Lightning Protection - Wood mast

Discussion in 'Boatbuilding' started by M&M Ovenden, Jul 31, 2016.

  1. M&M Ovenden
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    M&M Ovenden Senior Member

    Hi,

    Any experience running lightning conductors in a wood mast ? We will have a gaff rig, so no sail track.

    We will probably laminate a mast up, so it would be possible to run a conductor in the center, but I'm somewhat worried about heating and having the mast "explode" if hit.

    I'm considering having a conductor laminated in above the hounds, and connect it to one of the shrouds, but most standards say a "continuous" conductor.

    Thoughts ? Comments ? Suggestions ?

    Cheers,
    Mark
     
  2. PAR
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    PAR Yacht Designer/Builder

    The mast will not explode. Areas of concern are; connections and hard angle changes in direction and where it passes through things, like decks, bulkheads, etc. This are places the arc can jump, which will act much like an explosion. The idea is a continuous, as straight as practical run from masthead to ground plate. This directs the path straight to earth, which is what you want.

    FWIW, a gaffer without a track isn't very likely to get struck, but if you must, you can cut a rabbet (most common) and bury the conductor or put it inside the mast.
     
  3. CDK
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    CDK retired engineer

    Wood is a poor conductor that doesn't attract lightning. Any kind of metal conductor will make it a target, not just for full hits but also for induction when lightning is near.
     
  4. mydauphin
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    mydauphin Senior Member

    True but I have many a bust wood mast from lighting, may be it is all stuff people put on them like antennas, stays, lights.
    I would either put nothing at all on mast which defeats it's purpose, or a big welders cable from top to the ground on hull. But lighting though rare can be devastating. I saw a boat on the hard with a hole on its side, some one painted over the ground and voltage decide to remove it.
     
  5. M&M Ovenden
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    M&M Ovenden Senior Member

    Hi,

    I'd like to bump this thread again and see if there are any opinions considering the use of synthetic shroud material. This would mean no real conductive material overhead for protection.

    Thoughts, comments ?

    Cheers,
    Mark
     
  6. tom28571
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    tom28571 Senior Member

    It appears that lightning does pretty much what it wants to based on physics of the particular situation, which is seldom understood well enough for humans to predict. Wood masts can certainly be struck by lightning just as trees are commonly hit. If the mast is reasonable dry an interior or exterior conductor will likely reduce the damage of a strike.

    Paul is right about avoiding sharp or even small radius turns in the conductor but it might help to understand why. A turn or bend in a conductor acts like an inductance which acts like resistance to high frequency current. While a lightning strike may seem to be a DC current, it will have an extremely sharp leading edge followed by a slower decay which contains all frequencies. That is why a strike is always picked up by radio receivers. A poorly made splice or connector will be a point of resistance and may heat up and explode if subjected to a large current.

    If the bend or arc in the conductor occurs near anything that is grounded the resistance of the bend and the capacitance between the conductor and ground will allow an arc to ground through the low resistance of the capacitance. Consequently, running the conductor along the inside of a hull below the waterline is a common invitation to holes in the hull from a strike.

    As an engineer often working with systems exposed to lightning, I found that it was a difficult taskmaster and predicting accurately the result of protective efforts was sometimes a sobering realization of our imperfect knowledge.
     
  7. SamSam
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    SamSam Senior Member

    I've read in a number of places to only use braided wire, but this is the only explanation I've seen for why it's braided. Also, it's never covered in an insulator like welding cable is, and it's not recommended to even paint it. I think the braiding also helps keep the wires all together, since they aren't covered and held together with anything.

    Lightening protection seems a bit like black art or voodoo, so it might be a good thing to google 'lightening protection for boats' .

    Also, scroll down to the bottom of this page for 'similar threads', there have been a number of discussions.

    (23) Technical facts: Lightning Travels on the surface of the Lightning Rods and cable. The many strands in the specially designed braided cable adds greatly to it’s surface area. The BRAIDING in the Lightning Cable is very important as it has the effect of cancelling the Electro Magnetic Pulse (EMP). As current travels down each braided strand of wire and since the strands “cris-cross” each other, and “like charges repel”, The braided cable has the effect of cancelling the EMP. Compared to regular electrical wire, the EMP around regular wire can couple with other wires under roof and behind walls and can transfer very high voltages into the electrical wiring. EXCEPTION: On structural steel buildings and below ground, un-stranded cable of sufficient size is ok since the EMP will be absorbed by the steel in the structure and in the earth.

    http://www.lightningrodsupply.com/index_files/page0008.htm





    [​IMG]
     
  8. tom28571
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    tom28571 Senior Member

    Braided wire does may have some advantages at higher frequencies but I'd need to see some objective testing to know whether the overall effect is positive or negative for lightning protection. Higher frequencies do tend to run on the surface of a wire but all those loops do represent a lot of inductive resistance to high frequencies. I've not heard of using braided wire in lightning protection. I would personally prefer flat straps as probably having the most effective surface area per unit of conductor and the least inductance when run straight. Lightning is dark magic so I may be wrong.

    The reason paired communication wire is twisted is that each twist represents a "loop" that generates a field in opposite direction to its neighbor and the induced currents tend to cancel each other. The effect is the same whether transmitting or receiving.
     
  9. M&M Ovenden
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    M&M Ovenden Senior Member

    Hi,

    Any thoughts on running a large lightning conductor inside a wood mast? I'm wondering if anybody else has done this.

    thanks !
    Mark
     
  10. PAR
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    PAR Yacht Designer/Builder

    Inside the mast is just asking for a significant fire potential.
     

  11. M&M Ovenden
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    M&M Ovenden Senior Member

    yea, fire is my worry. In theory we have an ok idea of the lightning waveform, and could size accordingly, but the unknowns of terminals/connections means anything could happen.

    I'm considering running a 2/0 cable (much bigger than the 4awg normally spec'd) to solid copper bar to exit the mast at the top/bottom for other connects (drill/tap it for taking a lug). This would produce a couple sharp bends.

    It seems that if a wood mast is hit, it's not a good situation, so maybe doing this offers better protection at least for the crew on deck.

    I'm going to be laminating the mast in about a month, so I have time to stress about this a bit. Moving from wire rope to synthetic standing rigging changes the game.

    Cheers,
    Mark
     
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