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The Frugal Mariner
Saltwater Suzi and Cap'n Larry's "Boating on a Budget" 
How to's, Information, Education & Fun Stuff about Boats, Sailboats, and Cruising

Before you go sailing check your local National Weather Service forecast by "City, ST" or Zip Code



Lightning and Your Boat

Lightning!  The very thought strikes fear into the hearts of many mariners. 

There have been countless studies, myriad recommendations, disputes, assertions, speculations, contentions, disagreements, and even official policy  over what to do about it. 

And there are questions, many questions without definitive answers:

What do we do if we see an electrical storm approaching?  How do we recognize an electrical storm? How should we protect our boats? Will installing a lightning protection system protect my boat?  What are the odds that my boat will get hit?

What's the best thing to do to protect our electronics on board?   

How do we protect



If you are looking here for definitive answers, you've come to the wrong place.  We have done some considerable research and there are no definitive answers.  To sum it up, it's a crap shoot.

Here you will find some anecdotal evidence which, to the scientifically minded skeptic, is nothing but useless, however interesting, information.  With enough of it one may discern trends which may lead to tests or experiments which will either confirm or refute hypothesis.  Will this happen?  Not likely.

We would also like to offer some references and links to government funded studies, which, again, either disagree with each other, or offer information which may or may not work to protect you and your boat.

We will also offer you some advice which we feel may logically help you in your pursuit of protection from lightning.  Bear this in mind...  we can't prove anything about what we say here actually works.  But listen anyway. 

Most of the studies and articles we've read (and we list many links to them below) agree that a well  designed and properly installed lightning protection system on a boat can help minimize the damage once your boat does get hit.  Unfortunately, many sources indicate that that same system increases the likelihood that your boat will be hit.
Lightning and Boats - The Michigan Sea Grant  College Program
Lightning and Sailboats - The Florida Sea Grant College This one states, among other things, that boats with proper lightning  'protection' fare better than those with none.  ( NOTE: this comment edited after it was pointed out that my previous comment was in error.  See comment below by Ewen Thomson. )
Inadequacies in the U.S. Code for Lightning Protection of Boats attributed to E. M. Thomson, Department of Electical Engineering, University of Florida.  This paper seems to agree with the Florida Sea Grant College and uses some of the same charts.

Boating Lightning Protection by William J. Becker, also a Sea Grant, University of Florida, (May 1985). Are we sensing a pattern here?   While this one does not use the same graphs as the above two, there is an interesting paragraph, " Lightning protection systems do not prevent lightning strikes.  They may, in fact, increase the possibilities of the boat being struck.  The purpose of lightning protection is to reduce the damage to the boat and the possibility of injuries or death to the passengiers from a lightning strike."
Lightning Cone of Protection, U.S. Coast Guard.  This one is a fairly short read and it may have some good information - but speaks of the unsubstantiated 'cone of protection.'  It also says that grounding such things as shrouds and tankage will eliminate the possibility of side flashes.  This is just not so.  It may lower the likelihood - but will not 'eliminate the possibility.'

Lightning - Grounding your boat.  A brochure published by (you'll never guess) University of Florida Sea Grant: Lightning and Sailboats.

What follows is based on the recommendations for lightning protection provided by the American Boat & Yacht Council, Standard E4.

The primary purpose of a lightning protection system is to provide for the physical safety of all aboard your vessel. Prudent actions that should be taken during an electrical storm are:

1) If at all possible remain in the cabin of a closed boat.

2) No one should be in the water or have any part of their body immersed in the water.

3) Do not come into contact with any components connected to the lightning protection system of a properly protected vessel. Otherwise your body could act as a conductive bridge between any items connected to the lightning conductive system. For example, you should not be in simultaneous contact with a metal steering wheel and a metal stern pulpit.

A good lightning protective system ensures that all large masses of metal are electrically connected. This purpose should not be confused with that of the vessel's basic bonding system. A properly installed and isolated bonding system is there to provide a low resistance electrical path to reduce electrolytic corrosion and as a measure of personal protection if there is an electrical fault in the boat's AC/DC electrical systems.

