Monday, August 10, 2015

Lightning strikes...twice!

Dangerous cloud-to-ground lightning


by Daria Blackwell

A boat we once owned had the unlikely misfortune of having been struck by lightning twice. She had a dissipator on her mast, her rig was not unusually tall, and she was always moored in a crowded mooring field.  Yet, somehow, the lightning liked her best.

A study published last year in Science (ScienceVol. 346 no. 6211 pp. 851-854 DOI: 10.1126/science.125910) concluded that lightning strikes are predicted to increase 12 ± 5% per degree Celsius of global warming and about 50% over this century.  With the increase in likelihood of a strike, what do we need to know to protect ourselves and our vessels?

Here are a few things we learned from our experience.



Plenty of damage

Approaching storm
Lightning struck a boat we had some time ago while she was on her mooring on Long Island Sound. I took the picture of the approaching storm just before we went ashore. The yacht club staff, huddled in the dock house, said they felt the hair stand on their necks and heads and they heard a sound they never wanted to hear again. They saw it touch down in the mooring field but couldn't see which boat, or indeed if any boat, was struck. They were temporarily blinded by the light. There was no visible external damage when they later surveyed the boats.

Our boat wasn't so lucky. She took a direct hit. The damage to the keel where the lightning exited after the strike was pretty impressive. The lightning blew right out the bottom of the keel, fortunately only destroying the fairing and not blowing out a through-hull, which can happen.  Had she had those plastic through hull fittings, we were told they could have melted, and she might have sunk. 

All the electronics and electrical wiring and components had to be replaced (all covered by insurance, thankfully). Everything was fried including handhelds that were unprotected. Hence we now store handhelds in the oven (faraday cage) in lightning prone areas. Fortunately, the engine was undamaged (it has its own ground in the propeller and shaft), but the bilge pump was rendered inoperative. Fortunately, we don't get much lightning in Ireland where we are now even though we do get thunder. We always laugh when the forecast is for "risk of thunder". Never heard of thunder doing any damage. But if there's thunder there must be lightning somewhere.
Hole blown in the keel by lightning passing through

That was the second of the strikes she suffered. The first was a smaller electrical discharge nearby but not a direct hit. We were on board that time. It caused intermittent electrical problems that were difficult to diagnose and relate directly to a lightning discharge; ie, no insurance case.

After hauling and repair work on the second hit, she was good as new with all new equipment. We did remove the "toilet brush" dissipater which we felt (superstitiously) had become a lightning attractor. Instead, whenever we left her on her mooring, we clipped a wire fitting to the shrouds and ran the wire to a  metal "fish" which was hung over the side and into the water. It was supposed to conduct the lightning from the aluminium mast into the water, bypassing the hull.  As we never suffered another strike, we don't know if it worked or not.

What works?


The Florida Sea Grant estimated that lightning can be expected to hit from four to twenty percent of moored sailboats per year and that cruising sailboats typically get hit at least one time during their lifetimes in Florida, a region of high lightning storm activity. Worse, there is no way to protect against a strike. In fact, there is no sure-fire lightning protection. Lightning is notoriously unpredictable.

Those lightning dissipators can neutralize static in the vicinity but they do not protect against direct strikes.You can't stop lightning from hitting your boat or shooting through your boat.The best you can do is to help the lightning find the path of least resistance through your boat, one that you have set up to do minimal damage along the way.

Thunderstorms can form unexpectedly
Lightning experts say the right thing to do is ease the path of the lightning through your boat via a grounding system. Begin by getting a heavy copper plate and through-bolting that to the bottom which becomes your "ground." I'm no expert, but you need to run special copper wire from the base of the aluminium mast to that plate. To increase safety, also ground all stays to the plate.  That way, the electrical charge will follow your prescribed path and be conducted through your ground into the water. Advice on installing a grounding system can be found here.

What is lightning?


Lightning is a sudden electrostatic discharge. Cloud-to-ground lightning is a sudden electrical discharge between negatively charged clouds and the positively charged ground (CG lightning). It can also occur between oppositely charged clouds (CC lightning) or between different electrically charged regions of a cloud (intra-cloud lightning or IC).  Eventually the charges build up and the cloud sends out a bolt which is really a discharge of neutralizing energy.  It is completely unpredictable and doesn't follow any particular path or rules. The best thing to do if you see lightning approaching is to get off the water if you can. The next best thing is to go below and not touch anything metal.
Cloud to ground lightning

We used to see a phenomenon called heat lightning in the states, which appeared as a silent electrostatic discharge in the atmosphere. It's really just lightning that is far enough away (more than 12 mi or 20 km) that the thunder is no longer heard. But when the sky is hazy, as is quite typical on warm summer nights, the light from intense thunderstorms as far away as 100 miles away can be reflected off the haze and up into the night sky. That's probably what accounted for the name.

