A personal investigation into how and why a catamaran was hit by lightning

This article describes what my wife Heather and I learned over the five months it took to complete repairs on ‘Milliways’, our Solaris 42 catamaran, after being hit by lightning off Tioman Island in the South China Sea.

It was written in response to the many yachties who have asked us for our advice since the experience. Our account of the actual strike was published in Yachting Monthly July issue 2013. You can find a link to download that article at the bottom of this article.

Throughout the article, I refer to diagrams and images. These can be found in a gallery here.

How lightning works

First you need a puffy white cumulus cloud. On a sunny afternoon near the coast cumulus clouds appear and dissolve away with a life of about 20 minutes. However if conditions are right, a few of them will develop an upward circulation of warm moist air in the centre while condensing water droplets fall around the perimeter to be warmed again and recycled back up the core. A thermal bubble of hot air rising from the land may start or reinforce this circulation or the presence of a strong updraft on a cliff or mountain side called ‘orographic uplift’ can boost the rate of circulation. These clouds grow into magnificent towering masses, called cumulo-nimbus, often several miles high, and may develop the classic flat ‘anvil’ top if they reach a temperature inversion in the stratosphere.  

The base of the cloud turns a dark grey and it starts to rain, at which time the shape and direction of travel can be tracked by your radar..The cloud will be pushed by upper level winds and may not be travelling in the same direction as the surface wind. It is worth noting that once this direction is established the next thunder cloud will be travelling in the same direction.  

Inside these monsters the increasingly rapid circulation of water as droplets or ice acts as a dynamo. Gradually the top of the cloud becomes positively charged and the bottom correspondingly negative. Often adjacent clouds will balance their charges by an arc across from one to another. To a person at sea level this appears as a diffused ‘sheet’ of lightning high in the sky. This can reduce or turn off the charging effect and collapse the cloud, so the threat of a ground strike recedes. A medium band AM radio receiver tuned to any random frequency will detect unseen daylight lightning as crackly interference. The approximate distance to the storm, in miles, can be estimated by counting the seconds from flash or crackle to thunder sound, and then dividing by five.

The warning signs

A cumulo-nimbus cloud which continues to grow will begin to induce a positively charged zone in the sea beneath it, roughly delineated by local heavy rain and strong winds. A vessel passing into this charged zone may be given warning of its presence by ‘St Elmo’s Fire’. When blue sparks start dancing around the hull and rigging you are sailing into a highly charged zone which is almost ready to produce a strike. Eventually the potential difference between sea and cloud reaches the critical trigger voltage at which a ‘fork’ of lightning arcs downwards through the interior of the cloud to be met by an upward ‘return’ strike from the sea to the cloud base. The mechanism is similar to a capacitor gradually charging up and then suddenly discharging. Note the voltage required to produce an arc from fresh water is over 1,000 times higher than seawater. So a strike on a vessel in a river or lake is always more damaging.

The air is ionised along the lightning path and ozone produced which has a distinctive metallic smell. If you missed the last strike, but smell ozone you are much too close to the next potential strike location. Sometimes if the air circulation within the cloud is particularly vigorous the ionization path established by the first arc may open an electrical channel allowing several strikes to occur in quick succession, in our case three within about two seconds.

A vessel makes good electrical contact with the positively charged water around its hull, particularly through its metal propellers and stands a high probability of acting as the start point for the ‘return’ path from the sea back up to the cloud. 98% of lightning strikes on boats travel upwards. The path starts at the electrically connected equipment in contact with the water, runs through the hull, up the rigging and mast and ends in a visible arc flashing upwards from the masthead to the cloud base. (see fig 1). The ‘fireworks display’ is usually the VHF antenna vaporizing. A brush style conductor which is lower than a masthead antenna will not protect it.

Why us?

The inspiration for this section came from conversations with Jeffrey Leng the Manager of Raffles MYS boatyard Singapore, who said that ‘5 out of every 8 boats he repairs are catamarans’. Why a 60% hit rate when it should be only 20%? This set me thinking and researching figures from published reports, together with our own observations.

The determining factor of which vessel in the charged water zone below the cloud will provide the return start point depends on the relative size of the electrical contact ‘footprints’ of the vessels. Size counts, and so the largest footprint wins the prize by
becoming the favourite to be hit. Lightning chooses only one return path, so only one vessel in the charged zone will be hit each strike, any others must wait for the next strike! The height of mast is not an important factor in a lightning strike which will arc over several miles of sky, but its presence as part of the prime conductor does impart protection to the crew. While at sea fishing boats, powerboats and freighters all without tall masts are struck just as many times as sailing vessels.

To estimate the ‘electrical contact footprint’ area of your vessel, list all the electrically connected contact points with the water, then draw straight lines from the rear most metal contact point, usually either the rudder stock or propeller(s) to the cap shrouds, as these will definitely act as conductors during a strike. Include any bulges for anodes and Dynaplates outside the direct line, and calculate the area contained.

Monohulls result in a narrow roughly triangular wedge shape. (see fig 2). A trimaran will look similar to a monohull triangle but a lot flatter. Isolated seacocks forward of the mast and connected into the earthing system will increase the footprint.

Most Catamarans look like a square, (see fig 3). but a catamaran with water level stainless stays to the centre of the forward beam has two more contact points and increases the footprint forward of the mast to a rectangular shape.

A vessel lying to an all chain anchor will have an extra point of contact forward of the shrouds which will increase its footprint. Thus an anchored monohull footprint becomes a diamond shape, two wedges back to back, (see fig 4), and the catamaran rectangle adds a triangle ahead of it. A combined chain and rope rode will eliminate this risk.

A metal hulled monohull is the nearest to the theoretical best solution of a clear conduction path and a single electrical contact point with the water. It is always a diamond shape similar to an anchored fibreglass monohull.

Table 1 shows for a typical vessel of 13m overall length, the ‘relative’ contact footprints and thus the propensity to be struck, when in the charged zone.

Table 1
                                     Sailing      Anchored
Monohull   Metal hull        1            1.1
        Fibreglass       0.6            1.1      
Catamaran             1.5            2.0
Trimaran                    1.3            2.7 

I leave you to draw your own inferences from these figures; suffice to say we presented the largest ‘footprint’ of the three vessels within the charged zone.  

David Lyne

Call 0044 (0)1202 440 832 or click on this link to buy the original account of the lightning strike, published in published in Yachting Monthly July issue 2013.