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Old 14-12-2019, 08:56   #1
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Lightning grounding question

Quick scan of boats at the shipyard uncovered that many do not seem to have any external ground plates, with those that have them, use the dynaplate approach. On my boat, a ketch, I installed a 8' by 1" bar of copper running outside alongside the keel, with both mizzen and main connecting to the copper bar at different locations.*

My questions are:
1. Am I crazy, and should I revert to a cooper square (like a dynaplate but solid) solution?*
2. If using a* copper square, can I install 2 such squares, one for the mizzen and one for the main?
3. Would 2 grounding plates not directly connected with each other create some type of a loop?

Thanks for your thoughts.*
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Old 14-12-2019, 09:19   #2
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Re: Lightning grounding question

Sounds like you have it about right. Modern thinking about the seawater end of the lightning protection (protecting ship an occupants) is edge length is more importing than area. Long and skinny produces a lot of edge.


Typically you want the lightning dissipation system connected to the ships 12v negative at a single point to try to minimize common mode voltage throughout the craft.

Just keep an eye on all the connections, keep your insurance current, and your fingers crossed.


Always anchor near someone with a taller mast.


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Old 14-12-2019, 10:07   #3
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Lightning grounding question

Many Sportfishermen in the Fl Panhandle have ground plates that they connect to their tuna towers, which are of course pretty similar to a radio mast but made from aluminum tubing.
I think Dynaplates are often used just because they are available is all, most of these are fitted by yards, who will of course use off the shelf components.
Difference is people sit in those towers.
Iíve never heard of anyone being electrocuted sitting in a tuna tower or fly bridge, but have heard of boats sunk from the grounding plate being blown off, leaving a rather large hole in the boat.

I have no idea if 99% of boats are protected with a grounding plate and only 1% get it blown off or if itís the other way around.

Iíve come to believe that lightening is I guess similar to a tornado, very unlikely, but also no real way to protect from it, if your hit, most likely there will be damage, that you canít prevent.
Maybe thatís a defeatist attitude, I donít know, but surely if something worked there would be Insurence breaks if you had it?

But buildings are protected, so why canít boats be similarly protected? Is it the grounding point that is the difference?

I used to hang some SS chain from my chainplates in the water, but just quit.
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Old 14-12-2019, 10:36   #4
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Re: Lightning grounding question

We have a similar approx 8ft by 1.5in external bar. It has a separate lightning ground system attached to it, starting at the masthead rod and connecting major metal components.
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Old 14-12-2019, 11:51   #5
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Re: Lightning grounding question

NFPA 780 (Standard for LP Installation) actually has a section on "protection of watercraft". Tossed my copy years back but don't remember anything that hasn't been beaten to death on the boating forums.


As I have said many times "unlike gravity there are no absolutes dealing with lightning", but following the standard guidelines makes driving to the grocery store a significantly more hazardous event than being on Cbreeze in a good old Florida thunderstorm IMO.

As to Dynaplate (they have been around over 50 years) the single gold plated bolt connection never made me comfortable for handling a serious strike. The copper plate on Cbreeze is connected to the mast with redundant 1/0 copper connected to 4- 1/2 " silicon bronze bolts soldered to the copper plate. I worry about her electronics but not the craft herself.


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Old 14-12-2019, 11:54   #6
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Re: Lightning grounding question

Aluminum is a poor conductor compared to stainless steel, so ensure your chainplates are bonded to that bar with a substantial cable...at least 1/0 AWG. Also, ensure there are no sharp bends when routing your wires...high voltage and RF energy do not like sharp bends in their path.

The bonding everything vs non bonding discussion is here...http://www.cruisersforum.com/forums/...-not-1362.html

Lightning is a magical beast and while we can attempt to reduce exposure or try to minimize damage through past practices and best guesses....it's still just magic and luck. Frankly had good advice..."keep your insurance current, and your fingers crossed."

As for me...I bond my chainplates to my steel keel and engine, and and also have lengths 4/0 AWG wires bonded to my main shrouds and backstays that I deploy and drag in water if anywhere near a storm to provide a "path of least resistance." (I also cross my fingers, and pray.)
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Old 14-12-2019, 12:24   #7
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Re: Lightning grounding question

I think you have it backwards, aluminum is a very good conductor itís one reason itís used for wires, light weight helps too of course.
But SS is a very poor conductor.
https://www.tibtech.com/conductivite.php?lang=en_US
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Old 15-12-2019, 12:43   #8
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Re: Lightning grounding question

Looking back over this discussion, something that I take for granted maybe needs to be said.


Lightning Protection Systems are not intended to reduce the probability of being hit (building, tower, or yacht), but to potect the underlying structure from damage when they are hit. Although there are, have been, and will be different devices that claim otherwise, I am not aware of any scientific evidence to support such claims. Also there is no evidence that installing a proper LP system increases the probability of being hit.

As terrifying as watching a serious strike attach to something is, everyday over the world LPS safely conduct that energy to earth / water ground with no serious damage to the protected structure. Internal electronics another deal.

The significant variables at your control are location (Lake O is a lot worse than Lake Michigan), and relative altitude (thus my suggestion about achoring near a ship with a taller mast). Insurance and a rabbits foot round out most of the rest.


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