Lightning at Sea: Part One
Chris Whitelaw explains why lightning is such a formidable force, considers the odds of catching a bolt, and offers tips on how best to avoid it
Lightning is a violent and powerful force of nature. Of all the perils sailors face at sea, lightning is one of the most dangerous. It is highly unpredictable and far more common than you might think. Although a lightning bolt may last only milliseconds, a direct hit on an unprotected boat can cause catastrophic damage. There is no ‘safe’ place for a boat in a thunderstorm and sometimes lightning just can’t be avoided, but it is possible to take precautions against a bad outcome.
LIGHTNING 101
Lightning is usually produced by cumulonimbus clouds, huge formations with bases typically 1–2km above the ground, rising as much as 15km to flattened, anvil-shaped tops. Within them, warm, moisture-laden air rises rapidly, cooling and condensing into water vapour the higher it goes. The denser cold air falls and may leave the cloud as rain, or it may reheat and rise again to repeat the process. This turbulent cycle produces ‘charge separation’, an electrostatic imbalance in which positive ions gather near the top of the cloud leaving negatively charged particles near the bottom. As the negative charge increases, it induces an accumulation of opposite, positive charge on the surface of the water (and boats) below.
Sometimes, the static charge on a boat may dissipate harmlessly as a gradual flow of ions into the atmosphere, seen as an ethereal blue flamelike discharge from the spars and rigging, known as St. Elmo’s Fire. But if the concentration of ground charge is strong and rapid enough to overcome the resistance of the air beneath the cloud, the electrostatic difference is suddenly and violently neutralised by a massive flow of negative ions across the gap. This is cloud to ground lightning. The strike will seek the most direct route to the water and, if an electrically conductive object such as a projecting aluminium mast intervenes, it will strike there and dissipate in all directions through the boat.
The peak current flow of an average lightning strike ranges between 18,000 and 140,000 amperes, and produces an instantaneous release of about one gigajoule of energy. The flow of electrons along the strike path is incredibly hot, forming a plasma that reaches 30,000°C, five times hotter and brighter than the surface of the sun. This sudden explosion of energy produces a powerful shock wave which is heard as thunder.
THUNDERSTORM DISTRIBUTION AND FREQUENCY
At any given moment, there are some 2000 active electrical storms throughout the world, producing about 8,000,000 lightning flashes per day, of which only 25 per cent touch the ground. Lightning does not occur uniformly around the planet. Its distribution and frequency depend on many factors, such as ground elevation, latitude, prevailing wind currents, relative humidity, and proximity of warm and cold bodies of air and water.
Most thunderstorms occur within the Intertropical Convergence Zone (ITCZ), which has abundant tropical moisture, strong surface heating and vigorous trade-winds. They are more prevalent in the southern hemisphere during the summer months as the ITCZ undergoes a seasonal shift to the south. They are more common over land than over the oceans, and are generally more frequent near coastlines where there are sharp contrasts in air masses.
In Australia, thunderstorms are most frequent over the Kimberley, Top End and Gulf during the monsoon season (December–March). Curiously, Queensland’s tropical coast between Cooktown and Innisfail experiences relatively fewer thunderstorms because the moist, south-easterly air flow from the Coral Sea generates frequent showers with less electrical activity.
Thunderstorms are also common in the spring and summer months over southeast Queensland and along the New South Wales coast. An estimated 5–10 per cent of this storm activity is severe, producing heavy rain, hail and damaging winds. The incidence of lightning generally decreases towards the southern latitudes, with storms occurring in the cooler months (May–September) in association with active frontal systems sweeping in from the west.
WHAT ARE THE ODDS?
The odds of a person catching a direct hit from a bolt of lightning are about one in 700,000. It is estimated there are five to ten deaths a year from lightning strikes in Australia, with more than 100 people seriously injured. In a boat on the water, the odds shorten considerably to an average of around one in 1000, depending on the type of boat and its geographic location. That might not sound like a significant risk but in the United States, where there are about 12 million recreational boats, that’s 12,000 boats getting hit by lightning every year, in that country alone. Other international statistics show that more than 10 per cent of fatalities occurring on cruising sailing boats are caused by lightning.
The odds for motor boats are slightly higher than the average (1.5 in 1000) and higher still for monohull sailboats of comparable size (4 per 1000). The bad news for catamarans is that multihull boats are two to three times more likely to be struck by lightning than monohulls — due to their increased surface area in contact with water and the lack of a keel directly beneath the mast to facilitate grounding (earthing) — and the average damage bill is around 67 per cent higher.
Size and composition are also factors that affect the likelihood of lightning strike. Tall, high profile boats are more at risk than dinghies, and bigger craft are more likely to attract lightning because of their larger footprints, higher content of conductive material, and more electrical equipment. The number of strikes on boats is increasing, with one international underwriter estimating that the total number of such incidents each year has tripled over the past 15 years. Also, the cost of a strike has risen enormously as yachts carry more networked electronic devices and systems.
