What follows is based on the recommendations for lightning protection provided by the American Boat & Yacht Council, Standard E4.
The primary purpose of a lightning protection system is to provide for the physical safety of all aboard your vessel. Prudent actions that should be taken during an electrical storm are: 1) If at all possible remain in the cabin of a closed boat. 2) No one should be in the water or have any part of their body immersed in the water. 3) Do not come into contact with any components connected to the lightning protection system of a properly protected vessel. Otherwise your body could act as a conductive bridge between any items connected to the lightning conductive system. For example, you should not be in simultaneous contact with a metal steering wheel and a metal stern pulpit.
A good lightning protective system ensures that all large masses of metal are electrically connected. This purpose should not be confused with that of the vessel's basic bonding system. A properly installed and isolated bonding system is there to provide a low resistance electrical path to reduce electrolytic corrosion and as a measure of personal protection if there is an electrical fault in the boat's AC/DC electrical systems.
If your sailboat is a vessel with an aluminum mast you have the starting point of a well-grounded lightning rod. This will provide a zone of protection for a radius around its base equal to the height of the lightning rod. Due to some vessels overall length, it may be necessary to install another lightning rod to encompass any areas that do not fall within the zone of protection. Don't forget that the mast itself must be physically bonded or connected through to the common ground - one of the keel bolts or if a encapsulated keel, to the grounding plate, in order to provide optimum protection.
The apex of the rod should be a minimum of six inches above any masthead device. The end should be sharpened to a point. The base of the mast or the mast step if metal, should be connected to a keel bolt on externally ballasted vessels. The preferred wire gauge is No. 6 or even better, #4AWG stranded copper. In no case should such a connection be made to a vessel with internal ballast. The result could be a hole blown through the bottom of the hull. Boats with internal ballast should have a copper ground plate of at least one square foot in size installed externally on the hull bottom. The grounding wire should then be connected to the ground plate.
All wire conductors should be kept as straight as possible. All large metal objects above and below decks should also be electrically tied into the lightning ground conductor. This is a precaution against side flashes. Large metal objects include shrouds, chainplates, toe rails, sail tracks, winches, steering wheels, and bow and stern pulpits. These items can be tied into the ground conductor wire by a minimum #8AWG stranded copper gauge wire, or connected directly to the hull ground terminus.
A thorough inspection of the lightning protection system should be conducted on an annual basis as part of normal maintenance procedure. All connections should be maintained tight and corrosion free. Any corrosion will impede the flow of electricity and promote side flashes. For that reason it is important that the lightning protection system receive the same attention as the rest of the systems aboard the vessel. This should be included as a part of the annual lay-up and maintenance procedure. For additional details regarding the lightning protection standards readers should refer to American Boat and Yacht Standard E-4
Source: ABYC Recommendations on Lighting Protection in The Marine Advisor, Spring 2001
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