Metal grounding plate for steel hull

Discussion in 'Metal Boat Building' started by singleprop, Oct 28, 2010.

  1. singleprop
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    singleprop Junior Member

    Hi Guys,

    from another discussion about lightning strikes on steel boats the argument was made that the zinks would provide adequate grounding in case of a lightning strike.

    This contradicts the arguments from NA's and other specialists on this area that recommend dedicated grounding plates as well as zinks.

    I am somewhere in between the 2 arguements but will probably error on the safe side and weld-in some strategically positioned metal plates (underwater and next to each mast stay for example).

    The question is what kind of material would be best for this when it has to be welded onto the hull and be exposed directly to the salt water???
     
    Last edited: Oct 28, 2010
  2. Man Overboard
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    Man Overboard Tom Fugate

    I don't know if you know about this site but I thought I would mention it.

    You can find a tremendous amount of information about lightning strikes on boats at marinelighting.com http://www.marinelightning.com/index.html. There are several pages of discussion including articles and testing procedures.

    This link will provide you with information on grounding strips which may be of interest to you: http://www.marinelightning.com/GStrip.htm

    When designing a lightning protection system, there are two very important concepts to understand. First and foremost current from a lightning strike seeks to discharge at the water surface, not necessarily into the water. so for instance a grounding plate on the keel may actually be insufficient for protecting against a lightning strike. This is not always the case but it is lightnings preferred route. you may be thinking 'lightning travels the path of least resistance'; not necessarily so, electromagnetic current is still not completely understood, which brings me to the second most important thing to understand. 2) Lighting does not need a metal conductor to discharge to the water surface. It can for instance travel a short distance on a metal pipe and then arc through a fiberglass hull. (completely ignoring the conductive path you have bonded to the keel grounding plate, remember current prefers to discharge at or near the water surface.) Something to consider when placing your grounding plates. (does a metal boat really need grounding plates?)

    you can find more information in the article Lightning And Sailboats http://www.marinelightning.com/ECE/SGEB17.html
     
  3. singleprop
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    singleprop Junior Member

    Yep, I know that page and the theories involved...

    The mentioned zink strips can only be boltet onto the hull,.....I am looking into the best material to weld onto the hull, underwater.

    Yes, according to some NA's the grounding plates are recommended - even on steel hulls.

    As far as I understand the zinks provide good grounding "in general".

    But, in case of a ligthning strike, the above water dischargers and the strategically positioned underwater grounding plates (for example located at the mast stays) will ensure that the strike is guided (as much as one can guide a lightning strike) the most direct path......thereby preventing side flashes inside the hull etc etc (without any guarantie that it works due to the nature of lightning strikes)...
     
  4. CDK
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    CDK retired engineer

    On a steel hull the zinc anodes are all you need.
    Adding grounding plates to the hull will cause rapid wear of the anodes and/or the need for more of these.

    In case of lighting strikes a good contact between the mast and hull can make the difference between just a damaged top light and a complete catastrophe.
    Electrical discharged follow an unpredictable path when ionized air is the conductor. Where metal parts of sufficient diameter are present, the voltage differential is lowered to a value where ionization cannot occur.

    After a real hit, there will be some paint damage on bottom and keel.
     
  5. singleprop
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    singleprop Junior Member

    HI CDK.

    "Adding grounding plates to the hull will cause rapid wear of the anodes and/or the need for more of these"

    I asked Michael Kasten about the corrosion issue that you mentioned. He says this is not the case as long as the two metals are close on the corroison scale..

    Also, by reading his comments in his articles it is clear that the zincs are NOT sufficient since they do not have the required edge length (which increase the dissipation).

    And, when the zincs are new they are very good for grounding (as well as corrosion protection) but when they are old and half the size.....thats when the grounding plates/grounding strips are really needed.
     
  6. Ike
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    Ike Senior Member

    You have just encountered the Twilight Zone. This is the are aof boatbuilding where very few have any more than experience and what failed to fall back on. The website above is Dr. Thompsons http://www.marinelightning.com/index.html website and he is probably the number one expert on lightning protection for boats, but even in seminars he says that a lot more research needs to be done. For many years he was the head of research on lightning at the University Of Florida Lightning research center http://www.lightning.ece.ufl.edu/. So I would say, his advice is probably the best there is.

