This article by R. F. Billinger first appeared in GAM on Yachting November/December 1980. Reproduced by kind permission of the publishers.

Lightning struck the masthead. While most of the charge followed the aluminium mast to its step, some of it passed down by way of the standing rigging at least as far as the chain plates. Having no further fully conductive path to follow, the flash arced across the inside surface of the fibreglass hull to a point near the water line to exit through the many little holes that it blasted through the hull.

The heavy currents induced in the vessel’s wiring by the strong magnetic field surrounding the discharge applied excessive voltage to most of the instrumentation and equip­ment on board. In this particular case, the resultant current surge had an additional curious effect, that of energizing the starter on the diesel engine.

At the time, the boat was tied up at a marina dock in Florida. The owner, a Canadian, had flown back to Canada, leaving the boat unattended. Shortly after the storm passed, a marina employee noticed that the diesel on board the vessel was running, although the cockpit companionway was locked. Investigation of this uncommon situation led to the discovery of lightning damage on board, and probably saved the vessel from sinking.

That was the story told to me by the owner when I met him taking his repaired sailboat through the Welland Canal on his way to the North Channel. It’s a classic example of the type of damage lightning can cause. I doubt that anything could have been done to prevent lightning from striking that boat short of mooring it beside another vessel having a much taller mast, or moving to an area having less thunder storm activity. Florida enjoys approximately 80 or 90 thunder storm days each year compared to 5 to 10 in the Manitoulin area. Perhaps that is why the owner moved his boat!

A rather complex atmospheric condition exists when lightning occurs. It probably originates in radiation from the sun bombarding the atmosphere and producing unbalanced atoms, that is to say, ions having dissimilar charges. Lightning is sometimes caused by the rapid movement of air molecules, as the updrafts of a large cloud produces voltage in much the same manner as a high school Van de Graaff generator. Large voltages, as much as 100,000,000 volts, build up between the atmosphere and ground just prior to the lightning flash. The current along the lightning path is also high. It frequently approaches 10,000 amperes and some­times reaches ten times that figure, although this intensity lasts for only a few millionths of a second.

The lightning stroke begins when whiskers extend downward from the cloud in search of oppositely charged particles. They transform the air into a conductive path as they descend to within 300 or 400 feet of the ground. At this point oppositely charged ionized wisps rise up from the ground to meet them. This completes the ionized conductive path. Electrons will now start to flow from that meeting point towards the ground or water. The large flow of electrons will cause the ionized air along the path to get very hot and at the same time give off a very bright flash. This brightness moves upward along the iodized path towards the clouds, pulling electrons down from higher and higher sources. While the visible flash moves upward, the actual flow of electrons is downward. These electrons will follow along the best con­ductive path they can find during their plunge to earth. The amount of current will be high along that path, regardless of whether it is composed of air, metal, wood or fibreglass. The current is so high, in fact, that the resistance of the object struck becomes an insignificant consideration, except with regard to the voltage drop across that object and the power dissipated in it.

The sailor might receive some warning of an approaching lightning storm by observing the appearance of the sky or noting the build-up of static on his radio. In the Lake Ontario area, he can learn of the storm by listening to the weather forecasts from Environment Canada on 146.475 mega­cycles, or the marine weather on channels 21 or 836. Generally speaking, though, the appearance of large cumulus clouds in the west showing a marked vertical development, a dark base and perhaps the characteristic anvil top, suggests the possibility of an approaching storm.

Thunder storms can be classified as either convection storms or frontal storms. Convection storms, which account for the majority of thunder storms, are localized phenomena and, because of this, are sometimes missed by weather forecasters. Frontal storms, however, extend over a much larger area and may last for several hours. They are therefore easier to forecast.

In relatively flat areas such as a lake, the probability of lightning striking any given point is low, but an object such as a wood or aluminium mast rising above the average height of land or water will attract lightning that would otherwise have struck elsewhere. A sailboat mast will attract lightning within a circular area, the size of which depends upon the height of that mast and the potential intensity of the stroke. A 40’ mast, for example, will attract a 20,000 ampere stroke that would otherwise strike within a radius of 80’ had that mast not been present. This radius increases to about six times the height of the mast for a 60,000 ampere lightning stroke and ten times the height of the mast for a 150,000 ampere stroke.

Three distinct phenomena are associated with a lightning stroke; each can result in damage potentially destructive to the boat. The most obvious is the initial effect of a direct strike. Two years ago, we tied up to the visitor’s dock at the Royal Canadian Yacht Club for weather to settle. During a brief spell when conditions seemed to be improving, we ventured out into the lake, but decided to turn back due to the uncomfortable sea running from the east. Returning to the same dock, we found that a tall tree standing perhaps 20 feet from where we had been tied had been struck by lightning. The direct hit had caused sufficient heat build-up to boil the sap flowing in its trunk, resulting in an explosion virtually disintegrating the tree. The same thing could happen to a wooden mast or other moist object on deck.

