The current flow and heat from a direct lightning strike can cause catastrophic damage to an unprotected boat. A strike on the masthead may streak through the stays or arc from a shroud chain plate to the water, burning a mark down the side of the boat, and mast-mounted components (wind instruments, TV antennas, radar, lights) will likely be destroyed. If the current passes through an ungrounded mast to the keel, it may burn a hole in the bottom of the boat or start a fire below decks. If the current encounters poor conductors such as fiberglass, plastics, wooden bulkheads or teak decking, the heat can literally atomise, melt or ignite them.
On its way to the water, the current can arc across any gaps in its path or cause ‘side flashes’ to other conductive objects and equipment. A surge of current to or through delicate electronics, electrical devices or electric motors will either destroy them outright or disable them. Surge protectors (or transient voltage surge suppressors) may shield electrical equipment from damage by routing the spike to an attached earthing point but if this is not safely grounded outside the boat, further damage may occur elsewhere.
A lightning bolt is not a single, unidirectional event, but rather a rapid sequence of two-way discharges that involves ‘leaders’ and ‘streamers’ arcing back and forth through the strike path. The intermittent nature of this phenomenon creates electromagnetic pulses (EMPs) that radiate from the strike zone to the surrounding area. If they pass over conductive elements such as power cables and communication lines, the EMPs may induce a current that surges to electrical devices to which they are attached.
LIGHTNING DISSIPATORS
Many boats are fitted with lightning dissipators (LDs), small metal ‘bottle brushes’ on the top of the mast, as a form of lightning strike prevention, but their effectiveness has been questioned. Continuous static charge dissipation systems are often incorporated in laboratories that use electronics with sensitive integrated circuits and transistors. But what works in the controlled environment of an enclosed laboratory with very low amounts of static charge may not work on the open ocean in the middle of a violent thunderstorm.
The science behind LDs is simple. As a storm builds, it draws up ground charge which accumulates as static electricity on a vessel at its highest point — typically the masthead on a sailboat and the corners of the flying bridge on a power boat. If the static charge reaches a certain level, streams of charged particles (ion ‘streamers’) leap from these discharge points to invite a lightning strike from the cloud. Theoretically, LDs mounted at these points gradually bleed off the ions, which are carried away by the wind, before the charge reaches criticality, thereby reducing or eliminating the chance of a strike.
However, there are a couple of problems with this theory. First, in circumstances of extreme electrical imbalance between the water and the cloud, the static charge can build up faster than the LD can release ions and a streamer will develop at the discharge point. Second, if there is little or no wind to disperse the ions, they will simply accumulate over the boat, making the air more conductive and creating an attractive point of attachment for a bolt lurking above.
LIGHTNING PROTECTION SYSTEMS
Most recreational craft, especially sailboats made of wood and fiberglass with tall aluminium masts, are vulnerable to lightning strikes. The amount of damage a boat suffers depends on how much resistance it offers to the passage of electrical energy through it. A properly designed and fitted lightning protection system provides a low-resistance path — the best short circuit — for the electrical current to pass safely from the mast tip to the ocean. Such a system won’t prevent a strike, but it will certainly reduce the damage to the boat and the possibility of injury or death to the passengers.
Australian Standard AS 1768-2007 provides authoritative, non-mandatory guidance on the principles and practice of lightning protection for a wide range of applications, including boats. It is based on the idea that a boat can, and should, be protected in fundamentally the same way as a building, although the precise methods employed will depend on the type of boat and its form of construction.
The system espoused by the Standard, and indeed most commercial manufacturers, consists of three basic elements: an air terminal as a point of attachment for the lightning strike; a lowimpedance down conductor to carry the current to the earthing point (in the water); and an external grounding plate, typically on the boat’s hull below the water line, to dissipate the charge. Such a system can be, and usually is, augmented by ‘bonding’ the boat’s large metal fittings to prevent ‘side flashes’ and the installation of secondary components to inhibit power surges that might damage sensitive electronics and equipment.
THE AIR TERMINAL
The air terminal in a purpose-built grounding system is a vertical ‘lightning rod’ attached to the highest point on a vessel. On a sailboat, this is typically at the top of the mast, on a sportfish it could be bolted to the tower and made to look like an antenna, and on a powerboat, a mast-like structure of some sort is required. Optimally, the terminal should be a pointed copper spike about 15mm in diameter, mounted to project 15–30cm above the masthead so as to intercept lightning before it strikes some other unprotected part of the superstructure, like a VHF antenna
THE DOWN CONDUCTOR
The air terminal must be connected to a down conductor to provide a low-impedance path on the most direct route to the external grounding plate. On sailboats, an aluminium mast is a good conductor and usually sufficient to carry the electrical charge, but should be connected by a cable from its base to the grounding plate to complete the circuit. The lower ends of the standing rigging should be similarly earthed. An aluminium tower will work the same way on a sportfish so long as the legs are adequately grounded. Non-aluminium masts, such as carbon fibre or wood, and boats without masts should be fitted with an insulated, concentric-lay-stranded copper cable, with a minimum cross-section of 21sqmm, as the down conductor, routed well away from other wiring to prevent arcing.
