Thunderstorms on a boat: What to do if there is a risk of lightning
5 key immediate measures
- All hands below deck: If you don’t want to end up as a human lightning conductor – after all, people are conductive too – stay below deck.
- Keep your hands off metal parts: Even lightning strikes in the immediate vicinity induce high voltages in any metal parts, whether they be cables, steering wheels or stainless-steel coffee pots. Approaching thunderstorms can also produce similar effects (electrostatic discharges), known as St Elmo’s fire.
- Fire extinguishers on standby: The hand-held fire extinguishers (2 kg) should be kept in the cabin, within easy reach and ready for use.
- Switch off electrical appliances: Switch off all electrical appliances – or better still, disconnect them from the vehicle’s electrical system – so that any induced voltages do not damage them. This way, you can reasonably hope that they will survive the thunderstorm unscathed. The oven (a Faraday cage) is also a very safe place for valuable equipment.
- Disconnecting the shore power: When the boat is moored in the harbour, it is disconnected from the shore power supply. Thunderstorms on the boat: This is what’s important right now
Lightning protection is, quite literally, a hot topic. If you read the relevant literature, you quickly come to the conclusion that effective lightning protection on boats is almost impossible. You read about square-metre-sized earthing plates, lightning protection masts metres high and earthing cables as thick as an arm. Admittedly, lightning protection systems do exist, but they’re really intended for mega-yachts. And how, pray tell, is one supposed to fit such heavy equipment onto a medium-sized cabin boat? And who can guarantee that, in an emergency, a lightning strike will stick to its designated path?
When approaching the subject of lightning protection from a practical perspective, the focus is always on striking a balance between what is technically feasible and the level of protection that can be achieved. Our primary concern here is personal safety. Whether the echo sounder or the electronic chart plotter continues to function properly after a lightning strike is, in our view, of secondary importance and can hardly be guaranteed even at great expense.
This is how high the risk is for open boats, steel boats and plastic boats
Open recreational boats
To put it quite plainly: open boats have no place on the water during a thunderstorm. This means that even if there is a risk of a thunderstorm, the only thing to do is to seek a safe harbour with all your might; because the open design makes it impossible to protect the boat or its occupants from a lightning strike.
Steel cabin boats
Cabin boat owners are in a better position in this respect. Depending on the boat’s size and material, this type of boat inherently offers greater protection. Speaking of materials: steel boats are the safest. Much like cars, they form what is known as a Faraday cage, inside which (in the saloon or cabin) you are protected from a direct lightning strike. The superstructure and the steel hull reliably conduct the lightning along the outer skin into the water. It is important, however, not to touch any metal parts. It is best to keep a distance of at least 30 cm from them.
Plastic or wooden boats
In plastic or wooden boats, too, you are safest inside the cabin. Nevertheless, a lightning strike poses a greater danger here. Whilst the point of impact is relatively clear – typically the highest point of the boat (mast or equipment rack) – the point where the lightning will exit the boat cannot be predicted. It is entirely possible that it lies below or just above the waterline. In an emergency, this means water will flood in. If the electrical system is out of action – which is likely – the only recourse is the manual bilge pump, which one can only hope is available.
What really helps with lightning protection on board
Another hazard on plastic boats is the heat generated by a lightning strike and the associated risk of fire. The only solution in this case is a lightning conductor. This can be a pointed metal rod (copper, aluminium or stainless steel) about 50 cm long, mounted at the highest point of the boat, for example on the mast. From there, a cable as thick as possible (16 mm²) runs to all metal parts that are in contact with the water, effectively earthing the boat.
Special terminals, available from an electrician, are used for the connection. In total, a grounding area of around 0.10 m² is required for lightning protection in salt water. In practice, this can be the rudder, the Z-drive or even metal through-hull fittings. In fresh water, due to the lower conductivity, you would theoretically need a hundred times the area, i.e. around 10 m².
However, as this is completely impractical, the only advice is to connect all metal surfaces exposed to water to the lightning conductor, and to hope that, should lightning strike, it will accept this compromise.
Be careful with DIY lightning protection: if the wiring isn’t done properly, you could easily end up creating a ‘battery’. If the boat is then exposed to salt water, corrosion can occur at various points in the wiring. It’s therefore best to consult a specialist.
