Lightning Protection System
George Jones, #236, Ca Ira
The big black 2/0 cable attached to the base of the mast about 2' from the deck is the primary ground cable in the lightning protection system.
I've included a picture of the attachment point to the mast, the primary ground cable from the cabin top and disappearing into the hanging locker, and the primary ground point.
On my boat a very heavy 2/0 tinned copper cable made by Anchor is the primary ground ($72.00 by itself). This cable connects the base of the mast with the grounding keel bolt attachment point in "Ca Ira"'s bilge (lightning ground point photo). Cable is attached to the mast with a bi-metallic connector. In addition to this primary ground cable there are #2 tinned copper Anchor cables that connect the shroud chain plates and flow down to the primary ground point in the bilge from both starboard and port. In addition to these cables there is a #2 tinned copper Anchor cable that connects the primary ground point in the bilge with the engine block. The forestay is connected to the starboard chain plates, and thus to the primary ground point, with a #4 copper cable. The backstay is connected to the engine block with a #4 copper cable.
The system is designed to easily carry the largest lightning bolt that nature can deliver and exit it safely through the external lead keel. (Remember that lead keels only work as a grounding point if you sail in salt water; freshwater sailors need an external ground plate. ) A heavy lightning rod is attached to the top of the mast and extends 7" above the top of the VHF antenna. The 2/0 size wire is used to handle the multiple ( 3 to 7) 400 amp return strokes present in a really huge strike. This way I never have to replace anything in the system if I take a hit. The system is designed to deflect all strikes to the mast and then provide a straight path to ground. The rest of the cables in the system are just there to eliminate any side flashes between big pieces of metal on the boat and to keep the shrouds and stays from getting engerized. ( I do need to add a cable connecting the new stainless steel mast beam supports with the chain plates. )
The bottom part of the primary ground cable is further insulated with heavy wall fuel hose. This is to make sure that a really big strike doesn't get any ideas about jumping off the cable and burning a hole through the bottom of the boat. Shouldn't't even be a possibility with 2/0 cable but why take a chance :-) ( Note: sometimes a mast grounded with bare #4 cable will burn a hole through the fiberglass bottom of a boat if the cable is laying right up against the hull and there is a weak point or blister in the fiberglass near by )
I was also asked about any increase in the probability of the boat being struck if it is well grounded versus if nothing is done to ground the boat. IMPORTANT : There is no real world situation in which grounded vs. ungrounded alters the probability of a sailboat being hit. Only the final 3 to 18 foot jump in a lightning bolts stepped ladder makup, varies with voltage and amperage of strike, is altered by a ground being present is the equation. Since there is nothing higher than the top of your mast within 18 feet, you will be hit grounded or not grounded if a bolt is coming down in the area of your boat.
There is, however, a huge difference between a really well grounded sailboat taking a hit an ungrounded, or poorly grounded sailboat, taking a hit.
Really well grounded boat :
There is a big flash and a bang and not much else ( electronics may be damaged due to really high voltages being induced by the strike's magnetic field in lengths of wire connecting the electronics )
Ungrounded or poorly grounded boat :
The gap between the mast step and the water creates the problem. Since there is no direct path for the lightning to follow it makes up its own path or several paths (engine, head, bottom of the boat, through hulls, captain, crew, etc.). It also energizes all the shrouds and stays and creates massive differences in potential between large pieces of metal on the boat. Lightning between two objects on a boat that is created by induced differences in potential is called a side flash. ( A 100 amp side flash traveling through a person is a bad thing ) Also, watch out for red hot pieces of what used to be your VHF antenna falling on you.
It should be noted that it is very rare for a traveling sailboat to be hit by lightning. It is even rarer for a person on board a sailboat to be killed by a lightning strike. ( Your chances of being killed are much higher on a powerboat ) Lightning hitting a sailboat almost always finds a path to ground that doesn't involve a person. Almost always.