If your sailboat is a vessel with an aluminum mast you have the starting point of a well-grounded lightning rod. This will provide a zone of protection for a radius around its base equal to the height of the lightning rod. Due to some vessels overall length, it may be necessary to install another lightning rod to encompass any areas that do not fall within the zone of protection. Don't forget that the mast itself must be physically bonded or connected through to the common ground - one of the keel bolts or if a encapsulated keel, to the grounding plate, in order to provide optimum protection.

The apex of the rod should be a minimum of six inches above any masthead device. The end should be sharpened to a point.
(NOTE: There is some disagreement on this point.  Captain Larry)The base of the mast or the mast step if metal, should be connected to a keel bolt on externally ballasted vessels. The preferred wire gauge is No. 6 or even better, #4 AWG stranded copper. In no case should such a connection be made to a vessel with internal ballast. The result could be a hole blown through the bottom of the hull. Boats with internal ballast should have a copper ground plate of at least one square foot in size installed externally on the hull bottom.* The grounding wire should then be connected to the ground plate.

All wire conductors should be kept as straight as possible. All large metal objects above and below decks should also be electrically tied into the lightning ground conductor. This is a precaution against side flashes. Large metal objects include shrouds, chainplates, toe rails, sail tracks, winches, steering wheels, and bow and stern pulpits. These items can be tied into the ground conductor wire by a minimum #8AWG stranded copper gauge wire, or connected directly to the hull ground terminus.

A thorough inspection of the lightning protection system should be conducted on an annual basis as part of normal maintenance procedure. All connections should be maintained tight and corrosion free. Any corrosion will impede the flow of electricity and promote side flashes. For that reason it is important that the lightning protection system receive the same attention as the rest of the systems aboard the vessel. This should be included as a part of the annual lay-up and maintenance procedure. For additional details regarding the lightning protection standards readers should refer to American Boat and Yacht Standard E-4


The above is what we have found in our research to be the best 'official' advice.  It is not based on any scientific studies - of which there seem to be none,  It is, simply, what makes sense, and what, for many, has worked.  Bear in mind that it is only the survivors who tell us what works.  Those who did not survive may have done the same thing - who can tell? 

Some of the things we believe do



1) Hanging a chain off of a shroud and letting it dangle in the water.  First off, depending on your tack, it could be way out of the water.  Secondly, it does not offer a straight path to ground (water). 

2) Forespar Lightning Master™(Static Dissipater):  We're probably going to get into trouble here.  The advertising claims all kinds of things.  For three years in a row, I went to the Lightning Master™(Static Dissipater) booth at the Annapolis Boat Show.  Originally, they offered to cover up to a certain amount of your insurance deductible (I don't remember how much, exactly, it was somewhere in the neighborhood of $5,000.  Then the next year, three thousand, then, eventually,  the offer disappeared entirely. 

Every year, I asked the booth attendant what studies they used to support their claim that there was less of a chance of your boat getting hit.  I was told that there were no studies, they just knew.

Then, a friend of ours had his boat hit with lightning.  The boat was in its slip.  It was a wooden boat with a wooden mast.  The boats on either side were FRP boats with aluminum masts.  Both of the aluminum masts were considerably taller.  Our friend's boat had a Static Dissipater at the top of the mast and a large ground wire running down the mast and to the keel and was grounded in the water. 

Yes, you're right - it's only anecdotal evidence.  But where is any evidence to support their claims? 

So we e-mailed them and asked.  Here is the response we got:  "We have sold over ten thousand units world-wide and only have had four reports of them being hit."  I read one insurance report where one boat with one was hit twice

3) We have seen advertised "Lightning Prevention Systems."  There is no such thing.  Anyone claiming to have one or offering to sell you one is a con artist, a snake oil salesman. 

