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Arlyn Stewart
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USA
2980 Posts

Initially Posted - 10/11/2003 : 17:09:01

More theory for those who like it

http://www.stewartfam.net/arlyn/lightning.html

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

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ClamBeach
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3072 Posts

Response Posted - 10/11/2003 : 21:28:49

There was another strategy/theory regarding lightning protection that your posts didn't touch on... and that's the nature of 'charge dispersion'.

My understanding of 'lightning rods' is that one of their functions is to allow ground charge to be dispersed from the point of the rod into the surrounding air. (sharp points disperse charge more readily than flat surfaces)

This drains off electrical potential and reduces chances of a ground strike. There are some modern versions of lightning rods that look like a copper bottle brush. This provides thousands of points for better charge release.

Also remember that distance isn't the determining factor unless the resistance offered by the atmosphere is constant. The resistance of the path between the charges is what matters. "Bubbles" of charged/ionized/moist air (more conductive) may be the explanation why lightning hit "over there" and not "here"...

Currently maintaining two holes in the water...'77 Venture 23 and new to the family,
'78 Catalina 25

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Dave Bristle
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Djibouti
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Response Posted - 10/11/2003 : 22:15:09

For some scholarly research on the subject, see http://www.thomson.ece.ufl.edu/lightning/ .

The only thing I'm going to say about our choice is that we go to extremes not to be out in lightning storms--admittedly easier to do in CT than in FL. I won't argue for or against grounding for the same reasons Catalina and others don't ground our boats for us--I don't want anybody taking my advice on this issue. (Not that anyone would have...)

YMMV.

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

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Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 10/11/2003 : 23:09:54

Clam,

Your right... but charge dispersion is a necessity only on a tall grounded object. I'm advocating that the mast not be grounded. If a lightning protection system is wanted... ok, but it can be had without grounding.

I have absolutely no doubt your right about the single point dispersion and can give the next best thing to solid proof of that. Statistics. Numbers tell stories. In this case the story is told by Ham radio operators who with the advent of FM 2 meter repeaters purchased a very popular vertical antenna called a Ring Ranger and placed it usually on the top of whats called the stinger pole (the pole that mounts into the rotator and on which various beam antennas are mounted). The reason, was to avoid having to rotate the antenna stack to work repeaters in various neighboring comunities.

This antenna was loaded such that it had an electrical ground. Hams paid the price for such an indiscretion with literally a mass wave of lightning strikes to their shacks. What they were doing was allowing a single point dispersion to send up a feeler. The single point dispersion would send a feeler a long way which shortened the cloud charge potential path far below surrounding trees or terrain.

The solution was to get those frigging Ringo Ranger antennas off the stingers which didn't prove too hard because the lightning strikes took most of them down. Far better was a horizontal beam antenna as the upper antenna on the stack. Its horizontal surface would discharge or disperse the ion buildup without sending the long feeler. The dispersion device you mention is based on that concept and could be used in lieu of a horizontal antenna.

The single point lightning rod value... is to head off a lightning strike to sensitive equipment or facilities by inviting strikes to a well designed system capable of handling the loads.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>sharp points disperse charge more readily than flat surfaces <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

I don't know why this would be true. My thinking is the sharp point may in fact disperse with more difficulty than a horizontal surface or many fingers device and that difficulty culminates in a focused single point dispersion with the characteristic long upward feeler. This sort of ionization would not be wanted as it would attract not retard strikes.

If my mast were grounded, I would definitely not want a metz antenna on top... rather, I'd want one of the ion dispersing brushes that would harmless disperse.

If the mast is not grounded, it makes little difference whether there is a Metz, Windex or an ion brush on top.

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

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Douglas
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1595 Posts

Response Posted - 10/11/2003 : 23:22:28

It's all theory. Its either your time or it isn't. I have seen some wierd stuff with regards to lightning. I have seen it up close and personal. I have been hit when flying numerous times and its never the same. I have had one close call on a boat C-25. Hith the water a few yards in front of us. Lit up the water and made lots of steam. I have also been hit while on a boat. Setting at a dock in Newport Beach Ca. Everthing began to fizz and then Bang it hit. We were lucky and werent hurt. I do have a cool video of it. But once again if its your time its your time. No amount of gizmos and do dads is going to help.

