Lightning – Nature’s Fireworks

2 July 2009By Kate Hubert

Anyone who’s been out during a thunderstorm, especially on a tall-masted sailing ship, will know how vulnerable you feel when lightning starts to strike. Sure, you can navigate around the worst storm cells by eye or using radar, but sometimes you just can’t avoid a bolt from above.

You may feel more secure in harbour, especially with taller boats and buildings around, but even there you may not be safe. In January 2008, the lovely 93-foot sloop Keturah was struck in Falmouth Harbour, Antigua, and burned for 36 hours virtually to the waterline.

To try to avoid damage or injury from a lightning strike, you first need to understand a little about your high-voltage enemy.

It’s hard to talk about lightning without getting scientific, but essentially lighting is massive static electricity between the ground and a cloud. To build the charge, you need a big engine – and thunderclouds are the perfect vehicle. At the top of the cloud, water freezes into droplets (which are negatively charged) and falls, leaving the cloud with a positive charge at the top. Rising air also tends to carry positive particles upwards, leaving the cloud with a massive difference in charge between its top and the bottom.

The negatively charged cloud base then starts to repel electrons in the ground and sea below, pushing them deeper, making the surface more positive. Our atmosphere is a great insulator, but eventually the air breaks down into plasma and short-circuits between the cloud and the earth. That massive short-circuit is a lightning strike.

The thunder you hear is the shockwave caused when the air along the lightning’s path explodes. Because of the difference between the speed of light and sound, you’ll see the strike first. If you hear a thunderclap five seconds after the flash, the strike was a mile away. Ten seconds equals two miles, and so on....

A lightning strike does not always follow the straightest line or shortest path. Even if you’re moored next to a yacht with a great tall mast, your yacht could draw the strike. This is because lightning can jump around, seeking the least path of resistance. In marinas it’s common to get “side-strikes”, where a bolt hits a mast, travels down, then jumps to an adjacent yacht, even up to 20 meters away!

If a thunderstorm is unavoidable, the first priority is to keep everyone on board safe. It may sound self-evident, but this means no wandering on deck, touching metal rigging, swimming (duh!) or using electrical devices such as mobile phones. It’s a statistical fact that more people are killed by lightning than any other weather event.

The average strike has about 20,000 Amps (enough to power around 100 houses) and 90 percent of that energy is heat. While fire resulting from a strike is a major concern, your onboard electrical systems and electronics also are vulnerable. Turn off any nonessential electrical circuits as the storm approaches. It’s good practice to have a spare GPS, VHF and maybe even a navigation laptop on standby in case the main systems get zapped. Even better, put them in a Faraday cage – basically a metal box that will guide any charge around the objects inside, protecting them. If you don’t have a Faraday cage, a metal stove may work as a substitute (this is the same theory that keeps you safe in a car during a thunderstorm).

Commercial marine lightning protection systems try to provide the strike with the easiest path to ground – often the keel or a plate in the hull. Apart from offering this “sacrificial” route through the yacht, is there any more you can do?

Some sailing yachts seem to have sprouted odd brushes at their mastheads meant to dissipate the ions and make the mast less “attractive” to lightning. Received wisdom seems to be that they are far too small to make any useful difference. At least they may deter birds from roosting and pooping on the deck....

Lightning is a truly awe-inspiring force of nature – it can be quite beautiful when seen from afar. But I can testify that seeing the sea apparently on fire (I think it must have been vaporising water) close to my yacht during a thunderstorm in the Malacca Straits was not so lovely! Nature’s fireworks deserve to be treated with respect.