    But as I said, this is a gray area and many people have their own theories. Mine is simple. If you can, get off the water. if you can't don't touh anything metal, turn off all electrical and electronic equipment (not the engine if you need it). Stay as low in the boat as possible and wait for the storm to pass. If you believe, pray. Actually on a percapita basis very few boats are struck by lightning every year. However, when it happens usually everything electrical fries.

    lightning is not "attracted" It occurs where the path for discharge is easiest whether that be between clouds, earth and sky. If you happen to be in the path it keeps right on going. Actually most lightning discharges are between clouds and never reach the ground. But the ones that concern us do. So you have the right idea. provide the easiest (least resistance, and straightest path to ground. I am not going to enter the debate about whether a metal boat shold have a ground plate. Try the Metal Boat Society. http://www.metalboatsociety.org/. They would certainly have some insight on this. Of course a lot of the members of MBS are also here on Boatdesign.net
     
  7. CDK
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    CDK retired engineer

    What sort of material did you have in mind for the ground plates?
    Iron, steel, stainless and more noble metals will all speed up the sacrifice of zinc; aluminum, magnesium or zinc itself will not, but it will also be a sacrificial anode.

    And Michael Kasten is wrong about the edge length. Even if your paint coat would be a perfect isolator (which it isn't), the steel hull would be one side of a capacitor, the water being the other side.

    Electrical discharges are extremely fast events, where the capacitance alone protects you from the dangers of a nearby thunderstorm. But there is also a lot of current leakage through the paint and low impedance from anything unpainted like a prop, its shaft etc., so the whole behaves as a capacitor with a parallel resistor. I consider that a very safe place to ride out a thunderstorm.

    In the unlikely event of a full hit, only a weak link between mast and hull could confront you with fireworks in the cabin.
     
  8. michael pierzga
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    michael pierzga Senior Member

    Ive never seen a grounding plate on a metal boat. I believe since the whole hull is common and naturally bonded together , with multiple exit points like the anode field, shafts and all the wet internal metal, that it is not necessary. This yacht is aluminum, no dedicated ground plate and ive received several direct lighting blasts, with the only damage being vaporized masthead VHF antena.. . One of the blasts happened in the harbour and was so dramatic that the port police sped down to the yacht, in the driving rain ,to see if anyone was killed...no damage other than a missing VHF antenna and some blown fuses. The police told me that the yacht literally glowed from the strike. If its a metal sailing yacht ,Good idea to make sure that your mast step is grounded, many times an insulating layer is between the mast and its step to stop corrosion or aid in adjustment. Its assumed that lighting will travel down the rigging, ,or down the spar itself, into the hull and then into the sea..if the charge cant escape it could arc across ,then seek ground.
     
  9. apex1

    apex1 Guest

    You live in a dangerous area Michael, when you have already encountered "several direct lighting blasts". Even in Florida that is not common.

    And then, after a direct blast you have had just a damaged antenna? But a "glowing" boat?

    Some lower level of dramaturgy and a higher level of truth would probaly make your wild stories a bit better. Not that there was a demand anyway.

    A direct flash, freezes almost always every single electronic item on board, and the **** out of your @ss on top of that.

    Regards
    Richard
     
  10. Stumble
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    Stumble Senior Member

    In over 25 years of boating I have seen exacally one boat that I believe was actually struck by lightning, a number of boats that lived through possible near hits, lots of electrical spikes, and dozens of owners that think their boat was hit by lightning. In fact until I saw what lightning actually does during a direct strike I thought some of the others were hit as well.

    So what sets apart the boat that was hit from the others?

    1) Every piece of electronics on the boat was fried. From the chart plotter down to the computer control for the generator. Every piece of wiring on the boat was clearly damaged, and we couldn't find a singleelectrical system that was functional.

    2) There was a burned section of the mast where the aluminium had bubbled, and turned black.

    3) The chain plate tangs on the starbord side had crystalized the glass within about 2 sq feet. When you stuck an awl into what was left of the bulkhead it sounded like a broken window.