Doug&Ruth
Triska (Alberg 29)
Tacoma Wa.

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Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 10/11/2003 : 23:42:19

Scholarly <img src=icon_smile_question.gif border=0 align=middle> I think they got bats in their belfries <img src=icon_smile_big.gif border=0 align=middle> Seriously... they come right out front and almost admit that they are about to tell you why you need to spend mega bucks on protection. They are recommending every one feed the Florida economy.

I'm not against protection... but they have a simplistic one track mind that to be grounded is the answer... and I think they suggest why... because not to take a stand leaves them looking impotent and to officially take the other side to not ground, leaves them liable. So, they simplistically say ground... who could sue em for that... and plus that, they got the Florida boat builders, and yards lobbying for it.

Scholarly.... I doubt it. I've enjoyed some good electrical engineer friends over the years. A few of them were pretty darn bright... a couple of them couldn't find their own way out of an outhouse.

I agree... to ground or not enters an area that deals with such an unpredictable phenomena that there are no easy or fast answers.

I'm just saying that it's not necessary to take a stand one way or the other... both worlds can be had in the same package... remain ungrounded and still have protection.

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

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tinob
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USA
1883 Posts

Response Posted - 10/12/2003 : 11:53:16

Hi Arlyn/Tom/Dave/Clam,

Thanks for the ENLIGHTENMENT all of which puts me squarely in Daves corner. However there is always the unexpected and in preparing for such an occurrence I'd opt for devising a ground rod from mast to keel employing an air gap within a glass tube. Would cutting the ground rod and fiberglassing either end into a glass tube be sufficient? What do you think a suitable air gap would be?

Val on Calista # 3936

Val Bisagni]<img src="http://im1.shutterfly.com/procserv/47b3df11b3127cce94709c5ff2e90000000010" border=0>

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Arlyn Stewart
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2980 Posts

Response Posted - 10/12/2003 : 12:50:22

Val, the air gap device may be available commercially... I havn't looked. The old house and barn arrestors as I think they were called of course used this principle to bring the two ends of the lightning rod close together but not touching. The glass bulb served as the mounting for the above rod as well as the preventer of the strike blowing sideways to the roof instead of across the air gap to follow the arrester and the grounding wire attached to it.

I think the important factors here are fairly simple... an air gap is needed in a large enough cable and the cable needs to remain straight on either side of the air gap. Some people recommend copper tubing for ground cable because electrical theory suggest that electrons flow on the surface rather than interior of a wires volume. The tubing yields interior as well as exterior surface and is easy to flatten the ends for bolting. If tubing were used, the challenge would be to find something to use that would hold the tubes in line and maintain a proper air gap. Preferably something that didn't burn, though everything seems to burn when lightning hits it, so lets say... preferably something that wouldn't start a fire following a strike. Something that would either go over the tubing or within, something that would allow visual confirmation that the air gap was set properly.

I'm thinking a 2 inch section of hardwood dowel turned to the inside diameter of the tubing. A couple of small holes drilled near the ends of the tubing to screw to the dowel. Yes, wood burns but the section would be so small as to likely be inadequate to fuel a larger fire. This section should obviously be kept in the open away from a bulkhead or fiberglass surface. An air gap of perhaps 1/8 inch would do the trick.

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

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Dave Bristle
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Djibouti
10005 Posts

Response Posted - 10/12/2003 : 13:35:04

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
Scholarly <img src=icon_smile_question.gif border=0 align=middle> I think they got bats in their belfries...

Scholarly.... I doubt it. I've enjoyed some good electrical engineer friends over the years. A few of them were pretty darn bright... a couple of them couldn't find their own way out of an outhouse.
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>
Well, one thing presented on that site is an analysis of the statistics (as opposed to discussions of anecdotes); another is a discussion of the physics of a strike. Both points suggest why some "protection" that's offered out there is hooey. Florida is probably the lightning capital of the world--particularly involving boats--and the U. of FL has apparently been working on that issue for a while, and their work has been utilized by the . BTW, did you follow the link from that site to the paper on Lightning and Sailboats-- http://www.thomson.ece.ufl.edu/lightning/SGEB17.html#strike prob vs. grounding ?

