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Cruising in lightning

by Rob Murray, Bluewater Cruising Association 30 Mar 18:22 UTC
Cruising in the lightning © Bluewater Cruising Association

Lightning strikes fear into every sailor's heart. You never feel more powerless than you do when you hear and see a thundercloud filled with lightning bearing down on your vessel.

A ground strike, where two electrically charged regions equalize, one in the atmosphere and one in the water or on the ground, causes the instantaneous release of an average of one gigajoule of energy. The heat and current can melt metal, cause wood to explode, and will without failure strike fear in the heart of all living things nearby. While (apparently) 90% of people struck by lightning survive (at least according to Wikipedia), it still seems like something one should avoid.

Short of burying your boat in a pit, there is no sure-fire way to eliminate the risk of a lightning strike, but here I discuss some of the ways you can reduce the risk and reduce the potential damage in the event of a strike.

So, what can you do? It is all about preparation and awareness.

How can you know it's coming?

There are real time lightning tracking services available. A notable free one is

There are also commercial websites like WeatherBug that track lightning and integrate lightning reports with other weather data. Weatherbug also has an App you can load on a smartphone that can be configured to give you 'alerts' (as long as you're connected to the Internet).

You can also get hand held or portable lightning detectors like the Accurite 2020. These work by detecting the Electro Magnetic Pulse (EMP) of lightning strikes. Typically they have a range of ~25 miles, and will tell you if the lightning is getting closer or further away. We don't have one, and we can usually see and or hear a vigorous lightning storm when it's within 25 miles, so we're not planning on getting one.

These systems are all ground-based (as opposed to satellite based) detection systems and offer good short term warning for lightning events.

For long term safety, you can try to be in places with less lightning.

Where can you hide?

Well, you really can't 'hide' per se, but you can seek shelter. According to Boat US insurance claims, catamarans are struck twice as frequently as mono-hulls of similar length, so if you're in a mono-hull, getting close to a catamaran offers you some comfort in knowing that the catamaran is more likely to be the target of any lightning strike. Lightning also prefers taller targets, so getting close to a taller (larger) boat would also be in your favour. This is also true of buildings, trees and landforms in general, so moving the boat anywhere there is taller 'stuff' will offer some protection.

You can also try to cruise (or lay up your boat) in areas where there is less lightning, like the strategy cruisers use to avoid hurricanes. While lightning (unlike hurricane tracks) is not on pilot charts, and is only sporadically mentioned in cruising guides and pilot books (and then only anecdotally without any real scale of comparison), there is data available from satellites gathered over years and freely available on the Internet.

For 16 years, the Lightning Imaging Sensor (LIS) on the Tropical Rainfall Measuring Mission (TRMM) satellite gathered data on lightning strikes and the data is available to use on the internet. The data circles the globe from about 40 north to 40 south. While the geographic precision of the data isn't great, it is certainly more than enough to paint a picture of where lightning is terrible and where it is just bad. The TRMM satellite has been retired and the new Geostationary Operational Environmental Satellite (GOES) series satellites have even more sensitive Geostationary Lightning Mapper (GLM) instruments (since about 2017 or so), but the data is derived over a shorter period and has yet to be collated in a readily available fashion.

According to the website:

    "The TRMM LIS 0.1 Degree Very High Resolution Gridded Climatology data collection consists of a set of gridded climatologies constructed from individual observations made by the Lightning Imaging Sensor (LIS) on the Tropical Rainfall Measuring Mission (TRMM) satellite (data also at GHRC). Complex algorithms are used to estimate total flash rate density (number of flashes per square kilometer per year) based on the flashes observed by the instrument and the amount of time it viewed a given area.

    This Very High Resolution (VHR) Gridded Lightning Climatology Collection consists of five lightning climatologies (links to data products provided below): the full climatological mean (VHRFC), monthly (VHRMC), diurnal (VHRDC), annual cycle (VHRAC), and seasonal (VHRSC). These gridded climatologies include annual mean flash rate density, mean diurnal cycle of flash rate density, and the mean annual cycle of flash rate density with daily, monthly, or seasonal resolution." Retrieved from GHRC on March 20, 2022.

What that means is you can see when and where in a given region the lightning is better or worse, where it is absolutely terrible (the 500 hot spots), and what time of year it really begins to 'heat up'.

What can you do to prepare?