A lightning discharge is also surrounded by an intense magnetic field. The flow of electrical energy we may be able to conduct harmlessly to ground by providing a suitable highly conductive path, but the resultant magnetic field will still induce a voltage into any conductors nearby, such as the wiring to instruments on your boat. Damage to this electro-magnetic induction effect is more difficult to prevent.

Lightning will discharge through non-conductive material. If an attractive conductive path does not exist, it is quite capable of chewing its way through a fibreglass or wooden hull in search of ground, carbonizing or disintegrating a pathway as it arcs. The main hazard here is the possibility of a hole being blown through the sides of the vessel to let the lighting out which is followed by water coming in. 

Protection from lightning damage comes in varying degrees. A boat may be virtually disintegrated by a direct lightning bolt if it has not been protected in any way. Partial protection will result in less damage, and a really efficient grounding system may even allow the vessel to completely escape damage if struck by a small lightning flash. A carefully planned and installed lightning protection system will provide a significant amount of protection to the boat and its crew. The first priority is to send as much of the lightning energy as possible directly down the mast and into the water. The water, in this instance, acts as a ground connection. The greater the salt content of the water, or the larger the area of contact   with the water, the better grounding there will be. Short of having a metal hull, it will be necessary to have some metallic plate in contact with the water to allow electrical current to pass with little opposition into the surrounding water. A sailboat with an external keel may have a connection from the base of the mast to a keel bolt, but with a fibreglass encapsulated ballast, it would be necessary to affix a metal plate at the least one square foot in area to the outside of the hull. Care in the selection of plate material and through hull bolts is required. The prospect of galvanic action eventually insulating the plate from the bolt holding it to the hull must be considered. A commercial plate of porous bronze is available in which the heads of the mounting bolts are completely encapsulated within the plate material. The porosity of this particular ground plate increased the effective surface area and establishes a better ground. While expensive, it is worth considering this key link in the chain of protection.

The question of how large the wires connecting the mast or standing rigging to the ground plate must be is difficult to answer. In many installations, much heavier wire than necessary is used. Lightning current is of very short duration. As a result, the heat produced in a good conductor by the flow is not great. The wire must be large enough that it will not be heated to its melting point. Copper wire of 46 American Wire Gauge should provide an adequate safety factor. Aluminium wire, if used, should have a cross-sectional area about 1.6 times as large as the copper wire for the same current carrying capacity.

To pass as much of the lightning energy as possible directly to the water, the mast, standing rigging, chain plates, etc. should all be connected to the ground plate. If the mast on the vessel is wood, then grounding the track on the mast will have a useful effect. Alternatively a separate grounding wire could be run from the masthead down. The grounding wire should connect to the standing rigging from each chain plate; terminate the fore and aft rigging to the central ground plate on the vessel, It would be a good idea to install grounding wires from any other large metallic objects such as sink or refrigerator to the plate as well. Remember that lightning wants to go to the ground in as straight a line as possible, so any bends in the grounding conductor should be kept to a minimum and be very gradual. Bonding connections made between chain plates, sink, stove, engine, etc. will also help to maintain low voltage levels throughout the boat during the lightning stroke and minimize the risk of electrocution to persons below deck.

A word about masthead antennas might be in order. It is would be best to use a type having a built-in coil between the radiating section and its mounting system at the masthead, or perhaps having a lightning discharge device as an integral part of its design. Fortunately the coaxial cable connecting the masthead antenna to the radio is usually routed inside the aluminium mast down to deck level. This will tend to reduce the flow of lightning energy in the coax.

If the lightning discharge can be kept within conductors leading to the ground, a minimum amount of damage will occur. The greatest hazard occurs when the stroke leaves the conductor and jumps or arcs through a path of non-conductive material to reach the ground.

Lightning can be a complicated subject, but a system offering a significant measure of protection to the average sailboat can be installed by the owner without a great deal of difficulty or expense. The key to effective lightning protection is to create a highly conductive grounding path for the runaway lightning energy to follow directly from masthead to water and by bonding all large metal objects together, to maintain equal voltage levels throughout the boat during the lightning strike.

In a properly protected boat, the safest place for the crew is below in the cabin; the most dangerous spot may well be at the helm, particularly if one hand is on the wheel and the other on some other metallic object such as a stanchion or part of the rigging. If both of these objects had been grounded and electrically bonded together, even that hazard would be drastically reduced. You can install lightning protection yourself. I hope this article will give you the encouragement to get on with the job before the next storm.

Mr. Billinger is affiliated with Shortwave Marine Electronics Ltd.

Last updated  13 January, 2006 - © Aloha Owners Association