Arlyn, I think you might have a useful idea there, but diss and dismiss the experts at your peril. Boats are not barns, oceans are not fields, fresh water lakes are not oceans, and lightning is not what most people think it is.

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

Edited by - Dave Bristle on 10/12/2003 13:38:05

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Arlyn Stewart
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Response Posted - 10/12/2003 : 14:18:57

Dave,

They can't justify the kind of grant money that they get to pay their salaries and run their bastions of loftiness by saying... "put a piece of dowel in a copper tubing". No way! And, they are hamstrung to say anything less than what will be right around the corner...to require a friggin engineers stamp on the design and a certified installer to allow someone to go sailing.

I guess I need to look closer... I've read a whole bunch of reports lately that suggest boats get struck more often that are ungrounded...but I yet haven't been furnished the data.

got to go...more later

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

Edited by - arlyn stewart on 10/12/2003 15:42:48

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Arlyn Stewart
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Response Posted - 10/12/2003 : 17:45:35

Dave, I have followed the many threads of this link and have the following comments. I first read this study a couple of years back when it was brought to the forum. That I'm not impressed with it in true. Also, I have come across in an uncharacteristic personality to this issue for a reason. Because I'm a great skeptic of commercial interest who use fear as a tactic in unloading people of their hard earned money. I think they opportune themselves as experts without providing the logic or data to back up what they offer.

Every year there are multiple deaths of boaters in open boats caused by lightning strikes, but very few reports of sailors in sailboats killed by lightning.
Why? Could it be because many of these open boats are either aluminum fishing boats or aluminum pontooned party boats? Such, they would present a lesser dielectric resistance than the surrounding water and therefore would be more subject to a strike.

His teaching experience includes undergraduate physics at the University of Papua New Guinea, and the introductory electric circuits course and graduate electromagnetics at the University of Florida.
These are the makings of an expert? I would think that someone with vast experience who deals with lightning such as communications and transmission systems to have vastly more understanding of the characteristics of lightning activity. In fact, I know that it is exactly those types of engineers who initiated our current appreciations for ion dissipation and many of the other disciplines that are raised in this article. This guys perspective is not a critical study but rather about like mine...one mans opinion.

To dispel two dangerous misconceptions, a lightning protection system is designed to conduct the lightning current, not repel it, and grounding the mast as the codes dictate does not increase the probability of a strike.
I tried following the does not increase link...and it took me back to his general article that did not answer or address what he calls a misconception.... Why doesn't he answer or argue why its a misconception?

A. Since almost all lightning deaths occur in open boats, you will definitely be a lot safer after the modification. However, an ungrounded power boat with towers is an accident waiting to happen,
How can one just admit that almost all lightning deaths occur in open boats and then contend that even though one is enclosed... it is an accident waiting to happen?

Lower mast on trailer sailor?
Why is this link broken?

Ewen Thomson, an associate professor of electrical and computer engineering, and two South Florida surveyors say the problem results partly from sailing enthusiasts' and power boaters' ignorance of, or misconceptions about, lightning.
The link is established between the esteemed professor and the surveyors who stand to increase their business from a possible requirement that boats be inspected for lightning protection.

But it also stems from a lack of codes requiring boat manufacturers to install lightning protection systems, Thomson said.
Here the professor is calling of codes to validate his beliefs.

Tragedies occur on both sailboats and power boats, but lightning kills or injures more people on open boats because it is more likely to strike crewmembers when there is no mast.
So, once again, why then do grounding advocates think it is necessary?

A 14-year-old Hollywood, Fla., boy was killed last year after lightning struck while he was aboard a small open boat on a lake in Marion County
Again, this was a boat low to the water line... possibly aluminum.

Most boaters know little about the dangers of lightning or lightning safeguards, Crosby said. "The boaters themselves for the most part know diddly-squat," he said.
While examining the aftermath of a lightning strike may very give surveyors a unique perspective about what constitutes a good ground system, it does not make them experts at about how many times lightning strikes just beside a boat compared to hitting it . Also, to self proclaim that they are the knowledgeable agents in this long standing issue is a little bit of a stretch and leans on self serving.