The primary preparation is ensuring your vessel is properly 'grounded'. Grounding has two purposes:

    1. to ensure the electric potential of the vessel is the same as the surrounding sea, so its not more attractive to lightning than the open ocean
    2. to ensure there is a path to ground for a lightning strike should one occur

Proper lightning grounding should be done as follows (according to Stan Honey, an electrical engineer and sailor geek extraordinaire):

    "Lightning Grounds

    Connect a 4 AWG battery cable from the base of your aluminum mast to the nearest keel bolt from external ballast. If you have internal ballast, you should install a lightning ground plate. One square foot is recommended for use in salt water; fresh water requires much more. Do not rely on a thru-hull or a sintered bronze radio ground (e.g. Dynaplate) for use as a lightning ground. For additional comfort, also run a 6 AWG wire from your keel bolt or ground plate to the upper shroud chainplates, and to your headstay chainplate. Don't bother with the backstay if it is interrupted with antenna insulators. Have each of the cables that are used for lightning ground wires lead as directly as possible to the same keel bolt, with any necessary bends being smooth and gradual. Given that you have grounded your mast solidly to the ocean, your mast will be at exactly the same electric potential as the ocean." Retrieved from here on March 20, 2022.

If you can't find cable as specified, try to find thicker cable (i.e. lower number). Having your mast at the same potential as the surrounding ocean theoretically makes it no more likely a target than the surrounding ocean.

Because you're likely to be mixing copper, aluminum and stainless steel in the wiring for this grounding, be sure to use plenty of dielectric gel and keep the connections 'clean' with regular inspection. They will likely be in the bilge, where moisture will further complicate cleanliness and hasten their demise (see this article in Practical Sailor for more on this).

Aboard Avant, we go a step further when we lay the boat up and use jumper cables to increase the grounding.

If in the water, we buy a set of cheap but thick jumper cables. Separate into two wires. Remove one clamp from each wire, strip back a couple or six inches of insulation and 'fray' the end (or keep the clamp and clamp it to a 1'x1' metal plate) to make a better ground connection with the water. Attach remaining clamp to a top shroud or other bit of metal that connects to near the masthead and throw the frayed/plated end in the water. One cable port, one cable starboard.

Of course, when your boat is hauled out, the grounding system stops working because the keel and/or grounding plate is no longer in the water (and in most yards is insulated from the ground by blocks of wood). If we lay up on the hard, we separate the two cables and attach one to the top shrouds and jack-stands on the port side, the other to the same points on starboard.

Here is a photo of a boat that was struck by lightning on the hard. You can see the hole in the hull and the trail the lightning burned in the bottom paint from the exit point in the hull to the jack stand and hence to ground. By providing better grounding and a more direct alternate path for the charge we hope to avoid this kind of damage.

In either case, the cables will be trash at the end of the season, as they are not designed for continuous outdoor use. Brushing clamps with wax, Vaseline or any other topical protectant helps them rust less and look better longer. Even if you think your boat might be/is well grounded, these jumper cable tricks will ensure/increase it.

While most experts say the bottle brush style 'lightning dissipaters' such as the 'Lightning Master™" by Forespar designed to go at the top of the mast don't work, we have one on Avant. I picked it up on a sale table for less than $10, and I can attest that it does keep birds off the masthead, and so far, we haven't been struck by lightning. (For more reading on them, you can see this article in Practical Sailor).

What can you do to try to mitigate damage?

Aboard Avant, when we hear a storm approaching, we gather up all portable and easily de-mounted electronics (hand-held VHF and GPS units, EPIRBs, LED flashlights, cell phones, tablets, computers, radios, etc.) and place them in the oven as a kind of faraday cage. We disconnect all antennas and power connections from easily unplugged items like Chartplotters, AIS, VHF, etc. and leave them disconnected and just hanging loose to disrupt possible paths for lightning.

We also ensure all aboard stay away from the mast and rigging and any chance of forming part of the 'path to ground' of a strike.

When we lay up for the season, we do the same, and we also wrap portable items first in paper towel and then in a double layer of aluminum foil to make it a double faraday cage.

When we store electronic spares aboard, we wrap them in paper towel and then in a double layer of aluminum foil before they go in the spares locker.

In areas where lightning is endemic, we pray more frequently and sleep with a bible under our pillow.

There is no way to make any vessel 'lightning proof'. But you can take steps to avoid more lightning prone areas, and to mitigate the effect of a strike.

This article has been provided by the courtesy of Bluewater Cruising Association.

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