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Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

Lightning - prevention & surviving

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mikengina
Deckhand

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USA
7 Posts

Initially Posted - 06/07/2006 : 17:53:47

Gina & I are restoring 1981 fixed keel C-25T #2404. The boat has had several rather extensive repairs in odd places in the vicinity of the forward hatch and chainplate for the starboard upper. There is no sign of any interior damage. Our fiberglass expert opines that it likely is lightning damage. This brings to mind several questions.
-- Have any of you ever been on board when lightning hit? If stuck in a storm, what safety measures are suggested? How best to prevent a strike?

These boats are not grounded and I worry about how lighting bridges that gap between mast/shroud & water. From my physics classes 30 years ago, I recall a discussion where the professor recommended attaching a grounding plate to the shroud by a length of wire. Aside from logistic problems (ie, dragging a grounding plate), I feared that such an arrangement would actually increase the probability of a strike (it completes the circuit between the top of the mast and the water) but it might also reduce the potential difference between the atmosphere in the vicinity of the boat and the clouds (thus reducing the likelihood of a strike). Out of 9 years of college, electricity & magnetism was without a doubt my weakest subject! Help, please.

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 06/07/2006 : 20:41:03

Hi Mike... There have been several threads on this here--you can probably Search on "lightning" and find some. The net of it is, nobody really knows. I suspect, from your profile, that you're sailing on fresh water--that makes a strike to your boat slightly more likely than on saltwater, which is more conductive and therefore more attractive to lightning. It's very common in saltwater for a lightning to hit the water very close to a boat with a tall mast--although the result is often that the electronics on the boat are still fried by the magnetic pulse.

Proplerly grounding a boat is a significant project involving a heavy copper cable from the base of the mast to about a 1' square copper plate on the outside of the hull. Experts suggest that doing less than that can result in attracting a strike that damages the boat, while doing nothing might actually avoid the strike. Static dissipaters are dubious--I know somebody whose little metal furball was blown to smithereens by a direct strike.

I've placed my bet as follows:

1. Sailing in salt water, I'm guessing that the mast and rig, being deck-stepped, with dry air and plastic between them and the water, will not be a sufficient attraction to divert a strike from going straight through the wet air to the water. I am aware of some evidence that supports that guess.

2. Not being in a high-thunderstorm area (like Florida), my strategy is not to be out there when thunder-bumpers are forecast, and when I hear on NOAA that one is coming (or see the signs and hear the rumbles in the distance), I head for shelter.

Now, what to do if that strategy fails... The conventional wisdom is to anchor or, in open water, heave to, and sit aft in the cabin (away from the mast).

All of that said, I will deny ever saying any of it!! You should look into the discussions, including the research done by a U. of FL professor at http://www.thomson.ece.ufl.edu/lightning/ and decide for yourself given your sailing venue, tenendency to be out in weather, and what information you choose to believe. Obviously, it's a serious issue, but there's no single answer.

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Leon Sisson
Master Marine Consultant

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USA
1893 Posts

Response Posted - 06/07/2006 : 22:11:23

Here's a link to a thread about lightning and grounding: [[url="http://www.catalina-capri-25s.org/forum/topic.asp?TOPIC_ID=10081"]lightning protection - grounding[/url]]
Like Dave says, there's no one perfect solution to the sailboats and lightning issue.

-- Leon Sisson

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mikengina
Deckhand

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USA
7 Posts

Response Posted - 06/07/2006 : 22:38:21

Dave -- That is a great article. We have a sailing friend in his mid 90's who still races his Catalina 22 competitively in our club's PHRF fleet. I crewed for him for about 15 years. He has been sailing since the 1920's and has had his boat struck 3 -- as in "three" -- times with him on board. Ironically, twice he was struck racing his Lightning class sailboat and once struck while on a Sunfish. Thankfully, no one was ever seriously injured. I was never on board during any of these strikes and am not at all superstitious, BUT when thunder-boomers were on the horizon, I never had much trouble convicing him that he shouldn't try for four.

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 06/08/2006 : 00:59:48

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 ...Proplerly grounding a boat is a significant project involving a heavy copper cable from the base of the mast to about a 1' square copper plate on the outside of the hull...<hr height="1" noshade id="quote"></blockquote id="quote">

In an article about lightning protection by Don Casey, he states that lightning dissipates through the edges of the hull mounted copper plate. He further states that salt water boats need a plate with 4ft of edge (1ft square) while freshwater boats need a ground plate with 24ft of copper edge due to the less conductive properties of freshwater.