Misconceptions, meanwhile, are not infrequent. For example, some sailing enthusiasts believe "grounding" a boat by running a wire from the mast to a connecting plate in the water increases its chances of being struck by lightning, Thomson said. However, reports from marine surveyors do not support this contention,
Some examples of these reports please... The reports showing the logic thought of surveyors would make very interesting reading.... For surveyors to draw those conclusions, they would have to witness more sailboats without grounding receiving hits than those with. Then it would be necessary to factor in the ratio of the two to find and adjusted number. Please print and distribute these for all of us to see.

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

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OJ
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Response Posted - 10/12/2003 : 22:08:01

On a less technical note we had a sailboat struck by lightning a few years back. Interesting, the lightning exited the hull just above the waterline on port and starboard.

When the board returned to the marina the following spring it was renamed . . . you guessed it

"HIGH VOLTAGE"

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Dave Bristle
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Djibouti
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Response Posted - 10/12/2003 : 22:08:51

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
His teaching experience includes undergraduate physics at the University of Papua New Guinea, and the introductory electric circuits course and graduate electromagnetics at the University of Florida.
These are the makings of an expert?
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>
OK, I'm not going to defend his credentials, although the "graduate electromagnetics" part was essentially "graduduate lightning." However, just as I'm dubious of your credentials to trash Dr. Thomson's and the Sea Grant program at the University of Florida, I know I don't have the education or experience in the field to trash your theory. My simplistic understanding is that (1) lightning starts with a charge that goes from the ground up (Douglas's "fizz"), and (2) the resistance of potential paths determines which path it will take.

It seems to me your air gap device must be very carefully calibrated based on many characteristics of the boat, so that it does not either (1) act as an insulator that causes the the charge to seek other routes, such as through a crew member in the cabin, or (2) increase the conductivity from the mast to the sea to an extent that attracts a strike. Have you determined that calibration?

I have no doubt that a person in an open aluminum boat is KFC waiting to happen. A person in a fiberglass sailboat with an aluminum mast is part of a much more complicated calculation. I'm not convinced that the calculation of the potential for the strike is dynamic, as your air gap suggests. It appears to me that the resistance of the potential circuit is what it is, and lightning will select a path based on the resistance as it is. If the air gap is sufficient to keep the mast from being an attraction, is it also sufficient make the least-resistant path pass from the mast to the person in the cabin, and on to the keel? As long as the gap is below the mast, I'd be concerned about that.

I said I wouldn't express my opinion, but I guess I will, with the disclaimer that I am in no way an authority. My working assumption is that the cabintop-stepped mast of the C-25/250, insulated by several layers of fiberglass and about six feet of dry air and wood between it and the water, makes a less efficient path to ground than the air around the outside of the boat, and that the stainless steel stays are poor enough conductors to not change that. Adding conductivity to the mast-to-sea path is, to me, inviting a monster into the cabin--one that is very difficult to control, UNLESS, the path is very direct and very conductive (such as to a copper plate on the hull). So, first of all, I will play the coward--not sailing when the forecast is for thunderstorms, and heading for the dock when the AM radio crackles or the horizon goes black. That leaves the question of what happens at the dock, when I'm not there... For that, I've left the boat ungrounded. My "air gap" is my cabin.

YMMV.

However, I was appreciative of the Trailer Sailor poster who made me aware of the U. FL site several years ago. It's the only source of its kind I've come across, except for books by marine electricians who have no particular credentials in the extraordinary electromagnetic physics we're dealing with, but simply relate what they've heard from who-knows-where.

"But now I've said too much."

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

Edited by - Dave Bristle on 10/12/2003 23:00:12

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deastburn
Captain

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Response Posted - 10/12/2003 : 22:09:57

I expect to own a boat in Florida within the next two years. I will have it grounded, not because I think that will decrease the likelihood of a strike (it may increase it slightly from the research I have read), but because:

1) It will dissipate most of the energy in an orderly way (ie. without bowing a hole in the hull);

2) It reduces insurance premiums and that can pay for the installation all by itself.

Last summer I was fifty feet from a Catalina 34 when it was hit by lightning. It was grounded. Sparks everywhere, lots of noise, no major damage to the boat, but all electronics and electrics fried of course. No way to prevent that. It was a major striked, direct hit to the mast top, sparks flying off the stays and shrouds and lifelines. But the boat suffered not structural damage.

