Make a grounding system to protect your boat from lightning strikes

Metallurgist Vyv Cox explains how a heavy conductor can keep you safe at sea during lightning storms

A surprising amount of research has been carried out into lightning strikes on yachts and boats, much of it in Florida but some in New Zealand, Australia and other countries. The findings are somewhat confusing when it comes to the layout of protection.

To summarise, the optimum protection according to all authorities is provided by a heavy conductor, preferably copper of at least 21 sq.mm. running from 15cm above the masthead and all antennae in as straight a line as possible to an underwater plate beneath the mast foot (see Fig 1 ). This should be copper, bronze or Monel, not sintered and not embedded into the yacht’s hull, as electricity prefers to exit via an edge.

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This arrangement creates what is known as a ‘cone of protection’ within which people are relatively safe provided they are below deck, sitting as high above the waterline as possible and well away from the mast (see Fig 2).

The American approach is similar but greater attention is paid to the possibility of side flashes, which may be particularly damaging to electronics and may even blow holes in the hull. This is the reason for the USA preference for electrically bonding all underwater components, such as seacocks, engine, drive and rudder. The cable used is of considerably heavier gauge than is needed for purely galvanic purposes. It is believed that this practice forms a cage-like network of conductors, helping to protect anything within it.

Further problems arise in the case of secondary paths from the main, vertical, path of the current, where the electricity can follow unpredictable routes to the sea, resulting in serious damage and risk to life. Again, bonding helps to reduce the likelihood of secondary paths but may have an adverse effect on corrosion.

Grounding system to protect against lightning

 

A grounding system is designed to provide a very low resistance path to the ground. The air terminal should be made of copper rod of between 10mm and 19mm diameter, installed at least 150mm above all other objects on the boat. On a yacht, this is typically at the top of the mast. On a powerboat, a mast structure of some sort is required.

The down conductor cable provides a low-impedance path between the air terminal and the external grounding plate. An aluminium mast is usually sufficient to facilitate the necessary lightning flow, but should be connected by the cable at its base to an external grounding plate in order to complete the circuit. Non-aluminium masts (eg. carbon fibre and wood) need a conductor.

The external grounding plate should be located where it’s never out of direct contact with the water, and should be made of either copper, bronze or Monel (a predominantly nickel and copper alloy). A long strip is more effective than a square plate.

Some more useful websites on lightning

The National Agricultural Safety Database (NASD)
Discover Boating
Club Marine

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This feature appeared in Practical Boat Owner magazine. For more articles like this, including DIY, money-saving advice, great boat projects, expert tips and ways to improve your boat’s performance, take out a subscription to Britain’s best-selling boating magazine.

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