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 06/08/2006 : 07:52:26

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dlucier
 ...freshwater boats need a ground plate with 24ft of copper edge due to the less conductive properties of freshwater.
<hr height="1" noshade id="quote"></blockquote id="quote">
Yikes! It should be pretty much in line with the mast so the connecting cable doesn't make any turns--that'd be tough to do on a C-25.

The Sunfish essentially has a "keel"-stepped mast (close to the water), and I think the Lightning does, too... That might have been a factor in attracting the strikes, and it's a characteristic the C-25/250 doesn't share. But the fact that he's still alive might also be due to the mast going to the bottom of the boat. These are the issues you get into when considering grounding--do you end up attracting a strike or just handling the inevitable strike better? I know of several cases where a shorter mast was hit in a marina with taller masts all around--don't know the details of the configurations.

Edited by - Dave Bristle on 06/08/2006 07:53:48

Douglas
Master Marine Consultant

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1595 Posts

Response Posted - 06/09/2006 : 22:57:27

Set of Jumper cables and a hunk of diamond plate. Attach one end to the mast or stay and the other to the diamond plate. Throw the end with the diamond plate into the water and quit worrying about it.

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Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 06/10/2006 : 08:41:08

Doug... if it were only that simple.

The problem as Dave points out, the nagging question whether grounding increases the chances of taking a strike and if the ground isn't adequate, then is it wise to increase the chances?

There is good data suggesting that grounding triples chances of a strike. It is unfortunate that the study which Dave points to by Dr. Thomson is given credit as science because his conclusions are based on bogus data and are errant. He said ungrounded boats take strikes in equal or greater numbers than grounded, so there is no reason to be ungrounded and therefore to be ungrounded is negligent. By the way, part of his livelyhood is as an expert witness suggesting the negligence of owners without lightning protection systems.

A challenge exist to sort out the conclusions of Thomson... he concludes more properly that boats with no lightning protection systems are hit in almost the same numbers as those with. The problem here is his use of the terms grounded and ungrounded. He errantly refers to a boat without a protection system as being ungrounded and that is not true. Almost all sailboats with an inboard engine will have ground potential raised to the top of the mast by a ground wire aloft. Lightning is not seeking the tallest object, it is seeking closest ground potential and more specially if that ground potential produces an attraction spark.

Almost all inboard equipped boats will have ground potential raised to the top of the mast by an electrical ground or VHF antenna, regardless if the mast is grounded. Dr. Thomson failed to consider that when he applies his surveyor data and therefore falsely drew the conclusion that ungrounded mast are hit at same rate as grounded.

There is data from insurance damage claims suggesting that boats without a ground aloft are hit at considerably less rates than those with ground potential aloft. There is much evidence that the precurser of many strikes is the attachment spark produced by a raised ground potential feeding a single point air terminal. I think it more than reasonable to conclude based on what data and science is available that an elevated ground potential increases hit rates.

So, things aren't so simple. One is faced with weighing the three times greater chances of a strike against whatever reduction in damage the grounded effort will offer.

This question is really an outboard - inboard issue as outboards will usually be ungrounded aloft and inboards almost always grounded aloft. So, for Doug who is grounded... the question is simple... making the ground adequate and according to Dr. Thomson, Doug would be negligent if he didn't.

For those of us with outboards, do we wish to increase our chances of being struck if a grounding effort would be less than adequate?

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 06/12/2006 : 12:22:40

Geez... I keep forgetting about Arlyn's analysis of Thompson's work...

I should stop mentioning it.

Taking Arlyn's analysis a little further... A number of sources (not just Thompson) say that the grounding system must be very robust, including a heavy, straight-line cable from the base of the mast to the terminal point(keel or hull plate), in order to minimize the damage from a strike. Stainless steel is a relatively poor conductor compared to aluminum, so jumper cables attached to shrouds could be an invitation to a strike that ends up going down the mast, and therefore that will damage things and possibly people. Jumper cables from the base of the mast, across the deck, and down to the water could cause the strike to depart the cable and go through the deck and hull. (I've seen jumpers used that way for minimizing electrolytic damage in marinas with current leakage around the docks.) Of course, nobody knows for sure what's going to happen, but evidence abounds that a poor system can be worse than no system.

Edited by - Dave Bristle on 06/12/2006 12:24:18

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