Dave on "Wood Duck" (#2616 - SR, FK)

Edited by - on

Frank Hopper
Past Commodore

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Pitcairn Island
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Response Posted - 10/12/2003 : 22:11:07

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
Dave, I have followed the many threads of this link and have the following comments. I first read this study a couple of years back when it was brought to the forum. That I'm not impressed with it in true.
Arlyn C-250 W/B #224
N/E Texas and Great Lakes
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

So is the upshot of all of this that we lake sailors should go in when it starts lightening relatively close by and you blue water dudes who get caught out should put on your survival suit and sit on your closed cell foam cushion while you do a mental rundown the current state of your karma?

<img src="http://members.cox.net/fhopper/Catalina25/sigbow.jpg" border=0>Frank and Martha in Wichita KS. Lake Cheney

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Dave Bristle
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Djibouti
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Response Posted - 10/12/2003 : 22:26:01

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
I expect to own a boat in Florida within the next two years. I will have it grounded...
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>
I wouldn't argue with that, Dave... I know how thunderstorms are in Florida--if you won't sail when they're predicted, you might as well sell the boat! However, would the strike have hit that C-24 if it weren't grounded, or would it have hit the water 100' away?

BTW, if that 34 were mine and sparks were coming off the stays, I'd replace the standing rigging. Stainless is a poor conductor--thus, the sparks and potential damage to it.

I'll just say that if you ground your boat, have it grounded or inspected by a pro with the appropriate credentials (or alternatvely by Arlyn <img src=icon_smile_wink.gif border=0 align=middle>). Don't invite that monster in if it doesn't have a really good way out!

Fair winds and skies,

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

Edited by - Dave Bristle on 10/12/2003 22:34:15

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Arlyn Stewart
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Response Posted - 10/12/2003 : 23:28:46

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>My simplistic understanding is that (1) lightning starts with a charge that goes from the ground up, and (2) the resistance of potential paths determines which path it will take. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Thats a pretty sound perspective.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>It seems to me your air gap device must be very carefully calibrated based on many characteristics of the boat, so that it does not either (1) act as an insulator that causes the the charge to seek other routes, such as through a crew member in the cabin, or (2) increase the conductivity from the mast to the sea to an extent that attracts a strike. Have you determined that calibration? <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

No... but Its a good guess that a 1/8 inch air gap between two cables that are inline and have the momentum of the straight line are more likely to pass the discharge than me who is going to be hunkered as far from anything metallic as possible.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>As long as the gap is below the mast, I'd be concerned about that. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

The air dielectric gap needs to be low because ground is raised from the water level to the height of the keel side of the dielectric. If the air gap was at the top of the mast via the old barn arrestor... then ground level would appear to be closer to the cloud at that point making the electrical path for lightning closer to the mast than to the surrounding water... this is part of what were trying to avoid.... the other part is the avoidance of single point dispersions which can send up ion feelers that increasingly shorten the path.

Electronics use a variety of MOV devices to discharge high voltage/frequency transients to address lightning and surge damage. One of their design goals is to break down very quickly to facilitate the discharge so that the voltage potential has less rise and therefore less ability to flash all over the place. I don't have any idea if this kind of technology exceeds the air dielectric used many years ago when dealing with major strikes. If its there... use a gorilla MOV instead of an air gap.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>So, first of all, I will play the coward--not sailing when the forecast is for thunderstorms, and heading for the dock when the AM radio crackles or the horizon goes black. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

This is well and fine for the daysailer and to a much more limited extent the coastal cruiser but doesn't work of course for the offshore passage maker.

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>Last summer I was fifty feet from a Catalina 34 when it was hit by lightning. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

It's no secret that given a choice between being grounded and ungrounded, that I will choose ungrounded... mostly because of the stories that are related. I think grounded boats do attract strikes... and if there was a data base of near misses on ungrounded boats compared to hits on both grounded and ungrounded boats, I think the data would be interesting and helpful in proving or dispelling a lot... all we are able to do, is count the stories we hear for each... which leaves us all with a different data base.

