Thunderstorms at sea: How sailors and yachts can protect themselves

​What should you do if a thunderstorm breaks out at sea?

A stormy atmosphere. A term used as a metaphor for the precarious calm in which a storm seems inevitable. It symbolises a tense atmosphere that makes it hard to breathe. It points to an impending change, the course of which is just as uncertain as the question of whether or not it will cause lasting damage. A stormy atmosphere signals fear and uncertainty and – worst of all – represents a hopeless situation.

According to an old rule of thumb, the most effective way to protect yourself during a thunderstorm is to avoid being outdoors. To take shelter. A tip that isn’t much use at sea, however. Here, no one can hide from reality; different rules apply. Those who know these rules and follow them in good time may still not be able to escape the storm. But they can at least face it with a fair bit more composure.

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The 8 most important immediate measures

1. Reefing the sails Out at sea, thunderstorms usually bring strong winds. The ship and crew must be prepared for this.
2. Start the engine Even if its electrical system fails due to a lightning strike, a running diesel engine remains fully functional.
3. Lightning protection If there is a mobile device on board, it is deployed.
4. Record the ship's location If the electronics fail, you can continue coupling using the last fix.
5. Secure the crew Everyone must go below deck, but must keep a safe distance from the base of the mast whilst there. The helmsman must wear insulating clothing and must not touch metal parts with their bare hands.
6. Be prepared Keep fire extinguishers and signalling devices to hand. Disconnect electronic devices from the vehicle’s electrical system and, if possible, place them in a metal box or in the oven.
7. Avoiding manoeuvres Anchoring and sailing manoeuvres are strictly forbidden, as they may involve touching metal parts that could conduct lightning currents.
8. At the harbour Disconnect the shore power cable and the conductive mooring lines, and stow them away.

The first step is to reduce the sail area. The wet sails and the metal parts of the rig should not be touched later on in the charged atmosphere. The engine is then started. This ensures it is ready for use before its electrical system is potentially damaged by the thunderstorm.

The deck is now being cleared. Anything that is not secured is stowed away or lashed down. Tension straps are fitted, and the boat is made seaworthy – hatches, windows, vents, seacocks, the companionway and other boat-specific openings are sealed. The cockpit drains are checked to ensure they are clear, and the pumps are checked to ensure they are in working order. The crew go below deck, and the helmsman puts on his heavy-weather gear.

The position is determined and recorded on the chart and in the logbook. In the event of a failure of the navigation equipment, the navigator must be prepared to use alternative methods. And it is no exaggeration to report a position on land and to have fire extinguishers and rescue equipment ready.

​How sailors can spot a thunderstorm early on

Apart from the uncertainties brought about by strong winds that build up rapidly, thunderstorms have their own particular dangers. These include, for example, sudden wind shifts of up to 180 degrees. You must expect visibility to be extremely poor. The sea becomes rough from one moment to the next. And finally: a lightning strike.

Lightning is caused by differences in electrical charge within a thundercloud (see diagram below). The underside of the cloud is negatively charged, whilst the upper part and the ground beneath the cloud are positively charged. The equalisation of these charges results in current surges of several million volts, an average current of 20,000 amperes and a speed of 10,000 to 100,000 kilometres per second.

They are visible as flashes of lightning and audible as thunder, either from cloud to cloud or from cloud to earth. However, not much else is known. The worst thing about this phenomenon is therefore its unpredictability. Whether lightning will strike, where it will strike and what will happen as a result is simply impossible to predict. Everything that has been said or published on the subject amounts to little more than rules of thumb.

There is still a consensus amongst experts that lightning tends to strike elevated objects that are good conductors and are earthed. Applied to a sailing yacht, this means that lightning is most likely to strike in the area from the masthead to the spreader. However, if the yacht heels, the spreaders, shrouds or stays could also become points of lightning strike.

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 form.

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 of a passing front pushes beneath the warm, moist air. The subsequent course of events is the same for both types of thunderstorm. All the phenomena associated with them are underpinned by the activity within a cumulonimbus cloud, or 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

1. Lull The wind dies down almost completely. It’s getting hazy. Clouds are gathering.
2. weather vane It’s coming from a completely different direction to just a short while ago.
3. Gale roller A black wall of cloud appears over the bow. Even before it reaches the ship, the vessel is buffeted by a gust of wind well over 6 Beaufort.
4. Heavy rain Heavy rain, and occasionally hail, falls from the dark underside of the storm cloud.
5. 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.
6. Cooling down The air is cooling down dramatically as a result of the thunderstorm.
7. Strong, gusty winds Strong winds are set to continue, with their strength and direction remaining variable.
8. 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 massive 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

Recognising thunderstorms

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 15 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.

