Hull Electrode for Lightening

Discussion in 'Multihulls' started by Owly, Mar 9, 2017.

  1. Owly
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    Owly Junior Member

    I'm planning to convert a Searunner 31 to a free standing aluminum mast and a split cambered junk rig. The mast will be located forward of it's current position a few feet, just inside the dressing room, and offset to port about 10" to avoid interfering with the access into the dressing room and head. It will rest right against the main strength bulkhead at the forward end of the centerboard trunk. Needless to say the hull will be reinforced as needed for a suitable step, though the vertical load is a fraction of that of a stayed mast.
    An unstayed aluminum mast is for all intents and purposes a natural lightening rod. The hull is of course extremely vulnerable in a lightening strike. It's going to blow a hole right through the bottom of the boat as it looks for the most direct path to ground.

    logically the hull should be cut away, and a stainless steel plate mounted flush with the outer layer of plywood to serve as an electrode, the mast step also being of stainless steel. I'm imagining a piece of heavy wall stainless steel tubing passing right through the hull, welded to the inside of the stainless plate electrode to carry current that travels down the mast into the electrode. That tubing would best extend upward around the mast, forming a socket, and the load distribution portion of the mast step would be attached to fixtures welded to it. Basically bolting through the main bulkhead with suitable internal structure at the bolting locations would be about all the structure the step would need.

    There are countless reasons people will give that the junk rig conversion is a bad idea or inappropriate for the trimaran....... I've already heard them all........ If I was going to be talked out of this project, it would have happened some time ago. A lot of thought was given to mast location, and while it is not ideal.....not far enough forward, it is one of only two that make sense......... port or starboard, same place. I don't feel that the offset will present any real problems in this case.

    The real challenge is compensating for the center of effort being farther aft, but how much effect this has on the helm in real life will become evident only after the rig is on the boat. I have some interesting solutions in mind that are also fairly simple, though an oversized balanced rudder with an end plate is a given.

    H.W.
     
  2. Sparky568
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    Sparky568 Junior Member

    Other than steel or aluminum hull vessels, no boat I know of is completely immune from a lightning strike damage. It is the concentrated energy that wreaks havoc in FRP hulls. Your idea may have some merit by creating direct path below the mast providing the plate you install is heavy and has enough surface area to disperse the energy.

    However the unintended consequence may be that it will actually attract a strike. As such you may not be immune from damage or fire as the heat has the potential to affect components in direct contact with the mast.

    Lightning is a strange force, and it does and goes where IT chooses. Lightning has been documented traveling sideways (parallel to the ground) as well as hitting trees and jumping to houses half way down.
     
  3. gonzo
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    gonzo Senior Member

    I have surveyed several boats that were hit by lightning. The rigging did not carry most of the current, and often it didn't follow the path to the grounding plate. I saw one, with a grounding plate, where the burnt path zigzagged through the interior of the hull, passed under the fuel tank and exited through the rudder.
     
  4. Owly
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    Owly Junior Member

    I've seen photos............. Of course there will be no standing rigging on this boat. On any boat with an aluminum mast, the mast will be the conductor of least resistance. The rigging offers a convoluted path with all the hardware, the mast a straight extremely low resistance path, but normally it terminates at the coach roof where supporting structure beneath it may be a metal or wood column that may or may not be directly electrically connected. It's little wonder that lightening follows convoluted paths in this case. A direct low resistance path right to the water just makes sense. The socket made of heavy wall aluminum tubing will fit tightly around the mast base, and be welded to the plate on the outside of the hull. Assuming lightening wants to follow the outside surface from everything I've read... which doesn't make a lot of sense as it is DC, it makes sense that it should encounter this tubing leading it directly to the plate. The idea is that the outer 1/4" of the hull which is several layers will be removed by whatever means works, and the plate shaped and bonded into the hull with a suitable adhesive. It would be 1/4" material, and probably about a square foot of area. I'd rather not have metal fasteners penetrating the hull.

    Resistance generates heat, and also encourages lightening to blow or burn holes in things. Stainless is not an ideal conductor, but I would hope it would be sufficient.

    The thought of "drawing lightening" has occurred to me, but I'm not sure it's a real world concern. An aluminum mast with 1/2" of epoxy ply separating it from the ocean seems to me like a formula for blowing a hole right through the bottom of the boat when you have no shrouds or stays. I'd rather "dodge the bullet" than try to stop it.

    I believe in "insurance". Not the kind you pay monthly premiums for, but the other kind. The kind that involves planning and precautions.

    H.W.
     
  5. Sparky568
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    Sparky568 Junior Member

  6. rasorinc
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    rasorinc Senior Member

    I know zero about sail boats but if you are going to be operating in known lighting
    areas why attract it to you? Could you use a wood or carbon fiber mast?
     
  7. Owly
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    Owly Junior Member

    An interesting device....... I'd never heard of one before.......

    With a car or airplane, one might avoid thunderstorms..... A sailboat voyaging in the open ocean is a different matter....... I can just imagine trying to dodge afternoon thunderstorms in some areas............ The ITCZ for example where there often is no wind except for the storms themselves.

    H.W.
     
  8. Owly
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    Owly Junior Member

    I'm not at all sure you are going to "attract it".... but I do know that if it strikes, I want it to have a path of low resistance to ground..............

    H.W.
     
  9. CDK
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    CDK retired engineer

    From post #4:
    " Assuming lightening wants to follow the outside surface from everything I've read... which doesn't make a lot of sense as it is DC, it makes sense that it should encounter this tubing leading it directly to the plate."