4) The very famous and totally unsubstantiated "Cone of Protection."   (claiming an angle of 60 degrees - 99 percent protection;  or 45 degrees - 99.9 percent protection from the tip of a well grounded lightning rod.)  These numbers are made up - there is no scientific theory upon which to base it.  The earliest reference I can find is from a British lightning rod trade group - in the year 1874.
See this from the   National Lightning Safety Institute
**A Faraday cage is an enclosed hollow shell made of an electrical conducting material. If there is a large electric field outside the conducting shell, the electric charges on the shell will move around and rearrange themselves until the electric field inside the shell is zero. Therefore a Faraday cage acts as a shield for large electric fields or for electromagnetic waves. Even if a Faraday cage experiences the large electric fields of a lightning strike, the electric field inside the Faraday cage will be zero. Hence a Faraday cage makes an effective shield against lightning strikes.

That is true as long as they are not in electrical contact with the exterior. For the electric field to be zero inside the Faraday cage, the electric charges must rearrange themselves on the surface. These moving charges on the outside of the Faraday cage can produce strong electric currents. Hence the occupants of the airplane should not be in electrical contact with the outside shell of the aircraft.

Read more at Suite101: Faraday Cages and Lightning Safety: Physics, Conducting Enclosures, and Protection in Electrical Storms
We, at Frugal Mariner, will stick our necks out here and recommend that you install a lightning protection system on your boat, or having one already, check it to see that it is up to recommended standards.

Our reasoning is this:  while it is, as we mentioned above, a crap shoot, even when shooting craps, if there is a way to increase the odds in your favor, it is a wise thing to do. 

When we are dealing with the millions of volts and the tens of thousands of amperes that lightning contains there is a certain amount of math that can just get tossed out of the window.  For instance, there is an equation:
                                                        I = E / R 
which means Amperes (the current or amount of electricity) is equal to voltage (the pressure) divided by resistance (rated in ohms).

The human body is not considered an efficient conductor.  That means it has a high resistance to the flow of electricity.   If you were to hold  the leads of an electrical multi-meter between the thumb and index finger of each hand and took a resistance reading, it would show  many thousands of ohms (varying with, among other things, how much moisture is on your skin.)  If you put those same fingers on the two poles of a 9 volt battery, would current flow?  Yes, it would.  But it would be so small you would not feel it.  (putting it on your tongue is a little different.) 
This is because in the formula I = E / R,    'E' is 9;   'R' is say 2000, so 'I', the current, would be 9 / 2000ths  of an ampere.  Do the same thing in an electrical outlet and you have 120 / 2000 which is about .06 amperes - (we do NOT recommend that you try this - just take our word for it.)  It probably won't kill you, unless you have a bad heart or other  medical condition, but you'll feel it and probably will hesitate before trying this foolishness again.

Now let's scale this up to the voltages attributed to lightning:  let's be conservative and say it's only 20 million volts (hey, it just traveled 5 miles through air, a much higher resistance than the human body) 20,000,000 / 2000 = 10,000 amperes.  You, sir or madam, are most likely dead.  The one hope you have is that, because these high voltages and amperages last for such an incredibly short amount of time,  thousandths of a second, you may be able to be resuscitatedWe hope that a member of your crew has CPR training.  This is another reason we do not believe the prudent mariner will sail as a singlehander.  It is not likely that you will have the fortitude and tenacity required to perform CPR on yourself.
Captain Larry, conducting the experiment described at left.

Plod on, my reader.

You're halfway through the boring technical part that you need to read to understand this. 

So now you have determined that you do not want to be a conductor of electricity, even though you have discovered that you are surprisingly talented in this area. 

Now we are going to look at your boat, and what happens when (or hopefully 'if') it is struck by lightning. 

You may or may not already be aware how a circuit breaker or a fuse works.  For those of you who are not - here it is in a nutshell - electricity, when it passes through a piece of metal, heats up that metal.  (Go hold the plug to your vacuum cleaner or electric heater while it's in use and you will usually be able to feel its warmth.) The more electricity (the amperage) or the smaller the cross section of the  piece of metal the warmer the metal gets.  When the metal heats up sufficiently, it will first warp, then melt.  It heats up sufficiently in a circuit breaker the metal warps enough that it no longer holds the circuit breaker closed (its conducting position) and a spring opens it.  In a fuse, the element goes beyond warping and actually melts. 

