Expert sailing advice: How to handle a lightning strike on board

Pip Hare

Pip Hare shares advice from sailors who have experienced a lightning strike on how to avoid getting hit by an electrical storm

Lightning is the thing that scares me the most at sea. Having never experienced a lightning strike I think this is mostly a fear of the unknown, coupled with a sense of helplessness.

My lightning strategy has always been to sail in the opposite direction and hope for the best. The following is a combination of my own practice and observations from sailors who’ve experienced a lightning strike first-hand.

Avoiding lightning

Thunderstorms are created in conditions where there is great instability between the upper and the lower layers of the atmosphere. Typically, thunderstorms follow an extended period of warm, still weather, but lightning can also form along very active frontal systems – this tends to follow a sustained period of average pressure, with little gradient breeze when the new front moves in quickly.

Forecasters can predict where there will be increased potential for lightning to form, but not its actual occurrence or exact location.

Specialist forecast models such as the CAPE (convective available potential energy) and the LI (lifted index) show storm potential by highlighting areas of atmospheric instability.

CAPE and LI forecasts are available via specialist weather sites and CAPE GRIBs can be obtained through some providers. Satellite images can also be useful for spotting intense areas of cumulonimbus clouds.

If planning a sailing voyage in areas where lightning could be expected, include a CAPE forecast in your daily GRIB run.

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Flashes on the horizon

If you get caught out or have to sail through an area where electrical storms are expected, it’s important to prepare for all the weather a thunderstorm can dish out, not just lightning.

Thunder claps can be heard for around 25 miles, so if the sky on the other side of the horizon is alive with light but you can hear no noise then stay vigilant but don’t panic – the storm is still a way off. Keep moving.

Keep a 360° look-out: due to the immense height of thunderclouds they are pushed along by upper atmosphere wind, not the sea-level breeze. This makes it difficult to predict which way a cloud is moving, they can sneak up behind you while you are sailing upwind. The best way to track thunderclouds is using the radar or a hand-bearing compass.

Prepare for a squall: wind associated with thunderclouds can reach in excess of 40-90 knots in a matter of seconds, this will often be combined with torrential rain and drastically reduced visibility. If there’s lightning around it’s best to keep on-watch crew in the cockpit so make sure you reef early.

Preparing for a strike

Lightning can strike up to ten miles away from the cloud that generated it. Just because you are in the midst of a thunderstorm doesn’t mean you will get hit – I’ve spoken to two sailors who reported lightning striking the water next to their boat but not touching them.

Others that were struck reported varying damage to electrical equipment and none experienced structural damage or fire. Here are some of their recommendations:

  • Unplug all masthead units, including wind instruments and VHF antennas and ensure ends of leads are kept apart to avoid arcing.
  • As the storm gets closer turn off all electronics – modern kit has increasingly efficient internal protection, but manufacturers still advise turning it off.
  • Take a fix and plot it on a paper chart. Update your log using dead reckoning.
  • Avoid touching metal around the boat, such as shrouds and guardrails.
  • A nearby strike will be blindingly bright. Sit in the cockpit until your night vision returns.
  • Expect masthead units, VHF antennas and lights to be destroyed, so make sure you carry a good quality spare VHF antenna.
  • Fluxgate compasses can lose calibration following a strike. Check all electronic compass readings with a handheld compass.

Maximising protection

By providing a direct route ‘to ground’ down which the lightning may conduct you may be able to minimise damage.

Among my small sample of interviewees, only one had a lightning protection system: this was a sloop with a deck-stepped mast on which the chainplates were bonded to the keel bolts. The masthead unit on this boat was still totally destroyed by the strike but the remaining electronics suffered no ill effects. The same sailor had experienced a strike two years earlier with no extra protection installed – in that instance all electronics were destroyed.

The remaining sailors were all in boats of less than ten years old and reported varying degrees of damage to electronics and 100% destruction of masthead units.

The simplest protection system is bonding an aluminium mast to the keel bolts. On a keel-stepped mast this is easily done as the mast heel and keel bolts are close to each other. For deck-stepped masts this can be achieved by running an adequately sized cable through the deck head and down a bulkhead or supporting pillar.

Most modern boats have the mast bonded to the keel by manufacturers – if you’re not sure lift the soleboards to check. Masts made of less conductive materials such as carbon would require a conductor cable as well.

Air terminals at least six inches higher than any antennas at the top of your mast may save your masthead units. There is also considerable debate over the need for dedicated grounding plates – this appears to be more relevant to older boats as none of my interviewees suffered ill effects through grounding to the keel bolts.

Faraday cage

There is a theory that the oven on a yacht can act as a Faraday cage, protecting anything inside it from the effects of electrostatic discharge (ESD). Handheld or portable electronics can be temporarily placed inside a metal oven to protect them during a storm.

I have no conclusive evidence this works, but I’ve always done it, reckoning it can’t do any harm – just remember to take them out before dinner!