Date: 01 February 2007
Posts: 234

Daniel. On our Swans the path would be for the energy to pass down the stainless steel shrouds to the copper cable which is attached to the chain plates and then onwards to the keel.

The path recommended by Discovery is straight down to the keel, so presumably no faraday cage effect, but maybe less resistance to grounding.

Would this make any difference or is either route just as catastrophic?

16 November 2015 - 19:29
#7
Join Date: 02 January 2008
Posts: 1547

Dear John and Daniele
If lightning strikes the main part will come down along the aluminium mast, which is a good conductor with plenty of cross section. The shrouds have much less cross section, and stainless is not a good conductor, mentioned earlier on this Forum.
The shrouds and stays are grounded for additional protection, but it is the top of the mast which attracts the strike and also effectively creates a round conical protected area around the hull by offering a convenient path to the water.
More information on the subject can be found for example in NFPA 780-2008.
Kid regards
Lars

17 November 2015 - 10:08
#8
Join Date: 30 January 2007
Posts: 461

John,
as Lars pointed out in a much clearer way than I did, I never even considered the shrouds. The latter may indeed participate in creating a static iso-potential surface around the boat (a Faraday cage) but, since they are bad conductors, they will behave quite badly in a non-static condition.
This is true for most of the boats and this is also why I wrote that the Swans and the ones you mentioned are essentially based on the same principle.
Daniel, 411/004

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