Expert opinion
Marine surveyor Ben Sutcliffe-Davies Over the years I’ve attended a few lightning strikes suffered by yachts on behalf of insurance companies. In the UK it’s not such a common issue as, say, in the Mediterranean. That said, the ones I’ve seen have caused some spectacular damage.
Most production yachts that have iron keels do have earth straps from cap shroud and mast steps on to one of the keel fastenings. My own yacht, being encapsulated, isn’t so easy to deal with simple earthing to water.
I don’t have any proof that a dissipator is better or worse than an unprotected mast, but since sailing in the Ionian I’ve certainly seen more storms brewing with stunning lightning. What I do know – from sailing with my father in storms and also from chewing over the risks with my friend John Goode – is to carry out the following:
Disconnect everything electrical ASAP. And that’s everything – VHF and aerials, radar, deck sockets, nav lights. John also goes one step further and disconnects the batteries.
Get your kedge anchor out and wrap the chain around the cap shrouds and lower the anchor deep into the water (another good reason why you should have 10m of chain!). I have a keelstepped mast so we’d wrap chain around the mast as well, and then overboard; the last thing you want is for lightning to go inside.
My father and I have always carried a set of long battery leads. Being around boatyards you often find owners on swinging moorings have flat batteries so these are useful to jump-start a yacht. In the case of a lightning storm we would clip one end of the leads onto the cap shroud and the other end onto the kedge anchor. As you know, lightning takes the shortest route to earth so a strike on the top of the mast would (fingers crossed!) run down the cap shroud and overboard rather than down the mast.
I’ve had customers report when dealing with claims that they have used the oven to put the VHF radio and handheld GPS in to try and protect them. Unfortunately
I’ve no actual proof this works but the oven as a Faraday cage does makes sense.
Metallurgist
Vyv Cox
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 over the page). 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.
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.