There are many safety issues involved when boating. One to not take likely is the possibility of a thunderstorm at sea and dealing with the inevitable byproduct – lightning. Even boats safely moored in a marina are susceptible to the ravages of lightning and the significant damage it can cause to hulls, dock equipment, and electronics. 

No matter where it strikes, lightning, and the damage it causes, are hugely unpredictable. The National Oceanic and Atmospheric Administration (NOAA) estimates that a single lightning bolt can contain more than 30 million volts of electricity — a tremendous amount of energy that must go somewhere. If not adequately protected, vessels and their passengers become the perfect target of this dangerous force of nature. Even the most advanced lightning protection system cannot guarantee that lightning will not strike. 

The information and the tips below provide a basic understanding of lightning for boat owners and charterers.

What is Lightning?

Lightning is energy formed by the difference between positive and negative electric charges that strikes ground or water when the electrical potential becomes greater than the resistance of the air. The electricity conducts itself to ground or water, seeking the best conductor in the area — on the water, that is usually a vessel.

What Happens When Lightning Strikes a Vessel?

When lightning strikes a boat, it seeks the path of least resistance to water, where it becomes grounded, dissipating its energy into the water. If a sailboat is struck, the lightning typically travels down the mast, and in many cases, will fry anything attached to or near the mast, like wind instruments, TV and radio antennas, and more. Powerboats are usually struck on the VHF antenna or bimini top and can sustain hull damage, but strikes usually exit through the propellers and rudders. Electronic engine controls can also sustain damage.

The extent of lightning damage is often not immediately apparent on vessels. When exiting a boat, lightning can leave via a through-hull fitting or the hull itself. Even if the strike did not damage a fitting, a small, gradual leak in the hull could go unnoticed and sink the boat hours or even days after a storm. On a boat with an effective lightning protection system, the strike follows the path of least resistance and goes to the boat's keel or grounding plate. Even if a boat does not sustain a direct hit, collateral damage to nearby vessels can occur.

Lightning Protection Systems for Boats

A commonly recommended way to defend against a lightning strike is to install a lightning protection system, which helps to conduct the electrical charge to the water without harming passengers, the vessel, or equipment. The system does not prevent a strike — it merely helps to redirect the energy. A typical arrangement includes a lightning rod at the mast top or the highest point on a vessel, conductors that transmit the electrical charge from the rod, and a grounding plate on the interior of the hull that is connected to the conductors. Installing the system can be challenging — the best course is to have a professional design and install it to ensure optimum protection.   To deflect or redirect a lightning strike, some imaginative sailors have installed short, medium-diameter pieces of wire to the sailboat masthead, creating sort of a whisker appearance. If this approach has not been proven, a simple fact remains: no matter what sort of protection might be present, it will not prevent a lightning strike.  

The most effective (and common-sense) approach to avoid lightning strikes is to not depart on a voyage if conditions or forecasts indicate thunderstorms. Some weather-related tips to keep in mind: 

  • Long before departing, check local internet, TV, radio, or print weather/marine forecasts, particularly during summer months when storm activity is highest.
  • Thunderstorms usually strike in the afternoon; about 70% occur between 12 noon and early evening.
  • Even if not visible, thunderstorms contain lightning. Another way to verify if lightning is in the vicinity is to listen for static on an AM radio station.  
  • Pack gear you might need in case of bad weather. Check our guide for the bad weather gear you can bring to stay safe.
  • Watch for the development of soaring, anvil-shaped clouds with flat bottoms and/or large, well-defined dark clouds.
  • Be attuned to sudden gusts of wind or short gusts of markedly cooler air; both indicate an approaching storm.     

Stay abreast of changing local forecasts on chart plotters, cell phones, or NOAA weather broadcasts on VHF marine radio WX channels. 

If underway and threatening weather approaches or if in the thick of a thunderstorm, passenger safety is the first priority. Consider these tips: 

  • Keep an eye on the western sky — in the U.S., most weather patterns move west to east.
  • Make an immediate course change to the closest safe haven, at the fastest but safest possible speed.
  • On sailboats, close all hatches, secure loose equipment, hoist the storm jib, drop/reef the mainsail, and put on rain gear.  
  • In open sail or powerboats, have passengers sit as close as possible to the centerline at the amidships point, on the deck if necessary.
  • On vessels with cabins, move passengers inside and to the center. Instruct them to avoid contact with metal equipment/fittings, appliances, and electronics.
  • Make sure everyone on board dons a life jacket. 
  • Unplug all electrical appliances and electronics equipment. If possible, lower radio or GPS antennas that are not part of the boat’s ground protection system.
  • For the working crew, avoid elevated areas and tall objects on the boat.

What to Do After a Lightning Strike on a Boat   

  • Check passengers for injuries from falling objects or burns.
  • Inspect the mast and attached equipment for damage or burn marks.
  • Ensure the bilge and other below-waterline compartments are watertight and the hull above the waterline does not show burn marks or small, pin-prick size holes.
  • Check the electrical panel for blown fuses, which can indicate lightning damage.
  • If using shore power, carefully inspect connections. If in doubt about the connections, seek immediate assistance. 
  • Check the compass — if it is swinging wildly or indicating a markedly different heading than the vessel’s actual course, a hand-held compass should be used.
  • After confirming and documenting any lightning damage, contact the insurance carrier.  

An important note: Today’s marine electronics control almost every vessel function, including engines, navigation, and communication, among others. If lightning destroys electronics or affects the vessel’s ability to move, the operator still has to be able to signal distress in situations such as a passenger injury, or if the vessel is adrift, taking on water, or on fire. Non-electronic emergency signals include flares, the orange November – Charlie distress flag, an air horn, and related gear.

Understanding the unpredictability of lightning strikes, how to protect passengers and vessels during a thunderstorm, and knowing the steps to take if a boat sustains a lightning strike can save lives and property.