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#1
02-17-2014
 Frogwatch Senior Member Join Date: Jan 2011 Posts: 666 Thanks: 0 Thanked 17 Times in 17 Posts Rep Power: 4
Probability of a lightning strike

To try to get an order of magnitude probability of a sailboat getting hit by lightning, I assume the following:

An Average thunderstorm diameter of 12.5 km
An average of 500 cloud to ground lightning strikes /storm ( get these numbers from wiki)
This simple models than says the probability of getting hit is simply the ratio of the area of influence under your mast to the area of the storm times the 500 strikes/per storm. It comes out to about a 1.5% probability of getting hit if you are a lone sailboat sailing under an active thunderstorm.
Of course, this model neglects a lot of things but it is probably reasonably accurate. I suspect the probability is a bit higher but probably no more than 5%.
So, how much of a chance do you want to take sailing under a storm? I have terrible luck so for me a calculated 1.5% is closer to 15% for most people.
#2
02-17-2014
 Uricanejack Senior Member Join Date: Nov 2012 Location: BC coast Posts: 109 Thanks: 1 Thanked 2 Times in 2 Posts Rep Power: 2
Re: Probability of a lightning strike

Well to start with it will vary depending upon the frequency of thunderstorms in the are you are sailing in.
#3
02-17-2014
 sandy stone Junior Member Join Date: Jan 2014 Location: Slidell, LA Posts: 23 Thanks: 0 Thanked 1 Time in 1 Post Rep Power: 0
Re: Probability of a lightning strike

Empirically, the odds seem to be a lot lower than that (thank goodness). Summertime around here sometimes seems like thunderstorm central, but I can only recall one sailboat getting hit under way in the last couple of decades. I've had some pretty uncomfortable times myself, with strikes on either side of the boat, but no direct hit. On the other hand, I know of several boats that have been hit at the dock (one of mine, in fact). Can't say what the difference might be.
#4
02-17-2014
 Frogwatch Senior Member Join Date: Jan 2011 Posts: 666 Thanks: 0 Thanked 17 Times in 17 Posts Rep Power: 4
Re: Probability of a lightning strike

We are assuming you are sailing beneath a thunderstorm so the number of storms/yr does not enter this. Now, if you happen to go sailing during thunderstorms and you commonly venture under them then the probability of getting hit in a year would depend on the number of storms/yr.
I avoid T-storms to the point of not sailing in mid-summer, too hot anyway in FL to sail in summer.
#5
02-17-2014
 bacampbe Senior Member Join Date: Mar 2009 Location: DFW Area Posts: 225 Thanks: 0 Thanked 0 Times in 0 Posts Rep Power: 6
Re: Probability of a lightning strike

The original post mentions 500 strikes per storm. I assume that's over the lifetime of the storm. If so, then the results are based on an implied assumption that you sail under the storm for the full lifetime of the storm. I imagine that's rarely the case. Storms move, and at least part of the time so do boats.

This would put the calculated odds off by a wide margin, even giving all the other assumptions the benefit of the doubt (e.g. strikes evenly distributed, all strikes in your radius-of-influence actually hit you.) You would need to consider the number of strikes over a period of time verses the time spent under the storm.
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Last edited by bacampbe; 02-17-2014 at 08:57 PM.
#6
02-17-2014
 Frogwatch Senior Member Join Date: Jan 2011 Posts: 666 Thanks: 0 Thanked 17 Times in 17 Posts Rep Power: 4
Re: Probability of a lightning strike

bacampbe:

Right. I was simplifying as much as possible.
The reason why more boats get hit at docks is because most boats spend far more time at docks.
I actually think the 1.5% is low. If you actually stayed beneath a t-storm from start to finish, I think it should be higher. In fact, I think the radius of influence of your mast over a flat conducting plane (the water) should be higher and the probability goes as the square of this radius of influence.
#7
02-17-2014
 capta Master Mariner Join Date: Jun 2011 Location: somewhere in the Windward or Leeward Islands Posts: 1,228 Thanks: 11 Thanked 54 Times in 49 Posts Rep Power: 4
Re: Probability of a lightning strike

I can't count the number of times I have been in lightning aboard vessels of all kinds. Steel boats, wooden boats, steel masts, wooden masts, alloy masts and I've only been hit once, maybe (all the electronics gave up the ghost, but we found no damage aloft or to the vessel).
I've seen dozens of strikes around me within the horizon, in as many minutes. I've been the tallest thing for a few miles around and possibly a thousand. I've had lightning at sea so consistent that it was bright as day (at night) and I'm sure I could have read a book, though I was otherwise occupied. The lightning and thunder were simultaneous hundreds of times. I've been blinded for a few minutes by a strike so close my hair stood straight up.
I don't think it's possible to put numbers to this. You either get hit or you don't. I know sailing boats that have been struck several times, in different storms and others that haven't ever been hit.
I do, however, turn on my engine (not necessarily in gear) when I am in a situation where I might get hit, in the hope that the alternator's field might keep the vessel from becoming positively or negatively charged. I'm sure it's foolish and it may not have anything to do with my good luck, so far.
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#8
02-17-2014
 TQA Bombay Explorer 44 Join Date: Apr 2009 Posts: 2,425 Thanks: 0 Thanked 33 Times in 29 Posts Rep Power: 6
Re: Probability of a lightning strike

There are some figures around from one of the insurance companies [Boat US?] which say that a boat on the East Coast, Chesapeake to Florida, can expect to get struck every 10 years.

If this is true 8 years to my next one.
#9
02-17-2014
 tdw Super Fuzzy Moderator Join Date: Oct 2006 Location: Sydney Australia Posts: 14,328 Thanks: 5 Thanked 67 Times in 62 Posts Rep Power: 10
Re: Probability of a lightning strike

Been sailing boats off our coast since the mid 1970s. Have sailed down that coast with lightning hitting the water on either side of the boat, seen it hurling bolts horizontally across the sky, the night sky as bright as daylight, blah de blah de blah yet narry a hit.

Came home from a mini cruise January 2013, left boat on mooring, came back two weeks later to mass destruction ..... beyond AUD\$40K.

You simply cannot set odds on lightning.
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#10
02-17-2014
 JonEisberg Senior Member Join Date: Dec 2010 Posts: 3,397 Thanks: 0 Thanked 113 Times in 101 Posts Rep Power: 4
Re: Probability of a lightning strike

Quote:
 Originally Posted by tdw You simply cannot set odds on lightning.
Not to mention, what's the point of trying to do so?

If the OP became convinced the odds were only 0.75% instead of 1.5%, would he then stop attempting to dodge thunderstorms, and sail blithely into them, instead? :-)

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