    Lightning certainly isn't DC. It has a waveform that closely resembles a saw tooth, an extremely steep flank followed by decay, repeated several times in rapid succession until the charge has depleted and the arc can no longer sustain. During the discharge many branches of ionized air are formed which may be used by nearby charges as well.
    The fact that a Faraday cage offers protection proves lightning always follows the outside surface of conducting objects (skin effect).
     
  10. Sparky568
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    Sparky568 Junior Member

    Cars and planes are actually better off in a strike (faraday cage) and I'm not sure I would choose carbon either.

    https://www.youtube.com/watch?v=10PZ_0GK_bU


    I have worked on grounding systems for lightning protection in large commercial and industrial buildings as well as a weapons building for a military installation. The goal there is to provide multiple paths to disperse the energy safely to ground. Four point connection to steel structure (again faraday cage) is minimum. Then ground ring around building perimeter, sometimes two depending on soil conditions. Then connected to the base of each column.

    Not saying what your trying to accomplish won't work but I won't be volunteering to test it. Avoiding the situation is preferable.
     
  11. Owly
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    Owly Junior Member

    Avoidance is of course preferable.........but how do you avoid thunderstorms when at sea? I'd much rather have a defined low resistance path to ground than to try not to provide a path in hopes that Thor won't notice my mast sticking up.......... the highest thing for hundreds of miles..... I also don't believe attempting to have a non conducting mast is realistic. In a storm, everything is going to be wet. Other than the outboard motor, anchor and chain, there will be almost no significant conductive masses except the mast.

    H.W.


     
  12. Ike
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    Ike Senior Member

    Don't waste your money. just go buy an old AM broadcast radio. AM has always been plagued by lightning noise.

    Lighting is not "attracted". Lightning takes the path of least resistance. If your mast is the path of least resistance and you are close to the cell, well, that's the way it's going to go. Most lightning protection schemes are based on that simple principle. Provide a least resistance path to ground.

    Yes lightning does tend to travel on the surface of a conductor. It's a lot like microwaves which do the same, travel on the surface of a conductor. That's why most microwave systems use tubes (square tubes) rather than cables to transmit microwaves.
     
  13. David Cooper
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    David Cooper Junior Member

    I've heard that the electrons in a wire flow through the edge of the wire rather than through the middle of it because they repel each other as far apart as possible, and a Faraday cage works for the same reason, it being hard to push electrons further inside. I've also heard of people surviving lightning strikes if they're wet, the current flowing over their skin instead of diving deep inside. That leads me to think that the current is more likely to travel down the stays than the mast unless the mast has better connections onward into the sea, but more importantly, when the electricity reaches the hull, it should be possible to keep it on the outside if you have a conductive coating on it. Conductive paint is available (metallic paint may work, but it certainly doesn't with low voltages, so you would be relying on lightning jumping billions of gaps), and although it may be expensive (e.g. https://www.bareconductive.com/shop/electric-paint-50ml/ - £15 for a twentieth of a litre, though I'm sure it could be a lot less for large quantities), you wouldn't need to paint the entire hull with it but could make do with a grid. Of course, the paint I've linked to may not be strong enough to stay on the hull for any length of time unless it's painted in very thin lines and covered over with a more durable layer of paint. It would be worth experimenting with that, if someone has access to the right facilities to produce simulated lightning. The same applies to clothing - I'm sure a grid of metal fibres would save lives, but no one seems to have any interest in testing that idea.
     
  14. Owly
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    Owly Junior Member

    I've been within a few feet of lightening strikes several times. I knew two men, a father and son, who were killed along with the horses they were riding side by side in Wyoming many years ago, so testing the metal fabric lightening clothing does not give me a "warm and fuzzy" feeling.

    The boat in question is a junk rig....... That means .. no standing rigging at all... Other rigging all leads to the base of the mast, except for the sheet. The mast is fairly heavy wall aluminum tubing with a taper. The mast will be well forward in the boat, though that should not make any difference as far as damage to electronics, etc. It will pass right next to one of the sea berths, so the better part of wisdom would be to sleep far aft during thunder storms. There will be no inboard engine or prop shaft, or any other logical pathways to ground.

    Unfortunately "avoidance" is only an option if you are in port, and as this boat is intended for circumnavigation type voyaging, I expect to face many thunderstorms in places like the ITCZ, where one is likely to be becalmed..... as if that makes any difference. Running away or otherwise avoiding is really not a realistic option. A "lightening locker", a compartment rigged as a farady cage, perhaps with radios permanently mounted within it, and a closeable metal cage cover, and room to stash computers, chart plotters, GPS, Ipads, satellite com devices, and epirbs might be a very good design feature. There are actual lightening detection devices with audio alarms which detect lightening within 25 miles supposedly, at prices under $50. One or more of these around the boat could be lifesavers, as well as equipment savers. On passage, there is little reason to be constantly looking at instruments, in fact I consider many modern yachts with arrays of instruments to rival airliners more than a bit absurd. I'm there to sail, not stare at electronics! No reason to look at a chart plotter for example except a few times a day.

    I'm of the opinion that the direct path downward provided by the aluminum mast is probably where the lightening energy will go....... if it has a good ground plain.


    H.W.
     

  15. Ike
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    Ike Senior Member

    Actually that's a good idea. In fact a Microwave oven can be used as a temporary faraday cage, but it doesn't hold much. Back in the 80's when I was running a USCG industrial facility, the electronics shop had a room that was a very large faraday cage. All of the sensitive test equipment was kept in there. All equipment calibrations were done in there. The cage eliminated any outside interference. So having a small locker lined with copper mesh would probably protect your electronics. (of course the copper would probably corrode like mad at sea)
     
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