Gordon is right in that copper is the best lightning plate, but it is a real problem on an alloy hull
as it immediately sets up a half-cell and begins to corrode the boat or more usually, eats the zinc off your anchor
chain first then begins to eat your boat. A thick copper plate is very heavy, and since the mast
is a wider pipe than it needs to be for carrying the sails
, I have used the mast
whivch is alloy as part of the system and run the aluminium cable up the inside of the mast. If my boat had a wooden mast I wouold probably have gone with copper--
However--the weakness is not the specific resistance of the conductors but the size of the grounding plate that stuffs mopst systems. A copper plate of one square foot is fine for a radio
earth where only a few hundred watts at most will be discharged. It is simply of insufficienbt area, a little over two square feet, to allow for a discharge likely to occur from the locally charged atmosphere, which is where all small strikes come from. The bigger this plate can be, the better will be the area in contact with the salt water
, and it is this area that will determine how effective the system will be. If the resistance at the saltwater junction is twenty ohms, then there is no point in running heavy conductors with a resistance in total of less than this amount--and twenty ohms id pretty thin wire. IIf you want to be safe, run big wires and have big plates, and if one under the mast lowered into the water
is alloy and has an area of two square metres it has to be a heap better than one of copper half the size. The good thing about copper is it does not encourage marine
growth and will be easy to clean--but it is heavy and loves to corrode anything else it is connected to electrically--especially anchor
chain galvanising and zinc anodes.
I use aluminium because it is lighter and a good conductor--just not as good as copper--but it has so many other advantages for me. I used copper in the past and most of my electrical
plant systems I used copper--so I have nothing against it on boats except weight and its potential for galvanic action with alloy or zinc.
The only simple analogy I can give for lightning is a gas discharge tube lkike an electronic flash. At each end of the tube we have conductors, and between these conductors is a large electrical potential difference. Between thes conductors is a gas at low pressure. Around or alongside the tube is an electrode or piece of coiled thin wire. It takes only a small amount of currebt flowing in the small wire to ionise the gas--and whamo--a big discharge.
Now--it takes a bit of imagination to see the charged upper atmospher and the millions of volts of potential difference with the surface of the earth as one electrode and the earth plus your boat as the other--and the local ionising charges eletrostatically generated from cloud movements in the lower layers of atmosphere as the ionising current
to fire ythe big discharge--but if your lightning protection can prevent that ionising current
from turning the air between you vessel and the nearest area of high charge potential by earthing it before it can increasingly ionise the moving and partly ionised gasses moving around and above your vessel--then it can certainly go a long way yowards making your vessel proof against a large strike, which will certainly damage it severely and possibly injure or kill anyone aboard.
Now--my little theory might be rubbish--but it is the only one I can come up with which explains simply why big strikes do not hit protected plant. Small discharges the conductors can handle--but not the biggies--and it is the biggies we need to protect ourselves from.
If you have stainless steel
or glavanised shrouds and stays connecting them at the masthead will do to get rid of charges, but if you feel they may be outside the area of protection afforded by the mainmast collector then you do need additional grounding. Alloy plates dropped over the side connected by alloy strapping (chain is a bit risky--the resistance at the link contact faces is comparatively high) is a good idea. If propellor shaft and propellor etc is a better earth than your earth plate it too will form a path to the sea for any current in your dissipation system, so it ought to be protected by having good connections to any discharge plates forming an alternative path. Connect everything except your electronic equipment
. It requires a separate earth system unbonded to the lightning dissipation system.