Lightning Protection

Discussion in 'Boat Design' started by DavidG, Jul 20, 2006.

  1. DavidG
    Joined: Jan 2002
    Posts: 52
    Likes: 1, Points: 0, Legacy Rep: 60
    Location: Chichester, England

    DavidG Junior Member

    I am working for a client who intends to keep his yacht in Chesapeake. There are numerous papers written on Lightning Protection and no two experts seem to agree. The paper that makes most sence to me is by Nigel Calder and was published in Professional Boatbuilder.

    Firstly, Does anyone have a view as to whether it is better to ground to the keel (or in this case an iron ballast shoe) or to a sintered grounding plate? The lightning guys seem to think that the keel is best but the corrosion guys say don't bond to the keel. I have heard plenty of reports that the sintered plate can boil and blow through the hull during a lightning strike.

    I also wonder whether by cathodically protecting the keel and fastenings, lightning might disipate through the bonded hull anode with the same results as the sintered grounding plate.

    Secondly, The builder has fitted a lightning disipator (dish mop type). The VHF antenae is above the level of the disipator and a Glomex TV antenae is on a bracket outside the "cone of protection", this doesn't look good to me, any views of alternative arrangements? The yacht is sloop rigged.

    DavidG
     
  2. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 116, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    It's a mystery. I have never seen a consensus agreement on the best ways to achieve lightning protection. Calder's advice is probably as good as any. The details of how to implement this on an individual boat is never very clear.

    I am pretty certain of four things.

    1. The "cone of protection" is a statistically arrived at phenomenon and not to be trusted too far.

    2. Scintered copper plates are a marketing ploy that is based on wrongheaded science and, as you said, can be worse than a solid plate..

    3. Ditto for the bristle brushes on the mast. They don't harm anything but are probably no better than a single wire.

    4. Contrary to popular belief, lightning must be treated as very high frequency AC and not DC current even though the current may only flow one way. This is why sharp bends in any bonding wires must be avoided to keep inductive reactance to a minimum. Inductance will be resistance to the current flow and will make the lightning current want to go somewhere else. Same goes for running bonding wires near the hull interior below the waterline. Capacitance between a wire running along the inside and outside water is like a low resistance path through the hull to the water. Holes blown through hulls in this way have sunk many boats.

    I am convinced that a good protection scheme is better than not having one although don't always know how to achieve that "good" scheme.
     
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