Lightning At Sea

It is hardly surprising that the most feared of the Greek and Norse gods were Zeus and Thor, the gods of thunder and lightning. Certainly thunder storms and lightning strikes tend to frighten us all, especially if we are in a boat at sea. Unfortunately detailed forecasting is impossible, as such storms tend to develop randomly, but it is both useful and interesting to understand how these storms develop.

How Is Lighting Formed?

Lightning is a huge electrostatic discharge between electrically charged regions within clouds, or between a cloud and the Earth's surface. It mostly originates in thunderclouds, called cumulonimbus which are responsible for most of our severe weather including snow, hail, tornadoes and rough seas. A classic cumulonimbus cloud will be huge, often with an anvil shaped top, starting at about 3 – 4 miles above the ground and reaching up to over 9 miles in height. It is a convective cloud having colder air at its top and relatively warmer air below. Bubbles of air rise from the warmer sea or land; they will expand, cool and start to condense, forming cumulus clouds. If the water droplets in the cloud freeze, they give out more heat, allowing the cloud to grow and as the ice particles collide, the larger hail particles gain electrons, giving them a negative charge, while the smaller ice particles lose electrons and become positive. These charges are like two pieces of wire connected to the positive and negative terminals of a battery and if the difference between them is great enough or if they are close enough, sparks will be germinated; not small sparks but gigantic ones which we call lightning flashes. If the lightning hits an object on the ground it is called a strike. Lightning can occur from a cloud to itself (intra-cloud or IC), from one cloud to another (CC) and between a cloud and the ground (CG). This type may also occur in the reverse direction, that is, ground to cloud. If you see it strike directly from cloud to ground or between two clouds, you see the path taken by the strike as the discharge ionizes the air, whereas if the strike is behind or within a cloud, the light is diffused and we call it sheet lightning. Lightning is always accompanied by the sound of thunder although sometimes it is so far away that it can be seen but the thunder is not heard. It is thought that lightning occurs approximately 40 – 50 times a second throughout the world, resulting in nearly 1.4 billion flashes per year.

About 70% of lightning occurs over land in the tropics where atmospheric convection is greatest. Over the worlds oceans it is notably less frequent.

What Happens In A Lightning Strike

98% of lightning strikes on boats travel upwards. A vessel makes very good electrical contact with the positively charged water all around it, especially through its metal propellers and it makes an excellent start point for the return path of the lighting, from the sea back up to the cloud. The question of which vessel in the area will provide the return path is determined by size. The largest vessel will provide the biggest electrical contact and therefore the best route. This is why normally only one vessel at a time is hit by a lighting strike, but it may be followed by others which will take out other vessels in the vicinity. The height of the mast is not an important factor in a lightning strike.

Lightning ionises the air along its path, producing ozone with a distinctive metallic smell. If you are close to a lighting strike you might smell the ozone. Even if you were not hit, you could be in a very dangerous position and be the next potential strike victim.

Lightning produces heat and huge magnetic forces. It can vaporize the sap in a tree causing a steam explosion that bursts the trunk; it can melt the earth when it strikes the ground; it can severely damage buildings or tall structures and humans or animals may suffer severe injury or even death, due to internal organ and nervous system damage. It is not usually the strike itself which causes multiple death or injuries, but the shock wave or fire produced. Lightning protection systems can greatly reduce damage by safely conducting a lightning strike to ground.

What Damage Can Lighting Do?

Comparatively few boats experience lighting strikes. Nevertheless if you are one of the unlucky few, lighting can do a lot of damage to your boat; vaporize antennas, destroy electrical power and navigation systems, including bilge pumps, plotter, auto helm, radios and navtex, blow a hole in the hull and cause a fire.

What is even more concerning is the personal injury that may be sustained. In a worst case scenario, a lightning strike on a small sailboat in fresh water, without lightning protection, the situation may be life threatening. Even with a protection system in place you are in great danger. Electrocution is the main hazard. Even if the victim survives the nervous system could be severely damaged.

Often the most at risk boats are the small ones as they are frequently constructed of wood or fibreglass which are poor conductors. Sailing boats with portable masts and vessels with masts mounted on the cabin roof are also at risk.

How Can I Protect My Boat?

Lightning protection is intended to guide the lightning through a preferred path to the keel by means of a good conductor. It is designed to minimise the damage caused by the lightning, rather than to prevent a strike. It involves providing a continuous path, as vertical as possible, from any vulnerable masthead transducers to grounding conductors in the water. This is called a grounding system. On sailing boats the mast is usually used as it is normally made of metal. Heavy copper wire should be run straight down the mast, without any bends, to the metal keel or grounding plates. This should provide a cone of protection around the mast. A bonding system may also be used which is a network of mainly horizontal interconnected conductors attached to large metal fittings, including the grounding system. Another method of protection is the mast-head "bottle brush" a diffuser which reduces static build-up.

You should do careful research before fitting lighting protection systems. They need to be as short and straight as possible to minimise side-flashes. Large voltages develop between anything electrical or metallic which makes the boat a potentially hazardous area. It is unlikely that lighting protection will totally protect your electronics as just a few extra volts are enough to cause extensive damage. Turning electronic equipment off will not protect it from damage and even if your equipment is totally disconnected in advance it will not be completely safe.

Type of water; whether salt or fresh, also has some significance. Because salt water is a better conductor of electricity than fresh, damage is likely to be more extensive on boats in fresh water.

What Should I Do?

Once you have recognised an unavoidable approaching thunderstorm by the building cumulonimbus cloud, then reef down and be prepared for considerable discomfort. If you can hear the accompanying thunder you can estimate how far away the storm is. Count the seconds between the lightning flash and the thunder, divide that figure by 5 and that will be approximately the distance in miles of the storm. Be aware though that thunderstorms may be many miles across and that ground flashes can take place anywhere inside the storm. There may be severe gusts, as cold air in the cloud can suddenly be released as a downdraft. You are likely to see an area of more choppy water and white horses. Hail and heavy rain can ensue and if you are unlucky you may get a full scale thunder storm with lightning. Other signs may be buzzing sounds off the radio antennas or the appearance of St Elmo's fire around the top of the mast and the hull in a strong electrical field in the atmosphere. The blue sparks will tell you that you are sailing into a highly charged zone and a lightning strike could well be immanent.

If your boat has lightning protection down the mast get your crew to stay in the centre of the boat where they are better protected, avoid areas close to the water line, stop fishing and don't touch or go near anything metallic. In an unprotected boat there is a danger area beneath the mast and the boom. On no account should you get into the water. If you have hand-held VHF or GPS, radio, or mobile phone, put them into the microwave oven, where they may be safe from damage.

And Finally

Lightning strikes on boats are rare compared with the number of boats out there on the water, so don't be afraid to sail in inclement weather for fear of a storm developing.

Author – Dee White

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