THE GROUNDING PLATE
The purpose of a grounding plate (earthing electrode) is to receive the lightning current from the down conductor, and any bonded circuits, and discharge it into the water. Adequate earthing will be provided by any metal surface which is normally submerged in the water, such as propeller struts and shafts, rudders and metal hulls. However, the latter may be protected by a painted coating or encapsulated in fibreglass which will impede the dispersal of the electrical charge into the water. Ideally, a dedicated grounding plate should be used, made from copper, monel (a nickel-copper alloy), naval bronze or other low-resistance non-corrosive metal.
Experts recommend that the plate should be at least 30cm square and 5mm thick. As the current dissipates mainly from the edges of the plate, a long narrow strip is more effective than a square one and grooves may be cut along it to maximise the surface area. Even star-shaped plates have been used to good effect. On monohulls, a single plate near the base of the mast is adequate, whereas catamarans usually require a plate on each hull to complete the system. A ketch, yawl or schooner requires a plate for each mast and a long strip under the hull between them.
BONDING
The grounding system is designed to handle the full lightning current. But lightning is highly unpredictable, and a strike may exceed the system’s capacity to discharge it quickly enough, or the current may encounter high impedance somewhere along the down conductor, for example at corrosion, bent cabling or loose connections. In those circumstances, the current may seek alternative paths to earth by ‘side flashing’ to other metal fittings, structures or equipment, causing serious damage to them, as well as increased risk of injury to the crew.
One way to prevent side flashing is by a process called ‘bonding’, whereby the metalwork and major electrical components are cabled to one another in an internal (horizontal) circuit which is incorporated into, and earthed via, the main (vertical) protection system. It is recommended that the engine be directly bonded to the grounding plate.
ELECTRONICS PROTECTION
These days most recreational boats, even small ones, are fitted with an array of electronic equipment -chart plotters, VHF radios, depth sounders, engine control units/modules and alarm systems to name a few. In varying degrees, they are mission critical, expensive and/or difficult to replace. All of them are vulnerable to damage by a direct lightning strike. Damage can also occur indirectly from a voltage surge through connection to a shore power grid struck by lightning or a nearby water strike that flashes up the anchor chain.
Because modern marine electronics typically operate on low voltages, it only takes a few extra volts to cause extensive damage. Even if a boat is fitted with a lightning protection system that is properly bonded and grounded, this may not necessarily save the electronics. Additional protection from ‘overvoltage’ spikes is achieved by incorporating surge-protective devices (SPD) or transient voltage surge suppressors (TVSS) into the system.
There are many such devices on the market that go by different names, such as lightning arrestors, spark gaps, metal oxide varistors, silicon avalanche diode and surge suppressor zener diode. They come in a variety of voltage ratings, energy ratings and response times. Some are designed to protect whole distribution systems, while others are suitable only for individual equipment protection.
They all serve the purpose of cutting off excess voltage before it reaches the electronics. The devices are connected to the wiring that supplies current to a piece of equipment, and react to a spike either by ‘clamping’ the excess charge and shunting it to ground or by self-destructing like a disposable fuse. The former kind may still allow some current to pass and critical electronics may need ‘cascaded’ surge protection with several devices in line.
THE WRAP
A lightning strike on your boat may be a relatively rare occurrence, but the fact remains that a single bolt can potentially wreck your boat and injure your crew, or worse. A properly fitted and wellmaintained lightning protection system will not prevent a strike, but it will definitely reduce the chance of a catastrophic outcome. The technology and resources needed for a protective system are readily available and should be given serious consideration. An ounce of protection is worth a pound of cure.
[Disclaimer: This article contains general information intended to convey a broad understanding of lightning protection systems. It does not constitute technical advice intended to be acted upon by anyone. Any person requiring professional assistance concerning this subject should engage an appropriately qualified marine surveyor or engineer to advise on the specific matters relevant to their individual circumstances. Neither the author of the article nor the publisher accepts any legal responsibility for the correctness, currency or completeness of the contents of the article.]