In my theory... I'm saying that the controversy between ground or unground is really more appropriately a controversy between unground and having protection. And, if both of those can be had in the same package, the controversy is muted.

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

Edited by - arlyn stewart on 10/12/2003 23:32:55

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Arlyn Stewart
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Response Posted - 10/13/2003 : 08:20:22

Here is a picture to express where I'm coming from

<img src="http://www.stewartfam.net/arlyn/relative grounds.JPG" border=0>

As can be seen, the ground point of the bonded mast is at the top of the mast, perhaps artificially higher depending on possible ion leaders.

However, the relative ground of the unbonded mast is 38 ft below the boat.

This means that the surface beside the boat is a shorter, least resistance path for a lightning strike than going thru the mast and hull on an ungrounded boat.

Whereas, a bonded mast provides a closer ground for a lightning strike than does the surrounding surface.

Arlyn C-250 W/B #224
<img src="http://www.cox-internet.com/arlynstewart/rr4.jpg" border=0>
N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

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ClamBeach
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Response Posted - 10/13/2003 : 10:34:29

Your theory that lightning shouldn't seek an insulated body above the ground plane makes sense, but it assumes that the boat itself hasn't accumulated enough charge to provide the leader spark or isn't offering a ground path due to salt/moisture/rain on the hull etc.
(the saltwater factor again)

Remember that lightning strikes aircraft on a fairly regular basis and they may be thousands of feet from being 'grounded'.

So, I dunno. Flip a coin and take your pick.

a) Perhaps reduce your chances of a strike by being ungrounded...

b) or take action to reduce/eliminate your damage if you are hit by grounding the mast.

Lightning is fairly rare here, but when I go to Mexico (someday), I think I'll go with plan 'b' and take a grounding jumper made from welding lead and a coppper ground plate to hang over the side.

Would be nice to see some 'hard' statistics on strike ratios between grounded/ungrounded masts. So many variables.

Currently maintaining two holes in the water...'77 Venture 23 and new to the family,
'78 Catalina 25

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Dave Bristle
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Response Posted - 10/13/2003 : 19:44:31

Arlyn: I'm interested in that 38' depth figure... What does that represent, and is it different (i.e. substantially shallower) for salt water than for fresh?

Clam: (I'm sorry... Bruce:) From what I've read, there are a couple of issues about jumpers over the side... First, if they're attached to the shrouds, they have little effect--the shrouds are very poor conductors compared to the mast. Second, if you attach one to the mast, the path is far from straight to the water--the conductor may carry a fair amount of the charge, but much of it is likely to find another route from the mast. You could be providing an adequate conductor for a "leader" but an inadequate one for the strike that follows it. Protection systems are generally rigged so the connection from the mast to the external conductor (copper plate or whatever) is a straight and parallel to the mast as possible. Again, if you "invite the monster in", you want to give it the best possible way out. Otherwise, you're better off letting it hit the water nearby. We only want to keep you around!

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

Edited by - Dave Bristle on 10/13/2003 19:51:56

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Arlyn Stewart
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Response Posted - 10/13/2003 : 21:33:10

Here is where the math comes from... and remember... these numbers are very arbitrary.

The dielectric base number of air is 1.0 and polystyrene is 2.6. I reduced the dielectric of air by 2/3 because of the rain saturation.

Taking the 6 feet of hull height, again a very arbitrary number, then 6 x 2.6 = 15.6

Now to adjust to the adjusted air number... I mulitplied by three, 15.6 x 3 = 46.8

30 feet worth of mast subtracted from that because the mast should present almost a dead short compared to the dielectric of moist area... 30 x 0.3 = 9 , now 46.8 - 9 = 37.8 I rounded to 38.

The arbitrary numbers are only to illustrate the point. I find it interesting that in Don Casey's recent article on Sailnet that he discounts the logic that an ungrounded mast can be a deterrent to getting a strike because the mast has a lesser resistance than that of equivelent air. His logic is however flawed because he is not considering the dielectric of the totality of the boat... mast + hull.

One thing that I should point out... in presenting this theory I'm not discounting what I believe to be the primary cause of boats being hit. That is an ion leader shooting skyward from a single point dispersion at the masthead. If I had a grounded mast... there is no way I would have a Mets antenna or a windex as my upper mast protrusion. If I had a grounded mast... it would have an ion dissipating device. I believe that the ion feelers do present a significant invitation to strike.