Thunder is the sound caused by lightning. Depending on the distance and nature of the lightning, thunder can range from a sharp, loud crack to a long, low rumble. Because light travels much faster than sound, the lightning is approximately one kilometer distant for every 3 seconds that elapse between the visible flash and the first sound of thunder (or one mile for every 5 seconds). Having sailed in lightning prone areas such as Chesapeake Bay and Long Island Sound, we automatically started counting when we saw a flash to see if it was coming toward us or moving away. A very bright flash of lightning and an almost simultaneous sharp "crack" of thunder therefore indicates that the lightning strike was very near. A thousand and one, a thousand and two...BANG! 

Cloud lightning
Then there was the time we were making an overnight transit of Long Island Sound and a lightning storm hit out of nowhere -- completely un-forecast -- and "stitched" lightning all around us in a circle like a high speed sewing machine, but that's another story.  Suffice it to say, we were really scared. We were looking for anchorages to pull into but there was no escape. All we could do was call it into the weather service (for which they were extremely grateful and immediately issued a weather alert) and head for where the lightning had been, hoping that it was moving away from that location. We made it that time, but as storms intensify, it pays to be prepared.


If lightning approaches?

 

Get sails reefed or down early to avoid the sudden, intense winds at the leading edge . You don't want to be at the mast during the lightning storm nor do you want to be knocked down with full sail up.

Turn on an AM radio: An old boater's trick is to turn on an AM (not FM) radio to listen for static.  Small, portable battery powered radios are best. It will indicate if there's an electrical charge building in the vicinity of your boat.

Beware of metal: The cockpit is one of the most dangerous places in a sailboat because of its metal parts. If you have it, engage autopilot rather than hand steering. During a lightning discharge, high voltages could zap a helmsman if he or she has one hand on a metal steering wheel, for example, and the other on the metal engine controls or a shroud thereby creating a connection between two conductors. The lightning doesn't even have to strike directly. The old adage was to keep one hand in your pocket during lightning storms. But it's much safer to anchor and go below.

Avoid the mast:  On deck, avoid handling sails at the mast. When below, stay away from the mast-to-keel area. That's the primary route by which lightning will be seeking an exit.

Head for safety: Head for shore as soon as you see storm clouds gathering. The worst place to be is on the open water where your boat is the tallest lightning rod around. Remember, the leading edge will be associated with strong winds and waves, heavy rain, and even hail, making it potentially dangerous to head to shore in a dinghy. Do at least drop anchor if you can, which will make it easier to stay below and away from metal.

Go below: If you do see lightning getting closer, go below. That gets you away from the metal in the cockpit. The places aboard to avoid most are directly beneath the mast or the boom so stay away from the area of the cabin closest to the mast.  Stay dry.  A dry body conducts less efficiently than a wet one.

Stay away from the water: Avoid any connection between yourself and the water. Your body is a better conductor than air, so lightning will think your body is the easier route making you a human lightning rod. If you enter the water, electrocution is highly probable if lightning strikes nearby.

Ewen M. Thomson writes on behalf of Florida Sea Grant: "There is no safe place on an unprotected small sailboat and in a protected boat only places of relative safety. There is one place that is more hazardous than a small unprotected sailboat, it's a small, unprotected boat without a mast. Every year there are multiple deaths of boaters in open boats caused by lightning strikes, but there are very few reports of sailors in sailboats killed by lightning." So choose to stay on the bigger boat rather than hightailing it in the dinghy.

This resource has a good description of how to protect your vessel with a grounding system.

Cumulonimbus cloud forming. Classic anvil shaped cloud signifies storm potential.

What if your boat is hit?

The best thing is to have your boat short hauled for inspection immediately. Many insurers now cover a short-haul and some, like BoatUS, do not charge a deductible against this coverage.  Seeing where the lightning came out is critical and can help avoid a catastrophic failure down the line.

My question to manufacturers of boats is why don't they include a grounding system on every boat they build?




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