    4) The engine raw water intake had litterly blown out of the boat. There were a few pieces of it still attached, but the mushroom had completely seperated from nut. (This almost caused the boat to sink before being lifted)

    5) The paint over the entire keel turned to powder, and while the rest looked ok for its age, started to fall off in sheets.

    6) The interior had a pervasive smell of the dreaded blue smoke from electronics shorting out

    7) The back of the main circuit board looked like someone had taken a blow torch to it. It didn't seem to matter if the switches were on or not. The electrical path seemed to jump and cook everything when it hit.

    8) 2 Batteries that were cracked and dry, 2 more that were just melted.

    9) Did I mention every piece of electronics on the boat was fried? From the microwave to the chartplotter, the computer (on a surge protector) to the AC.

    10) Every antenna on the boat (GPS, VHFx3, SSB, Iridium, TV) were either melted, or dropping pieces onto the deck.



    Now this was on a big heavy cruising sailboat that had the latest in lightning protection. Everything bonded, with a 50' grounding strip bonded to the hull. Personally I don't think there is anything on gods green earth that would have mattered. If you get hit by a strike like this, the boat is toast, and the only option is a complete refit. In this case the owners sold the boat to the insurance company for the policy limits and the rest was sold as scrap metal.
     
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  11. CDK
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    CDK retired engineer

    What he described probably was St. Elmis fire during a nearby thunderstorm. To the spectators this can look really scary, but because there is no actual discharge, the currents involved are low and damage is minimal.

    When my neighbor's house was hit a couple of years ago, several bricks vaporized, leaving a ragged hole in the wall where the fuse box had been. The discharge killed everything electrical inside the house, with damage to a lesser extent (TV-sets, computers etc.) in surrounding houses within a 300 ft radius.
     
  12. Lurvio
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    Lurvio Mad scientist

    This is not related to boats, but might add a bit to the discussion. In my time, I've seen multiple trees that have taken direct lightning strike. The most common damage is a 1-3 inches wide channels where the bark has blown of the stem, top to bottom. But then there are more serious hits, about ten years back I was walking in the forrest about a kilometer from home (adult spruce trees at a bottom of a hill side) where came upon a very weird site. There was wood pieces stuck to the ground everywhere, probably 100 meters diameter area. The pieces were up to two meters long and 3 inches wide. A lightning had struck this adult (skandinavian scale - about half a meter dia., 20 some meters tall) spruce. From the tree itself remained a needle like stump maybe 4 meters high.

    What I'm saying that not two lightning strikes are the same. The strike that had blown up that tree, could very well make the kind of damage Stumble mentioned.

    Just my 2c, Lurvio
     
  13. apex1

    apex1 Guest

    That would be a plausible explanation for you and me Cornelis. But this guy does not accept plausible arguments.
    He is just a bigmouth with a pc.
    That goes on and on since he signed up.
    Have some nice laughs on the "motorsailor" thread, where he spread his "knowledge" for free!
    http://www.boatdesign.net/forums/mo...ood-anything-just-motorboat-sail-35204-6.html

    A loudmouth
     
  14. michael pierzga
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    michael pierzga Senior Member

    Well CDK, Ive never actually seen the St Elmo fire phenomenon . I trust that it exists. I have seen lightning behave in strange ways. When I was a kid I worked in a shipyard. A yacht in the shipyard, a wooden classic sailing yacht , was struck by lightning and heavily damaged while at the dock. The strange thing about the strike was that is was a smaller boat...not nearly the tallest mast in the shipyard, yet the lightning choose to ground itself on this rather low target ? Hmmm... Unpredictable stuff. So unpredictable that when sailing, I'm careful to use the radar to spot "cells" at 24 or more mile distance, simply so that I may alter course early and stand clear of any potential thunderstorm trouble. .
     

  15. Ike
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    Ike Senior Member

    I have seen St Elmos fire on several occasions. ir's weird and eerie and very recognizable. I have also been out in thunderstorms and fortunately never been hit, but had one near miss. St Elmos fire looks nothing like lightning. By the way even on a wooden boat touch nothing metal when lightning is around. You'll get the shock of your life.
     
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