Meekos’ thread about lightning striking the ocean reminded me I’ve been wanting to ask this.
While I was wandering around the marina yesterday, it dawned on me that a sailboat must be a near-perfect lightning rod (50ft metal pole sticking out of the water… what more could a lightning bolt want?). Yet I never read about them being destroyed in storms (well, not by lightning, anyway). What gives? Is there some sort of “grounding” thru the keel that isn’t obvious? Or are they frequently damaged, just not enough to make the news?
The site seems to agree with others as to the frequency of sailboat strikes. However you should ignore their descriptions of lightning strokes. The main discharge of electricity is from the ground to the cloud in what is called the “return stroke.”
Here is Britannica’s description of the formation of a lightning stroke.
“Cloud-to-ground lightning is initiated by a preliminary breakdown process within the cloud, typically between the centre region of negative charge and the small positive charge below it. This process creates a channel of partially ionized air—air in which neutral atoms and molecules have been converted to electrically charged ones. Next, a stepped leader (initial lightning stroke) forms and propagates downward, following channels created by the preliminary breakdown process. The leader is highly branched in the direction of its propagation. Most leader channels are negatively charged. When the stepped leader nears the ground, an upward, connecting discharge of opposite polarity rises and meets it at a point typically about 30 metres (100 feet) above the ground. When the junction is complete, the cloud is effectively connected to the ground, and a very bright return stroke propagates back to the cloud at a speed about one-third the speed of light, following the leader channel. A typical lightning flash to the ground contains three or four leader-return stroke sequences in rapid succession. Occasionally,when there is a strike to a mountain or tall building, the first leader will start at the ground and propagate upward …”
In short, there are little lightning strokes, leaders, that gradually work their way from cloud to surface. When the leader gets close to the surface the remaining air-gap breaks down suddenly and the excess negative charges on the surface discharge back to the cloud rapidly in three or so sequences of leader-return strokes.
I remember reading the story of a sailor who made the first sail from the lowest body of water on the planet to the highest. He told a tall tale of his boat being hit by lightning, through the mast, and that the lightning caused all the cans of beer he’d been keeping in his bilge to fuse together. He had to take a hammer and chisel to this block of misshapen aluminum to get a beer out of it.
As David Simmons said, lots of sailboats are struck by lightning, with varying degrees of damage. There are a whole passel of different theories and methods for prevention of damage. Some are dead simple, like a big chain wrapped around the mast and dangled into the water. Some have the mast wired to a big bronze plate on the outside of the hull. Some have the mast, engine, keel, and all the electrics wired together to a common ground. Some ground through the shrouds (those diagonal wires from the mast to the outer edges of the deck to wires trailing in the water.
A popular, recent device is a crown of spikes at 45 degrees from vertical, bolted to the top of the mast. It is said to “disperse the charge without a strike.” Sure, it sounds like bad science, but theories about lightning are a dime a dozen. Two hundred years after Ben Franklin, there are still arguments about whether, and how, lightning rods work.
My stepmom’s boat was struck at dockside. It vaporized the wind indicator atop the mast. It also fried the radio. Everything else was OK.
Franklin noticed that a sharp point discharged a Leyden Jar (a capacitor) from a greater distance than did a blunt object. This made him theorize that a grounded spike mounted on or next to a building would allow the charge from the sky, or cloud, (Franklin considered the movement of charges to and from the Leyden Jar to be the motion of positive charges) to leak off at a lower potential difference between surface and cloud. He thought this would at least mitigate the strength of the lightning strike and might allow the equalization of the charges without any strike at all.
In the same fashion the mast, being a vertical spike, can allow leaking off of the negative surface charge at a lower potential when it is connected to a conductor immersed in the water. So when strikes do happen they are much weaker than would otherwise be the case because they happen at much lower potential and after a considerable amount of the charge as already equalized.
At least that’s a possibility. As As Nott said, lightning rods and lightning protection in general seem to work but there is some controversy over just exactly how, and indeed some dispute as to whether. I go with Franklin’s speculation.
Wetren’t old-time sailboats frequently illuminated by St. Elmo’s Fire (corona discharge)? It seems to me that the masts act as lightning rods, drawing off the charges and conducting them to ground. St. Elmo’s fire is described as a pale blue or yellow light ,emenating from the tops of mats, ends of spars, etc. Occasionally, even the crew would see their shipmates enveloped in a ghastly glow! This must have scared the shit out of old-time sailors.
Do any photos of St. Elmo’s Fire exist?
I don’t have any photos, but I was fortunate enough to see St. Elmo’s Fire up close. We were sailing towards Jamaica one night about 7 years ago, and there were lightning storms in the area. I was hanging out on the bridge and was watching what I thought was the luminous lights of a cruise ship beyond the horizon. After about 20 minutes of seeing this glow, I took a closer look with binoculars, and realized that what I was seeing was the top of the jackstaff glowing bright green - about 25 feet away.
Soon we were all out on the bridgewings gawking at all the glowing items on the cutter. The tip of every antenna, mast and staff was glowing bright green. Very, VERY cool looking. Then someone on the bridge read what “The American Practical Navigator” had to say about St. Elmo’s Fire: “…an object from which St. Elmo’s Fire emanates is in danger of being struck by lightning, since this discharge may be the initial phase of the leader stroke.”
:eek: :eek: :eek:
Not cool. Not cool at all. The weather decks were pretty much secured after that.
I can really see how this would have led sailors back in the day to believe something supernatural was going on.