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 Revisiting Lightning and a NOAA study		  New Topic  Topic Locked
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windsong
Captain

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USA
318 Posts
Initially Posted - 08/03/2009 :  08:58:46  Show Profile
I read all the lengthy threads here going back to 2003 on lightning and the relative merits of grounding your boat. Good stuff. I did not see any links to a NOAA-funded 1992 study "Lightning and Sailboats." (old, yes, but so is lightning).
http://nsgl.gso.uri.edu/flsgp/flsgpg92001.pdf
Very practical, layman's advice. Very readable, I thought.
My one takeaway is that grounding will not increase the likelihood of a strike, and it will minimize damage to crew and boat. In other words, "Get it to ground!"
4-20% (wide range!) of all sailboats in FL have been struck by lightning at least once. One Sarasota, FL sailboat has been struck 5 times. With those statistics, grounding your mast seems like a worthwhile effort.

Michael Hetzer
"Windsong"
2009 Catalina 250 WK HN984
Myrtle Beach, SC
Edited by - windsong on 08/03/2009 09:04:33

redeye
Master Marine Consultant

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3476 Posts
Response Posted - 08/03/2009 :  09:10:27  Show Profile
What is the name of that boat? Iron Maiden...
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JohnP
Master Marine Consultant

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1520 Posts
Response Posted - 08/03/2009 :  09:39:47  Show Profile
No, it's named "$#!% Happens".
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aeckhart
Master Marine Consultant

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USA
1709 Posts
Response Posted - 08/03/2009 :  10:26:51  Show Profile  Visit aeckhart's Homepage
I've been struck once in the twenty years I've owned Gallivant. She was unoccuppied at our mooring. The strike destroyed the radio, one battery, the Windex, the electrical panel, and my speed/depth instruments. The lighting exited on the boot stripe, making a quarter-sized hole and a dime-sized hole right next to each other. There were also about 50 pin-sized holes in the bottom. No water entered the boat and I have not gotten any blisters, and this happened about 8 years ago.

I have not and will not go through the time and expense of grounding all the shrouds/stays and electrical equipment when there hasn't been any information published that says it definately works. Grounding does not stop a lightning strike. With that kind of power hitting the boat, somethng is going to happen no matter how much grounding is built into the boat.
Edited by - aeckhart on 08/03/2009 10:28:21
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DaveR
Master Marine Consultant

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USA
2015 Posts
Response Posted - 08/03/2009 :  13:42:34  Show Profile  Visit DaveR's Homepage
In a thread on this subject 6 to 12 months ago I posed a question;

If you have 2 wire leads (one grounded, one not grounded) the same distance from a third opposing lead and you charge the third one to the point that it makes the jump to the other leads, which one will it jump to? I'm betting it'll jump to the grounded one every time.
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windsong
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USA
318 Posts
Response Posted - 08/03/2009 :  14:22:41  Show Profile
I'm a EE, but I must have missed the course on lightning. :)

Dave, the greater the potential, the greater the likelihood of a discharge, so I agree with your point. But the NOAA description of how lightning interacts with a boat is pretty interesting, how it forms a positive spark at the tip of the mast and how the lightning then "attaches" to the mast. This mechanism seems to dispel the old sailor myth. They write: "There is no support for the argument presented by some sailors that they should not ground their sailboat since it will increase the chances of it being struck by lightning."

And then it cuts to the chase: "The difference is what happens where the conducting path, the mast, ends. Since current cannot flow from the ground to feed the growing attachment spark, a negative charge accumulates at the base of the mast and eventually arcs across in the general direction of the water or a nearby conductor. (For this exericise, crew members are conductors!). The result is an unharnessed electrical discharge between the bottom of the mast and the water."

Ouch! I don't know about you, but to me, that sounds pretty nasty.

As a final thought, I will not argue with the naysayers who point out that the chance of a lightning strike is so slim that there are far more important and mundane things to worry about on a boat - such as changing our oil or checking the keel bolts. But there's just something about being out in a lightning storm, isn't there, that makes you wish you had done something, anything to improve your chances?
Edited by - windsong on 08/03/2009 14:24:53
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Voyager
Master Marine Consultant

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5378 Posts
Response Posted - 08/03/2009 :  16:30:13  Show Profile
Michael
I advanced a very simple scenario some time ago. The mast has a few wires in it. These wires lead back to the power panel. From there, a wire leads to the depth meter and or knot meter. From there, a wire goes to one or another through hull fittings, a depth transducer or a paddle wheel. Some have a wire to the outboard, or to the inboard engine.

Basic fact is that the mast IS grounded to the water.

Problem is, the lightning will pass through #12 wire, #16 wire, something that is not really up to carrying 1,000,000 A of electricity. In terrestrial installations, lightning rods are usually grounded using #4 or thicker wire connected to an 8 foot long copper grounding rod that's pounded into the earth.

I would assume that an adequate ground for the mast would consist of a piece of #4 wire or even copper pipe connected from the bottom of the mast to the keel bolts. While this may not be optimal, it could save the boat from being holed.

The recommended grounding plate is approximately 1-2 square foot of copper plate attached to the bottom of the hull. The piece of #4 wire should be connected through the hull to the plate.

Some people also carry a set of jumper cables with a chunk of copper plate on one set of alligator clips. In case of a storm, you could connect one end of the cables to the mast step, and drop the other end with the copper plate overboard.
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Dave5041
Former Mainsheet Editor

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USA
3758 Posts
Response Posted - 08/03/2009 :  17:25:50  Show Profile
It's interesting stuff, but the 12 +/- 8 (4 - 20) is eye catching. You can choose to <u>ground</u> or not to ground, but don't kid yourself that a marginal and haphazard attempt will give you any protection.
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
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