Lightning at Seaby Kurt Kupper*

Lightning at Sea

Lightning usually occurs in the afternoon or early evening. During the course of the day evaporation takes place and moisture laden air rises to form first fluffy cumulus clouds, then darker thunder clouds. While this occurs in all areas of the world, it is clearly most prevalent in the warmer, tropical regions.
The upper portion of the cloud develops a positive electrical charge, the lower a negative charge. The negative charge at the bottom of the cloud causes positive charges on the earth below to be attracted to and concentrate at high points of objects on the surface. On a boat that may be a mast, radio antenna, a fishing rod or a person.
Lightning is a flow of current from negative to positive. It occurs when the difference in the negative and positive charges becomes great enough to overcome the resistance in the path between them. The potential difference may be up to 100 million volts.
It can occur within a cloud, i.e. between the bottom and the top of the cloud, or between two clouds, or between the cloud and the surface of the earth beneath it. In the latter case, an electrical discharge takes place through the air to objects on the surface and then to the earth itself.
When lightning strikes, it will usually strike the highest object in the vicinity. On a body of water, this is often a ship or a boat. Once it has struck the highest point of the boat, it will take the path of least resistance to the water. The path of least resistance will be along the best electrical conductors.
These are primarily metal objects, ie masts, stays, metal hulls, keels, engines, propshafts and some rudders. Not such good conductors are wood and fibreglass. Unfortunately, the human body is a pretty good conductor of electricity, so the current will prefer to pass through a body rather than through poor conductors.
The path could be through the mast of the boat to a stay, through the body of a person touching the stay, to the wet deck of the hull and then to the water.
It could be through a radio antenna, down the antenna cable to the radio set, through the body of a person touching a part of the radio and again through the hull of the boat.
Or it could be to the tip of a graphite fishing rod, through the body of a fisherman holding the rod, to the outboard motor that he is holding onto, to the water.
But is not even necessary to be in direct contact with a good conductor to be sought out by the charge. This is because side flashes can occur. These happen when the current reaches a point where there is no easy path through a good conductor. The charge will then jump through airspace to the next best conductor to find a path to earth, and this could be a human body, especially if it is wet.
If lightning does pass through a human body, serious burns and even death can occur. The path from one hand and arm to any other limb passes almost directly through the heart, and heart failure is a real danger.
How do you reduce your risk of being struck by lightning?
The most obvious answer is to not be on your boat during a thunderstorm. Before going boating, check the weather reports, and stay on land if thunderstorms are forecast.
These reports are very reliable, but in some cases localised thunderstorms were not foreseen. So watch the clouds. When you see those ominous dark clouds forming, head for home. If you can hear thunder, you are probably within 10km of the storm.
But it will not always be possible to avoid being on a boat during a thunderstorm.
Try to stay in the middle of the boat, below deck, avoid touching any good conductors, and stay as dry as possible. In case the worst happens, it is good to have someone on board capable of administering CPR, as that can save lives.
But it is also possible to prepare the boat to minimise the risk of injury and damage to the boat in case of a lightning strike. This will be discussed in our next article.

*Kurt Küpper is director of Aquavolt Electric Boat Parts. Tel: 02 9417 8455