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  #1  
Old 12-11-2008
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"Pointed" Lightening Protection Question

To quote a subset ot the seamanship article on lightening, posted here: Lightning Strike!

Quote:
...
To Not Protect
The thinking of the "unprotected" school is that by installing a direct path to ground, as in a lightning protection system on a boat, an invitation is being issued for a strike to come aboard. This is the great irony of deciding on lightning protection systems—unprotected boats may actually be struck less often, but when they are struck, they usually suffer more damage. A boat with a good lightning protection system, on the other hand, may actually have a greater likelihood of being struck, but the strike is dissipated and directed away, usually with minimal resulting damage.
I remember playing around with a Van de Graff generator in high school and the teacher showing us how a lightening rod works. He put a small house between the 2 parts of the generator, with a grounded needle at the top of the house -- a pointy needle.

The lightening rod worked because it provided a path for static electricity to ground, BEFORE a build up of static could occur. Thus no lightening happened in the area. Actually the amount of lightening betweeen the arms of the generator went down drastically, as the needle drained away the static.

This also lines up with a story I heard somewhere about how the English didn't like the American way of installing lightening rods and so installed them round side up (instead of pointy side up). This resulted in many more fires than installing it the right way. Apparently, the pointy tip of a lightening rod allows for the most charged air around that point, and so it grabs the static out of the air better.

Now I've always wondered after all that, why modern lightening rods on sailboats are not pointy. Instead they are blunt. And I wonder about the accuracy of the Seamanship article claiming that a lightening system will INCREASE the probability of a strike to your boat (albeit a less damaging one). Could it be that because we don't use pointy lightening rods, the Seamanship article is actually correct? Could it be that if we were using pointy lightening rods, we would get lightening protection with a drastically LOWER chance if geting hit?

Thinking about this another way, surely lightening rod on houses aren't increasing the chance of getting hit. And their rods are always pointy.

House lightening rod (pointy):


I looked for a picture on the web of the blunt lightening rod that I've seen on sailboats. Can't find one right now. If anyone has one, please post it here.

I propose that we are not using the right design in our lightening rods. They should be pointy at the top.
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Bene, I think you will find that if you look in the bilge of your boat that the rig is grounded to the keel. That would be the green wire running to a keel bolt.

I lighting rod on top of a mast of a sailboat never made much sense to me as the mast itself is a lighting rod.
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Old 12-11-2008
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Bene, as I understand it the lightning rod does not "grab" static from the sky, but rather it allows the GROUND CHARGE to bleed off UP into the sky. When there is a trike, normally the ground charge goes UP creating a path of ionized air, and then the cloud strike comes DOWN that same (more of less) path, creating the damage.

A pointed rod apparently can discharge ions better than a rounded or blunt rod, which is why some makers were using "bottle brush" designs with many fine wires at the tip, since each fine pointed wire end allows another point where ions can bleed off.

If someone on a sailboat had a blunt lightning rod--it wasn't for performance reasons.
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Quote:
Originally Posted by hellosailor View Post
Bene, as I understand it the lightning rod does not "grab" static from the sky, but rather it allows the GROUND CHARGE to bleed off UP into the sky. When there is a trike, normally the ground charge goes UP creating a path of ionized air, and then the cloud strike comes DOWN that same (more of less) path, creating the damage.

A pointed rod apparently can discharge ions better than a rounded or blunt rod, which is why some makers were using "bottle brush" designs with many fine wires at the tip, since each fine pointed wire end allows another point where ions can bleed off.

If someone on a sailboat had a blunt lightning rod--it wasn't for performance reasons.
This is CORRECT. Lightning isn't attracted to the points, per se (though they certainly can be hit)... but static dissipation is the point (no pun intended) here.

As a charged area in the sky, a cloud usually or an area of clouds, moves past, it DRAGS ALONG BEHIND it an opposite charge looking for a place to go. In a static discharge (which lightning is that exactly) the ground charge will pull up a "grounded system" and the points on several lightning rods will force a static discharge, slower than a direct lightning discharge into the space around it.

This is why lightning rod systems on buildings include many, many points, all connected electrically together, and then tied to a ground path.

