Lightning Protection

2nd_apprentice

2nd_apprentice

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Plenty of information on that one but I've been quite surprised to read the following:

[ QUOTE ]
Along with most boat yards and boat building operations we suggest that people do NOT install any "lightning grounding" system UNLESS it is led completely outside the boat in its entirety. Even then our personal preference is to avoid grounding systems entirely. We have never seen a vessel without a grounding system hit by lightning and have seen many with grounding systems struck and heavily damaged. You should absolutely never ground to your rig and down into the boat to a grounding plate or keel bolt. This is extraordinarily dangerous. You should never attach anything to a lightning grounding system that you would not want to see damaged. For more on this see: "Living Aboard - Frequently Asked Questions" by Thomas A. & Nannette M. MacNaughton. This is available through our marine publishing order form. See the question: "Should we install a "lightning protection" system?

[/ QUOTE ]

/forums/images/graemlins/shocked.gif
 
Yes AFAIK that is absurd. The fact is you have a lightning conductor in an Al mast (or brass track on a wooden mast.
This conductor will be connected to the sea by moist deck /hull or wiring of instruments or lights to negative to engine ground to sea.

What you need to do is to redirect the huge current away from those resistive and easily damaged conduction paths to a low resistance high current capacity path to the sea.

The mast will be quite a good low resistance conductor ie it will probably carry the required current without too much heating but SS stays are resistive and easily damaged by high current and resultant heat.
So you need to connect the base of the mast to the sea. The best way to connect to the sea is to a steel keel. You need wiring that will carry a lot of current without fusing. Connection to the keel should be low resistance but any damage by resistance will probably be bypassed by arcing and hopefully this will not cause a fire.

How much current. Well the only guide I can give is the many old churches which have copper strips down the outside to carry lightning current. Usually several inches wide and 1/4 inch thick. Presumably this size has been proved adequate by time and many strikes.

If your source quote is correct I would imagine a church less inherently likely to be struck than a boat due to high resistance/non conductivity of stone structure. Yet they do provide lightning conductors.

Yes it would be far more desirable to have an external conductor from mast to sea. An internal conductor if it actually conducts will probably fill the cabin with smoke and possibly get very hot. But an external conductor would just get in the way. ie too much triouble

So Bollocks olewill
 
As a stinkie with only 10' of airdraught I have found the perfect lightning protection system, moor next to a raggie /forums/images/graemlins/grin.gif

As Said before I would have though with a dirty great metal pole poking skyward out in the sea some protection would be a good idea but also agree with the taking it outside the hull bit, where lightning would just travel through a metal hull I bet it would punch a big hole through grp once the other routes had melted.
 
The Jury is still out on lightning protection and another jury is still waiting a definitive account on how lightning strikes are initiated.

One school of thought suggests that the lightning strike is initiated from Earth to the base of the thundercloud. So if you have a good connection to Earth you may get an small ionising discharge at the top of the conductor which then gives a lower resistance path for the real strike.

The other school of thought is that the local electric field builds up to the point of electric breakdown and the lightning strike from the cloud goes to the nearest Earty object, ie the tallest.

Contrary to poular belief that "lightning never srikes twice" there is evidence that when lightning does strike, the trail of ionised debris will direct further strikes to the same spot. As I recall, 400strikes have been recorded !!!

So, in conclusion, I have read the books , cogitated etc but I still don.t know what the right thing to do is.

One thing that seems semi-sensible is to include a spark gap in the lightning conductor system which gives a discharge but limits the current.
 
[ QUOTE ]
How often are yachts struck by lightening? Or am I more likely to win the lottery?

[/ QUOTE ]

Not that infrequently, there have been a number of first hand accounts in yachting mags in recent years.

I am sure I remember reading that you are more likely - in the UK at least - to be struck by lightning than win the lottery.
 
[ QUOTE ]
If there were two yachts next to each other one with a lightning system and one without which one would get hit .
My guess would be the one that is grounded....

[/ QUOTE ]Which goes to show how wrong you can be.

Lightning protection DOESN'T conduct a lightning strike safely to ground. It stops the growth of the beginnings of lightning. ie it grounds out the feeders of ionised air that precedes a proper 'strike'.

Even the largest thickest strip of highly conductive copper will be ionized instantly by a full strike. The copper is there to try and stop the strike in the first place...
 
Whether you go for the dissipation theory or the leader rising from earth the clear fact is that even a high electrical resistance is the same as a low electrical resistance in the initial stages.Trees attract lightning quite well. So 2 yachts one with protection one without I would suggest have equal chance of strike. The one closer to the electrical build up will get it.
The real problem is once the strike current flows it can do a lot of damage in resistive material. ie trees that boil the sap and explode. Lower resistance equals less damage by diverting current.
I think a spark gap will possibly allow or divert current to other electrics so do damage.
JeremyShaw's strike shield sounds good if you set it up in time. I think I would rather a permanent internal system. Fortunately we don't get a lot of lightning here (perhaps one storm per year) so I will risk no protection.
And i reiterate church and tall building protection has been around for a long time apparently successfully.
olewill
 
Once upon a time in mid atlantic, in the middle of a thunder storm, and getting rather worried that it was going to clobber us (as lightning was going beserk all around us), we attached a length of battery cable to (I think) a cap shroud (using bull dog clamps) and hung it over the side in the water.
We did not get hit by lightning, but on reflection now, after reading the above, and thinking about it, I suppose that if we had been struck, this battery cable might have melted - or would it conduct thousands (?) of amps to the sea quite happily?
Or perhaps another component in the rig might have melted first?
 
[ QUOTE ]
How often are yachts struck by lightening? Or am I more likely to win the lottery?

[/ QUOTE ]You don't actually have to be struck by lightning to have sensitive electronic equipment damaged. The induced voltages and currents near a lightning strike are high enough to blow up lots of yacht instruments. A full blown strike atomizes metals and cabling.
 
[ QUOTE ]
Whether you go for the dissipation theory or the leader rising from earth the clear fact is that even a high electrical resistance is the same as a low electrical resistance in the initial stages.Trees attract lightning quite well. So 2 yachts one with protection one without I would suggest have equal chance of strike. The one closer to the electrical build up will get it.
The real problem is once the strike current flows it can do a lot of damage in resistive material. ie trees that boil the sap and explode. Lower resistance equals less damage by diverting current.
I think a spark gap will possibly allow or divert current to other electrics so do damage.
JeremyShaw's strike shield sounds good if you set it up in time. I think I would rather a permanent internal system. Fortunately we don't get a lot of lightning here (perhaps one storm per year) so I will risk no protection.
And i reiterate church and tall building protection has been around for a long time apparently successfully.
olewill

[/ QUOTE ]I agree with much of what you say - except that a low resistance has been argued to alleviate the leaders more efficiently - and its what is provided on churches and other tall buildings.

We have our mast connected to the keel with a very heavy copper braid. It was put on at time of manufacture and I am happy to leave it there and check its condition now and again.
 
Thanks for the fruitful discussion. Not that my humble vessel was be quite as grand as Westminster Abbey though they might share some similarities when going to windward.
I'll stick with more conventional wisdom instead of relying on Mr. MacNaughton "never having seen an unprotected boat being struck by lightning".
 
I have a polyester keel so I can’t use it as a ground.
Sometimes when getting in bad weather I use a chain attached to the backstay and the other end in the water.
Theoretically this:
-Bends the equipotential lines and creates a stronger electric field and this should attract the lightening by creating an ionic path
-On the other hand this creates a kind of cage of Faraday and protects the electric equipment inside the boat.
So what’s the best solution?
 
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