Just how dangerous lightning strikes really are for yachts

It is extremely rare for lightning to strike a yacht. Most damage to boats caused by lightning is due to induced voltages, which occur whenever lightning strikes nearby – usually in the water. This damage usually involves damaged electronics, rather than structural damage to the yacht.

Should the rare event occur that lightning finds its way into the vessel via the rigging, various scenarios are conceivable. The underlying factor in all of them is that the lightning ultimately seeks a path into the water. If it is not effectively channelled there, damage usually results. This can range from the failure of all the electronics and cable fires to full-sized holes in the hull.

There are effective measures that divert lightning into the water, thereby preventing damage to the vessel and its crew. From a technical point of view, a distinction must be made between the various measures.

Lightning protection on board: what makes sense and what doesn’t

Firstly, ensure that there is a good lightning protection system in place. If the mast is made of aluminium, it can act as a lightning conductor in its own right. If it is made of wood, it must be fitted with a sufficiently thick conductive cable and a lightning conductor at the top. A lightning conductor is also advisable on an aluminium rig if it towers above the array of antennas, thereby reducing the risk of lightning striking there and causing damage.

The mast, shrouds, stays, and the bow and stern platforms must be carefully earthed as potential strike points.

1. Masthead A lightning arrester should extend approximately 30 centimetres above the aerials. On wooden masts, it is connected to the keel or earthing plate via a 16-square-millimetre cable.
2. bow seat It is earthed directly using a 16-square-millimetre cable.
3. forestay It is earthed directly using a 16-square-millimetre cable.
4. Mast base This is where the main load is concentrated in the event of an emergency. It is earthed using a 35-square-millimetre cable.
5. Püttinge The terminals are connected together and earthed using a 16-square-millimetre cable on each side.
6. keel bolt If the metal keel is used for earthing, all the earthing cables converge here.
7. metal keel A lead or iron keel bolted to the hull is the best lightning conductor. However, it must not be completely isolated from the water by a seal.
8. Stern platform It is earthed directly using a 16-square-millimetre cable.
9. Swimming ladder It folds down into the water and is earthed inside the boat, thereby acting as an additional earth connection.
10. Backstay It is earthed directly using a 16-square-millimetre cable.

On metal ships, this lightning earth is provided without any further action being required. Due to the nature of the material, should lightning strike the rigging, it will find its way through the hull into the water. And the crew of steel or aluminium yachts have another advantage: inside the vessel, they are in a Faraday cage and are therefore safe from a direct lightning strike.

Unlike on wooden or plastic yachts, here the lightning earth connection must first be installed. Shipyards often omit this and install only an earth connection via the shrouds. What needs to be done in each individual case should be discussed on board with an expert, as every yacht has its own specific characteristics.

The principle, however, is always the same: the shrouds, stays, mast, bow and stern platforms are connected to the water via suitable cables and clamps. This can be done via the keel bolts, if the metal keel is not sealed by a watertight GRP or painted structure, or via a connection to an earthing plate. This should be fitted to the outside of the hull and have an area of at least half a square metre – the larger, the better.

The recommended conductor cross-sectional area for lightning current-carrying components is 8 square millimetres for copper and 16 square millimetres for aluminium. A ‘better than nothing’ solution is a thick copper cable run under the keel, which is securely fastened to the mast and stays using clamps.

Be careful with DIY lightning protection: if the wiring isn’t done properly, you can easily end up creating a ‘battery’. If the boat then enters salt water, corrosion can occur at various points in the wiring.

Another step in effective lightning protection is to ensure equipotential bonding between all metallic components that are not connected to the lightning earth. These may include railing supports, the steering column, the engine, fuel tanks, the rudder housing and similar components.

These components should be connected to the ship’s service earth busbar to prevent different voltages from being equalised via crew members who come into contact with them. This is because it must be assumed that, following a lightning strike, these parts will become charged to varying degrees due to stray voltage, and this voltage must be discharged. Of course, the entire installation also has disadvantages, as it requires maintenance and is only effective if conductivity is not impaired by creeping corrosion. Furthermore, it promotes electrical corrosion and pitting.

Last but not least, attention must be paid to protecting sensitive electronic equipment. It can be destroyed in an instant; even inductive interference is enough if lightning strikes nearby. The only solution is a comprehensive set of measures. This begins with shielding the cables using metal cable ducts. Next, lightning current and surge arresters must be installed, both on the mains supply and on the signal lines. During a thunderstorm, removable equipment should be stored in tin tins or in the oven.

Anyone who has done everything possible can rest assured in two respects should they find themselves caught in a thunderstorm at sea. Firstly, because statistically speaking, winning the lottery jackpot is more likely than being struck by lightning whilst at sea. Secondly, because a properly installed lightning protection system is highly likely to prevent the worst possible damage to the vessel.