In a very powerful build up of static, many hundreds of thousands of coulombs of energy can be ready to discharge. Sometimes, even a lightning rod system can't stop it from happening!

I've had, over the course of about 19 years, about 6 or 7 very close strikes. I have a 20 foot tower, plus another 10 or so feet of antenna above the house. I've had wooden fences hit, trees next door and even the house down the hill from me (about 30 feet lower in altitude) - but my tower hasn't been hit (knock wood).

I usually keep my antenna systems grounded through a switch when not in use.

But you never know!

I was present at a friend's house many years ago when his CB antenna was stuck. Fiberglass antenna. It was pretty much vaporized, the coax cable melted and the radio was thrown across the room. His desk, the curtains and nearby electrical outlets caught fire. The breakers in the house and several neighboring homes were tripped. The phone system was pretty much obliterated in the neighborhood as well.

I've been "involved" in two near misses myself, both of which hurt like hell. Lightning hits what it wants to hit, when it feels like it, and the rest of the time it will spare you and your equipment.

Even with protection - you can still be struck. BUT, remember this, protection is BEST used to protect your equipment and persons than ignoring it.
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Originally Posted by N0NJY View Post

I was present at a friend's house many years ago when his CB antenna was stuck. Fiberglass antenna. It was pretty much vaporized, the coax cable melted and the radio was thrown across the room. His desk, the curtains and nearby electrical outlets caught fire. The breakers in the house and several neighboring homes were tripped. The phone system was pretty much obliterated in the neighborhood as well.
His CB antenna was probably not grounded.
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Originally Posted by hellosailor View Post
Bene, as I understand it the lightning rod does not "grab" static from the sky, but rather it allows the GROUND CHARGE to bleed off UP into the sky. When there is a trike, normally the ground charge goes UP creating a path of ionized air, and then the cloud strike comes DOWN that same (more of less) path, creating the damage.

A pointed rod apparently can discharge ions better than a rounded or blunt rod, which is why some makers were using "bottle brush" designs with many fine wires at the tip, since each fine pointed wire end allows another point where ions can bleed off.

If someone on a sailboat had a blunt lightning rod--it wasn't for performance reasons.
I agree. I found the brush design on the web and will probably install it or something else that's pointy. No sense having (to paraphrase bubb2) a blunt lightening rod.
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Quote:
Originally Posted by bubb2 View Post
Bene, I think you will find that if you look in the bilge of your boat that the rig is grounded to the keel. That would be the green wire running to a keel bolt.

I lighting rod on top of a mast of a sailboat never made much sense to me as the mast itself is a lighting rod.
That is correct IF your boat is grounded. Catalina's are not.
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That is correct IF your boat is grounded. Catalina's are not.
It is a Beneteau thing. It least all I know of including mine.
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The brush like lightning dissapators don't make much sense on a boat, since they require a fairly decent ground connection, which most boats lack.

There are basically two schools of thought on lightning protection on sailboats.

1) Ground the boat, and bond all the major metal structures to the lightning grounding system. This will give lightning a path to the water in the case of a strike and also act to protect the occupants as much as possible. However, grounding the boat as such will increase the chance of getting struck slightly.

2) Don't ground anything on the boat. This reduces the chances of you actually getting struck. However, if you do get struck, the chances for catastrophic damage go way up, as sideflashes may occur and there is greater risk to the occupants of said boat.
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Quote:
Originally Posted by sailingdog View Post
The brush like lightning dissapators don't make much sense on a boat, since they require a fairly decent ground connection, which most boats lack.

There are basically two schools of thought on lightning protection on sailboats.

1) Ground the boat, and bond all the major metal structures to the lightning grounding system. This will give lightning a path to the water in the case of a strike and also act to protect the occupants as much as possible. However, grounding the boat as such will increase the chance of getting struck slightly.

2) Don't ground anything on the boat. This reduces the chances of you actually getting struck. However, if you do get struck, the chances for catastrophic damage go way up, as sideflashes may occur and there is greater risk to the occupants of said boat.
Regarding #1, if you are using a pointy lighting rod or one of those brushes (which are also pointy or many-pointy), and you have a grounded mast, then you are "sucking" static electricity out of the air. How/why are you getting hit more often?
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