So we learn from this that the larger the cross section of a piece of metal, the more electricity it is able to conduct.  Different types of metal conduct electricity differently.  A certain size copper wire will carry more electricity than than equivalent sized aluminum wire.

Further, you may have heard the phrase "the path of least resistance" in regards to electricity.  This may lead you to believe that of, say, five conductors, four of them small and one of them large, all of them connected to the same source of electricity, that the path of least resistance would be the large one and all the electricity would flow through the large conductor and ignore the small ones.  'Tain't so.  The path of least resistance is all of the conductors.  To draw an analogy, let's take a fire hydrant with four hose connections.  We take a 4 inch diameter fire hose and connect it to one of the fire hydrant's connections.  Using adapters that we bought at the specialty hose adapter store, we connect garden hoses to the other three.  You would not expect that all of the water would come out the four inch hose and none out of the three garden hoses.  It would be in a proportion - the four inch carrying more, of course, than any one of the others.

Now, understanding this, picture lightning striking the top of your mast.  Assume you have a copper ground wire of adequate size running from the top of your mast in a perfectly straight line to a ground plate (again, of adequate size) on your keel.  This is the system most often recommended - with many caveats, ifs, ands and buts.  I have read many of them, because I am (justifiably, I believe) afraid of lightning.    So lets take a look at the math of this situation:

Warning: Math Ahead, next two paragraphs. (maintain speed - no passing)

Assume the same 20,000,000 volts has hit the top of your mast.  Assume  60 feet of # 4 gauge wire comes down to ground. Resistance of the wire is .01491 ohms (I looked it up -
  and did the math - ohms per 1000 feet = .2485; 60/1000 X .2485 = .01491)  Pretty precise.  We'll add a couple of ohms for connections so we have 2.01491  And since that last is a little less precise we'll round off for the sake of simpler math to 2 ohms.   Stainless steel is approximately 42 times more resistive than copper.  Your shrouds and stays, again, lots of assumptions here, but stick with me, will each have, with connections - maybe 100 ohms each.  Since you have a minimum of 4 of them, the math works out to about 25 ohms total (they are in parallel - not series.) 

So of our 20,000,000 volts, which is looking at a total of 27 ohms - we're not even counting your mast - which makes the total even less - parallel circuit, remember?)  25/27 of that heads down your ground wire.  20,000,000 x 25/27 =  18,518,518 volts sent harmlessly to ground - we'll see the fallacy of that a little later.  That leaves - let's see, 1,481,482 volts potential for the four shrouds - we'll assume they are grounded.  Now, there are 4 of them - leaving  370 thousand plus volts coming down your shrouds and stays.  The math may be off - most assuredly is off - too many assumptions.  But the fact remains that we are dealing with a lot of voltage here.   (Note: in a comment below, by Ben, more accurate math is brought into evidence - so if you are a stickler for accuracy please read Ben's comment. )

Math ends here:

An electric chair, reportedly, puts out about 2,000 volts.  Suffice it to say that you do not want to be any part of a 20,000,000 volt circuit even for a thousandth of a second or so. 

So, you make darned sure that you and your crew, unless you have deemed them to be disposable and don't mind the smell, do not touch anything metallic.  There can also be arcing between things which are metallic - remember this bolt of lightning laughed at the distance between the cloud where it originated and the top of your boat.   It can just as easily arc between, say, a backstay and the rudder shaft on its way to the water.  If you are in between, say hello to sizzle.  It can also choose part of the big metal wheel at your helm.

What can you do to protect yourself and your crew during a severe lightning storm where lightning is striking all around you?

We ourselves have thought about it and our plan is to park ourselves somewhere on the boat which is away from metal and away from any area which is between metal and any other large grounded metal.  This is when we are at anchor or tied up at a dock. 