Whether you’re out on a steel or plastic boat, you should always follow these basic rules during a thunderstorm!
How thunderstorms form
As regards their formation, a distinction is made between thermal and frontal thunderstorms.
heat thunderstorm
Heat thunderstorms usually occur over land during the second half of a hot summer’s day. The Earth’s surface gradually heats up, the air becomes warm and humid – muggy – and rises. At higher altitudes, the air is considerably colder. When this temperature difference reaches what is known as the ‘trigger temperature’, thunderstorms develop.
Over water, thermal thunderstorms occur only rarely, and then only at night. The classic summer thunderstorm therefore mainly affects inland areas. If a sailor encounters lightning and rumbling thunder at sea, it is usually a frontal thunderstorm. These are far more unpleasant, as they are accompanied by violent gusts of wind.
Frontal storm
As the name suggests, frontal thunderstorms occur when the cold air from a passing front pushes beneath the warm, moist air. The subsequent sequence of events is the same for both types of thunderstorm. All the phenomena associated with them are underpinned by the activity within a cumulonimbus, a thundercloud.
Before a typical summer thunderstorm hits land, the formation of the thundercloud can be clearly observed. It slowly forms from the billowing cumulus clouds – the fair-weather clouds. These grow upwards like towers and become fringed at the top. A veritable anvil takes shape. This also reveals the direction in which the storm is moving. This is because it is blown in the direction of the wind, and it moves in that direction too – with the wind. However, the wind can blow quite differently up there than it does near the Earth’s surface.
The typical progression of a thunderstorm
- Lull The wind dies down almost completely. It’s getting hazy. Clouds are gathering.
- weather vane It’s coming from a completely different direction to just a short while ago.
- Gale roller A black wall of cloud appears over the horizon. Even before it reaches the ship, it is buffeted by a gust of wind well over 6 Beaufort.
- Heavy rain Heavy rain, and occasionally hail, falls from the dark underside of the storm cloud.
- Lightning and thunder The discharge from the cloud manifests itself as flashes of lightning between the clouds or between the clouds and the ground; these can be heard as thunder.
- Cooling down The air is cooling down dramatically as a result of the thunderstorm.
- Strong, gusty winds Strong winds are set to continue, with their strength and direction remaining variable.
- Reassurance At some point, calm finally returns.
From a physical point of view, all types of thunderstorms follow the same pattern: warm air rises ever more rapidly within the cloud, whilst cold air shoots downwards alongside it. This leads to various effects: in front of the dark underside of the thundercloud, the cold air escapes in a powerful roller of gusts. Directly beneath this, heavy rain showers fall; there may even be hail. Lightning and thunder are always present. The air cools dramatically, and the wind blows strongly from varying directions before the weather finally settles down again.
As well as clouds, distant flashes of lightning – which can be seen as weather lights – are also signs of an approaching thunderstorm. Furthermore, thick layers of haze often form. When such signs are observed, the southern and western quadrants should be monitored closely. Thunderstorm clouds usually approach our sailing areas from these directions.
How lightning is formed
How to spot a thunderstorm in good time
Via the internet
Thunderstorms are easy to spot using the wide range of online services and apps available for this purpose. They provide a very clear indication of the intensity, direction of movement and progression of thunderstorms over time.
On the radio
If you prefer the old-fashioned way or don’t have an internet connection, you can tune your radio to the medium wave band. An intermittent crackling sound is also a sign of a thunderstorm in the vicinity.
As a rule of thumb
If you can see the lightning and hear the thunder, you can also work out how far away the storm is – because sound travels at 330 metres per second. So, if you divide the number of seconds between the flash of lightning and the clap of thunder by three, you get the distance in kilometres.
Just as it is easy to spot thunderstorms on and over land, it is difficult at sea to make out the gust front of a frontal thunderstorm. Often, the accompanying dark cloud bank can only be glimpsed ten to fifteen minutes before the storm breaks, because it lies so low and only emerges from behind the horizon at the last moment. During this time, the crew must prepare themselves and their vessel for gusts exceeding 6 Beaufort.
Torsten Moench
Editor in Chief BOOTE