DODGING THE BOLT
To state the obvious, the best way to prevent lightning damage is to avoid thunderstorms, if you can. It’s always a good idea to monitor the weather forecast before heading out. Short-term forecasts are fairly good at predicting bigger storms, but small, localized storms might not be reported. That’s when local knowledge comes into play. Be flexible in planning your passage and be prepared to stay off, and definitely get off, the water whenever stormy weather threatens.
It's also important for boaters to be able to read the weather, recognise a growing storm and track one in the vicinity. The development of large cumulonimbus clouds is a good indication that a thunderstorm is brewing and that it’s time to head for shore or move out of its path, if possible. Don’t wait for the rumble of thunder or the flash of a strike because by then it could be too late to take evasive action when the storm begins in earnest.
If clouds are forming directly overhead, with only a grey overcast visible, one way to get an idea what is going on in the area is with a cheap AM radio. (The FM radio band doesn’t work nearly as well for this purpose.) Tune it to, or slightly off, a local radio station, and listen for static which indicates electrical discharges in the atmosphere. The louder the static the closer the storm. Other signs of electrostatic build-up around the boat include buzzing sounds off the radio antennas and the appearance of St Elmo’s Fire around the top of the mast and rigging.
Once a thundercloud starts to produce ‘ground flashes’ and thunder, both can be used to track the storm’s course and determine how far away it is. If the bearing to the lightning does not change with progressive compass readings, the storm is likely heading your way and it’s time to adjust your course. To estimate the distance, count the seconds that elapse between the lightning flash and the sound of the thunder. As sound travels at about 340m per second, a 30-second lapse time means the storm is about 10km away.
If running for shelter is feasible, do it — there are few situations more perilous for boaters than facing lightning in open water. If you think your boat has the power to outrun a storm, consider this: fronts generally advance at an average of 20–25km/h, and supercells may move at 65–80km/h; engines can malfunction at the most inopportune times; big lightning storms can leave no room to escape; and lightning can sometimes strike up to a mile ahead of the storm front.
NOWHERE TO HIDE
If running is not an option, or shore and shelter are too far away, it’s time to hunker down and make preparations to ride it out. With or without a lightning protection system, it is crucial to take precautions for the safety of those on board in the event of a strike.
Drop and furl the sails early ahead of the sudden, intense winds at the leading edge of the front. If possible, lower the mast and secure it with the boom. Be aware that in an unprotected boat the areas with the highest risk of injury are at the base of the mast, directly under the boom, below the steering pedestal and near the engine. Lower, remove or tie down the radio antenna and other protruding devices if they are not part of the lightning protection system.
Pull in fishing lines and stow the rods. If anyone is in the water, get them on board quickly as electrocution is highly probable if lightning strikes nearby. They should dry off and change as soon as possible. To protect portable electronics (laptop, mobile phone, tablet, headlamp, handheld VHF or GPS, etc) put them in the microwave which will act as a ‘Faraday cage’.
If the boat has an enclosed cabin, get everyone below decks, away from the sides of the vessel, clear of electrical cabling and electronic instruments, and well away from the mast-to-keel area. If no cabin is available, move the crew toward the middle of the boat and under a hardtop or Bimini (without touching it). They should remove all metal jewellery and stay low in the boat.
Everyone should be careful not to touch, or go near, any metal objects or surfaces. If you have one, engage the autopilot rather than steering by hand. Otherwise, the helmsman may have to steer with a wooden object or wear rubber gloves for insulation. At the least, keep one hand in your pocket so as not to complete a circuit for the current to pass through your body.
Disconnect aerials such as the VHF, anemometer, navigation lights and radar. This may not preserve them, but it limits the risk of the electric field spreading to the rest of the boat. Also, disconnect all power and interconnecting cables to the electronics and refrain from using electrical equipment, including the VHF radio, unless it’s an emergency (handhelds are OK). As the circuit breakers may not be sufficient to prevent a power surge, the cables must be physically separated from the batteries when disconnecting them.
If the boat sustains a direct hit from a lightning strike, check the crew for any casualties. Immediately begin CPR on unconscious victims if a pulse and/or breathing is absent — there’s no danger of getting a shock from someone just struck by lightning. Break out the first aid kit and treat burns or minor injuries. In the meantime, have someone check the bilges for water. If the strike has blown a hole in the hull, plug it and get the bilge pumps working.
Once the danger has passed, check the operation of the engine and all electronics. Compasses and navigation instruments should be checked for calibration. Repair any obvious damage, if possible, and head for the nearest port. If the situation is dire, make an emergency call on the VHF. At the earliest opportunity, have the boat hauled out to check for hull damage and arrange for a full inspection by a marine surveyor. Advise your insurance company and keep any damaged parts for assessment before being thrown out.
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