Statistics we have seen indicate that fewer boats are hit when sailing than when they are still.  But then again, at any given time, especially in a storm, more boats are still than sailing - and we haven't seen statistics that show these odds as a percentage.  

If we are out and see a storm coming - (see our weather page) we head for shelter if we can.  We get the sails down and get the engine going and set the autohelm.  I don't want to be at the helm holding on to a big metal wheel which is just above a big engine with a metal shaft heading out to a nice hunk of brass in contact with saltwater.   I do stand watch, though, in our companionway.  If we do get hit and it takes out our autohelm and it becomes imperative that I man the helm, I will steer with a big wooden spoon so I don't have to touch that wheel.  It may not be enough, but it's better than nothing. 

It is a fact that a lightning rod at the top of your mast will attract lightning.  That's what lightning rods are for - supposedly to attract the lightning and conduct the lightning safely to ground.  It may work.  Especially on a building.

Our main mast is grounded - bolted directly - to the keel.  We do not have a lightning rod.  We don't have a ground wire from the top of the antenna.  Our reasoning is this:  Lightning rods attract lightning.  We do not want to attract lightning.  If lightning does hit our boat, the large mass of metal in our aluminum mast should conduct the majority of the charge directly to ground (seawater).  If your mast is not grounded to the keel, you should probably consider a grounding wire to connect the top of the mast directly to the keel - 4 gauge copper wire.  Many recommend a copper plate be connected to the keel outside the boat with the ground wire connected to it.  I have also heard stories where the lightning blew a hole in the boat as it exited to the plate.  Your call - do your own research, make up your own mind.  Until tests are conducted or scientific surveys taken, there are no rules or guidelines and we are not going to recommend anything - only mention what we have done.
Quote from Seaworthy magazine: "Note: If you disconnect your VHF cable from your radio during lightning season, like some boaters do, be aware that anything near the connector, including you, can get zapped during a strike."

Want to read more:

The Boat US Marine Insurance and Damage Avoidance Report In my opinion, an excellent report - it correlates with much of what we have presented here.

Electronic Equipment Protection

During that fraction of a second that the lightning is passing through your boat - by whatever path it chooses - it is carrying with it a massive electromagnetic pulse - which induces currents in anything in which a current is capable of being produced - which is almost anything - you can even feel it in your teeth fillings.  Depending on a.) how close and b.) how conductive an object is (and other factors) it can generate a very small current or a very large current.  

Back in the ancient days of tube electronics, unless the bolt of lightning was very close, the electromagnetic pulse usually did not generate a current strong enough to do any damage and your electronics survived.  In today's electronic integrated circuits with tens of thousands of transistors built inside tiny integrated chips and responding to signals of thousandths of volts, the current created by the electromagnetic pulse of a passing bolt of lightning can indeed do extensive damage.   There have been reports of electronics damage when a boat was not hit directly; lightning hit a nearby boat or hit the water nearby.

So what do you do?
If we have time, we try to protect our electronic equipment.  We disconnect the antenna from our VHF radio, and our GPS, disconnect the power and other connecting cables, remove them and stick them in our microwave oven.  Our belief is that this acts as a a Faraday cage (see sidebar to the right)  - the electromagnetic pulse is conducted around the metal box and sent to ground through the microwave oven's AC ground.  I stress that this is our belief - it has not been tested as we have never been hit.  Any handheld units we have - our VHF walkie-talkie, our handheld WHAM microphone, and our notebook computer get tossed in there, too. 

Anything else we trust to our insurance company.  Off premises back up of critical computer files should be considered, too - though we are somewhat remiss in this - (another thing to put on my things to do list - sigh).


As we said at the top of this page, the rumors abound, the scientific evidence is lacking or shaky.  Lightning and what to do about it is a crap shoot.  That part is a fact.  We, obviously, are not the final arbiters, the experts in the field.  I would venture to say that there may be no experts.  We will all just have to read what we can, believe what we will and react accordingly, hoping our judgment is not faulty and / or our luck holds.

