Boat Lightning Protection

Boat Lightning Protection looks at the causes results and precautions of lightening.  What is a lightning strike?   Lightning is formed by the difference between positive and negative charges. Lightning strikes when the electrical potential becomes greater than the resistance (insulation) of the air, and forces a conductive path to earth (ground or water surface) and will use the best conductor in its proximity.
A boat is a better conductor than air, so if you are close to a storm, lightning will look to you to find an easier path to ground.
What are the chances you will get struck?

BoatUS Lightning strike boat statistics

What are the chances you will get struck?

Lightning strike boat statistics    
Type Chances per 1,000 Severity (10=Highest)
Multihull Sail 9.1 10
Aux Sail 4.5 8
Cruiser .86 7
Sail Only .73 5
Trawler .18 6
Bass Boat .18 2
Runabout .12 3
Houseboat .11 4
Pontoon .03 9
PWC .003 1


What can happen when you are struck by lightning?

Note the highest incidents are on sailboats, due no doubt to the tall masts. Catamarans are even worse due to the width falling close to the cone of arc from a strike.  Large merchant ships that are made of steel, are good conductors, and so when these vessels are struck by lightning as they often are, the voltage is dispersed to water by the vast quantities of steel plate. These vessels are relatively safe from lightning.

However for small boats, once a lightning strike hits you and electrical charge gets into the boat, side flashes can destroy electrical instruments and engine controls, and knock holes in the side of the boat, as they find their way to water. This is not only expensive to fix it is a real safety issue.

To help prevent damage to you and your boat, we need to provide a better conductor than the boat itself, so from the highest, or most extreme point, we need to provide a path to ground i.e. the water.

Before we look at what you can do to prevent damage in case you are hit we will take a look at how you can avoid getting hit and what precautions you can take while out on the water.

Warning of impending strike or thunderstorm

When on the water or going out on the water Check weather status. Thunderstorm generally appear in the afternoon, with 70% occurring between noon and early evening. Watch for development of large well defined dark clouds.

Tools that will help you follow weather patterns

• Thunderbolt the personal lightning detection device
• XM weather on a chartplotter
• Cell phones can show weather radar
• NOAA forecast via VHF

Precautions to take if a storm is imminent

Depending on what kind of boat you are in, planning for a lightning storm or potential strike should include Protection of Human Life and essential boat equipment.

• Preferably get off the water
• If not keep low, and in center of boat
• Keep inside and in middle of cabin
• Avoid high areas or tall objects
• Keep body parts out of water
• Stay away from metal especially grounded metal
• Do not touch any part of a lightning protection system
• A sailboat mast forms part of a lightning protection system
• Disconnect electrical & electronics equipment
• Put any movable electronics in the oven. This may sound silly but an oven is a faradays cage.
• Lower antennas that are not part of ground protection system
• Turn off Engine controls
• Lower outriggers, outriggers are perfect for attracting a strike
• Learn CPR. CPR can resuscitate a victim of a lightning strike

What to do after a strike

  • Turn off all electrical devices as shorts could cause a fire
  • Check the bilge, as a side flash could blow a hole through the waterline
  • Do not trust compass or any electronics.

Lightning Prevention Systems Old School

A prevention system does not prevent a lightning strike it helps to dissipate it. A lighting protection system is designed to help conduct the electrical charge to ground (in this case water) without harming the vessel and contents.

A typical lightning protection system consists of an air terminal on top of mast or most upper point, a conductor to transfer the electrical charge from the air terminal to a ground plate which will disperse the charge into the water.

It used to be recommended the lightning conductor be a 8AWG wire and that is should be connected to a 1 sq ft ground plate.

Lightning Prevention Systems New School

It is now recommended that the conductor be more like 4AWG and the ground plate also be larger. Lighting rods (air terminals) are connected to the conductor and go to the ground on the outside of the vessel. This protects the contents inside the vessel.

Ewen Thomson is one of the specialists in lightning protection and has been studying the issue since the 70s and was instrumental in writing the lightning code for NFPA watercraft standard and the American Boat Yacht Council ABYC standards. His Company marine Lightning Protection today specializes in making parts and systems for protection of power and sailboats.

Lightning Prevention Systems

The images are from Ewen Thomsons site marine lightning protection

Thomsons system works like a faradays cage. A Faraday cage or Faraday shield is an enclosure formed by conducting material, which will protect the items inside. A “faradays cage” attracts and carries any lightning strike to ground.

There is a vast amount of information on the Marine Lightning protection site.

Marine Lightning Protections System consists of


Lightning conductors which transfer electrical charge from the Lightning rod, preferably on the outside of the vessel not down the middle. Passing on the outside keeps crew and electronics safely on the inside of the Faradays cage.

Grounding terminals (immersed) near the waterline and around the perimeter. A minimum of 1 sq ft.

Siedarc electrodes mounted just above the waterline, work like the grounding strips and discharge the current to the water. These help with sideflash protection.

Projects completed by Marine Lightning Protection include; TAG 60 ft catamaran, Seadoo jetski, Nordhavn 55, Northcoast 125, Hatterras 60, PDQ 36 cat 46 Outbound to name a few.

Resources & Information

National Fire Protection standard 780

Cantalupi ADP system uses a similar system to Thomson.

CASE Report; Sailgemini gives this report of a strike Here you can learn what they did in preparation and what damage occurred. They spent 3 months repairing the damage.