A metz antenna has a loading coil resulting in a grounded antenna. It is an ideal single point dispersion for ion feelers to burn upward and in so doing greatly carry ground potential closer to the cloud as an ion trail has very little resistance.

This distance then equated to a ground potential that is less than the water surface is my argument. An ungrounded mast, doesn't present ion leaders which originate from ground so a Metz or Windex on top isn't an issue.

Clam, the theory presented is not an attempt to discount the ion leaders or to over simplify a very complicated subject. Its just that I was trying to focus on the dielectric issue of the mast and hull.

Also, let me say that the dielectric theory... may be insignificant... who knows. My main reason for thinking it applies is that most lightning fatalities on boats happen on small open boats. Boats which may be alluminum or the fisherman having feet on a wet sole. Boats where the victim is close to ground potential rather than having a fair amount of hull form between them and the water.

Arlyn C-250 W/B #224
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N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

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deastburn
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Response Posted - 10/14/2003 : 12:44:47

Yes, anecdotal evidence seems to suggest that grounded boats are struck more often than ungrounded boats. I mentioned the C34 that was struck by lightning before my very eyes. I did not mention the Nonesuch 30 that was struck in the outer harbor on a mooring (I did not witness it) and partially sank (copper thru-hulls blown out). Point is, grounded you may be more likely to get hit, result fried electrics, electronics and maybe rigging to replace); ungrounded, if you are hit, you may be shopping for a new boat, or the Admiral may be shopping for a new hubby.

Dave on "Wood Duck" (#2616 - SR, FK)

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Arlyn Stewart
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Response Posted - 10/16/2003 : 08:30:07

Most of my reading the past few days has been trying to gain more knowledge about lightning strikes.

It occured to me that that we do have some data that can be of interest perhaps. One of those is that the largest category of locations in databases is the one listed as unknown location... however, most statistics give the location of the strike, and open areas are the largest identified areas to get hits. Also, very high up on the list is farmers on tractors.

Open fields, ballparks, playgrounds, etc.
This is the next largest category in every region and for the US (Table 34). The same Storm Data categories were used by Ferrett and Ojala (1992) for Michigan to find that ball parks and playgrounds were the most frequent locations of lightning victims from 1959 to 1987.

Why do people get hit in open areas? Is it because they present themselves as the tallest ground object? How much more would be a grounded aluminum mast?

Then I wondered, why are farmers on tractors at high risk? Cars are not a high risk to strikes and there are far more cars than tractors. Could it be that cars are isolated by the tires and therefore have a higher dielectric value... where the tractor with its plow, or field implement working the ground is not isolated?

At any rate, I continue to struggle with the notion that grounding the mast does not increase the hit percentages.

Also interesting ares the Boat US insurance statistics...

What are the Chances of
Lightning Striking Your Boat?
The following statistics are based on all of the BoatUS Marine Insurance claims for lightning damage over a five-year period. The percentages suggest the chances of the various types of boats being struck in any given year.
Auxiliary Sail .6% Six out of 1000
Multi-hull sail .5% Five out of 1000
Trawlers .3% Three out of 1000
Sail Only .2% Two out of 1000
Cruisers .1% One out of 1000
Runabouts .02% Two out of 10,000
Source: BoatUS Marine Insurance Claim Files

Those stats suggest that sail only boats which would not have a prop shaft or outboard in the water to provide a possible ground for the mast, are 3 times less likely to be hit as sailboat with an auxiliary.

Also, runabouts that don't have a mast sticking up are 30 times less likely than a sailboat with auxiliary.

Yet... we have Dr Thomson and those who echo his perspective, that say that grounding the mast doesn't increase the percentages of hits.

Again, I'm not advocating not having a protections system on a boat. My question is more, is there a proctection system that doesn't include grounding the mast and inviting the strike. And, it seems to me that there might be.

Arlyn C-250 W/B #224
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N/E Texas and Great Lakes

[url="http://www.stewartfam.net/arlyn/"]Arlyn's C250 Mods n Cruising Stories[/url]

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