The last thing we would like to offer you are some of the results of various studies for which most likely your tax dollars paid much.  Read them at the risk of your own sanity.  We did, and there is no doubt that our sanity is in question here.

Some anecdotal evidence???

Some things we have seen:  One boat, a new boat at the time, on Lake Erie was hit with lightning.  We don't know the details - this happened almost two decades ago.  The lightning arced through the hull all the way around the boat, leaving tiny, slightly charred pin holes, six inches to a foot above the water line.  Strange.

One 'head' boat in Fort Pierce, Florida (a head boat is a charter fishing boat) was hit many times.  So many, that some of the charter boat captains refused to take it out if there was any chance of a storm.  Why was this boat hit and not others?  No way of knowing.

We have heard stories about boaters being struck by lightning when "there was not a cloud in the sky."  This is possible because lightning can strike several miles from the storm clouds where it originates.

There are many boat / lightning stories.  If you are looking for more just put "boat hit by lightning" in your favorite search engine, pour yourself a glass of your favorite libation and kill an afternoon or evening.

What to do after your boat has been struck with lightning:

You will want to have the boat pulled. Inspect the hull for damage, particularly around through-hulls.  They may not leak immediately, but the heat generated in the lightning strike may have compromised the bronze fittings or the fiberglass surrounding them.

You will want to do a careful inspection of all of your rigging - particularly any connections.  Where the shrouds and stays are connected to the aluminum mast should merit a particularly close look.  Again, the heat generated at these joints may have compromised the metal - particularly the aluminum mast which has a much lower melting point than the stainless steel. 

Check all electrical connections - look for blackened wires near connectors.  Stress your circuit breakers to make sure the are still functioning.  Much of the damage can be hidden - or marginal - things may work when you test them but won't last long.  This is important to discover because when your insurance claim is made, and your claim paid, and you later discover more damage, you may have to pay the deductible again - as some companies may consider it a separate claim. Also, you will have one more claim on your record which may affect your rates.  Though some companies may simply reopen the original claim and take care of it.

Some, probably most, insurance adjusters are not trained to assess all of the damage on boats.  See if your insurance company will cover the cost of a rigging inspection, a boat surveyor and the hauling of your boat for the inspections.  If not, it may be worth it to pay for these things yourself.

Sometimes, the insurance company will pay for all replacement costs; sometimes for replacement cost less depreciation.  You may want to look at your policy and discuss these things with your insurance agent.  Collisions, groundings, fires among other things which could occur to damage your boat and its contents will have deductibles and areas which are simply not covered under various conditions.  Check out our Insurance for Boats page.

After the damages have been assessed, repairs made and checks and payments processed, you should as soon as possible take the boat out and stress all of the systems to make sure that all damages have been discovered and that all repairs have been made properly.  Again, discuss this with your agent.

What are the odds of my boat getting hit by lightning?

According to Boat U.S. "The feature reports that in any one year the odds of your boat being struck by lightning is about 1.2 in 1,000, with 33% of all lightning claims coming from the sunshine state, Florida. The second most struck area in the country was the Chesapeake Bay region (29%), while on the opposite side, 13 states had no lightning-related claims, including states such as Idaho and Nebraska."

The odds are, of course, higher on a sailboat, and for some reason, higher still for a multihull sailboat.
From Seaworthy Magazine - an excellent article about lightning.
Study on the Effectiveness of Single Lightning Rods by a Fractal Approach by Xuewei Zhang, Lin Dong, Jinliang He*, Shuiming Chen and Rong Zeng.  This may be an excellent article.  It sounds very scientific and uses a lot of diagrams and calculus.  I stopped reading it when I got to the word 'stocastic.'
* Having worked in marinas on the east coast,  I have worked on the bottoms of hundreds of boats. I have NEVER seen a "copper ground plate of at least one square foot in size installed externally on the hull bottom."  I have seen a few of the 'sintered bronze' plates attached.  I have also heard of cases where those 'blew a hole in the bottom of the hull' ostensibly because the lightning could not escape fast enough before suddenly heating the plate and / or hull to the breaking point.   (Captain Larry)
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