grounding for lightning

[Paul45c]

Rickysa suggested a new thread on what I did to "101" for lightning grounding. Here goes.

Innocently enough (and with a few sailboats in my background -- I've since reformed! ), I asked my surveyor where the grounds were for the outriggers and/or bridge halftower. He looked at me blankly and said I wouldn't find any. Don't do it on motorboats, that is except for regular bonding. I was always used to pulling up the center cabin floorboards over the keel and seeing a fat grounding wire coming down from the mast to one of the big keel bolts and that was that. I know they work because I've been on two sailboats that got whacked -- nobody got hurt.

That's all I was after was safety for those aboard, not knowing really what would happen to an ungrounded 35' triple-spreader outrigger just begging for a blast from above. I had some bonding work to do as a survey flagged item, anyway, so I had a company come aboard and talk options with me about lightning protection. They said there's no question I could improve on the protection w/r/t lightning strikes by grounding to separate long narrow lightning plates we installed when she was on the hard. They're about 4' long, maybe 3" wide and 2 or 3" thick. The important thing that distinguishes them from other bottom grounding plates you'll find is that they are solid forged and not sintered (the pebbly looking stuff) bronze. One on each side, o/b of the o/b stringer and as close as possible to being below where the grounding wire drops down from the base of each respective outrigger. This was key, my electrician said -- too much of a bend, and the lightning will just go to ground straight out of the cable instead of where you're trying to send it.

BTW, the solid vs. sintered is key, too -- the sintered stuff will basically vaporize a lot of the water trapped between the "pebbles" and blow a big hole in your boat. Important safety tip, Egon!

The outriggers were my main concern, and I feel pretty good about their ability to channel the spike; as an added safety, the electrician also bonded the half-tower and stainless rails on the bridge to the same grounding plates. By necessity, given how much those cables have to curve around, I'm from Missouri on whether they'd do much of anything, but it wasn't much extra $$ to add them.

That's it.

 

[Rickysa]

Thanks Paul....I've always heard about a "halo" of protection around a boat at sea, but have never felt very confident in it. I'd rather be pro-active.

Rick

 

[Paul45c]

Speaking of halo, that reminds me of a lightning demo I saw at the Boston Museum of Science years ago. I forget how many volts they were zapping a protected cage with, but they made the point that lightning will travel around the exterior of any metal it encounters (hence your safety inside a car). They actually had a guy inside the cage that was getting arced and he was safely able to touch the inside of the cage without even his hair standing up. Pretty convincing!

Anyway, Florida gets its share of thunderstorms, so I felt the proactive route was better, myself.

 

[Trojan]

I don't buy it. There generated static Lightning is not the same. Real lightning does not make turns. If it runs into a turn it doesn't. All the cables coming off my towers have 90 degree turns 10 ft from the ground. I ground nothing for lightning except the towers. I do have disconnects on the cables if I'm home. My 5 repeaters are never disconnected from there antennas and they are mounted on the very top of the towers. Why make yourself a lightning rod. A sail boat is a whole lot different than a power boat. I have 5- 70ft. towers in my yard at the farm. I ground nothing coming from these towers. These towers have been struck many times and nothing has entered the house. I don't believe in making myself a lightning rod. If there is no ground for the lightning it will find someplace else. Just my opinion and practical experience. My other hobby is storm chasing for the national weather service. My dock friends wanted me to name my boat Storm Chaser.



BILL

 

[Mike36c]

As for the outriggers, My Rupps have nylon washers at all bolted connections isolating each aluminum part and the ss bolts (for corrorsion protection). I'm not sure how that works with lightning prevention but I'm sure if a rigger WERE to get struck those little washers would do nothing to stop its path to ground.

 

[Paul45c]

As for the outriggers, My Rupps have nylon washers at all bolted connections isolating each aluminum part and the ss bolts (for corrorsion protection). I'm not sure how that works with lightning prevention but I'm sure if a rigger WERE to get struck those little washers would do nothing to stop its path to ground.

I've got the Rupps, too, and the base of the spar itself is what the ground wire is connected to. Hey, I hope it never comes into play, but now at least there's something.

 

[67hat34c]

Here is my Lighting story. About 5 years ago or so my house got hit. It was a discharge out of the ground instead of comming from the sky. My front porch on my house is concrete, the living room on the other side of the porch wall has a sunken floor so there was a metal pan or flashing running the length of the porch wall. Lightning blew the concrete away at the end of the porch by the house wall, hole was about 24"x24"down to the dirt. the bolt entered the low voltage landscape lights which go around a long curved side walk. that is how it got into the house electrical system. however at the apex of the curve the bolt decided to take a detour and went under the sidewalt and made bee line to my oak tree. at the base of the oak tree there was an iron park bench. the bolt left a run out trench in my yard about 15' long a 12" deep and 12" wide, oak tree roots ripped through and some were on the roof of my house. any how the bold did not go in the tree, it went up one of the legs of the park bench across the top of it, down the legs on the other side and then there was a second run out trench to the street. blew the iron legs off of the bench, broke them in half. stuff in my house was another story but not as interesting as the run out trench. State Farm kept the photos and put them on their board of strange but true.

Anyhow i have photo proof of how lighting goes around corners, by the way my oak tree is fine, park bench saved it. Dont know why my house did not burn down but i guess God saved that.... I have another story about that if anyone wants to hear i will post on PM.

 

[Trojan]

Lightning goes from the ground up before it goes down. That is one of the reasons why your hair stands on end before a strike. Sure it can turn once on the ground but not when its coming down. It follows the least resistance once on the ground until it dissipates.


BILL

 

[Paul45c]

Lightning goes from the ground up before it goes down. That is one of the reasons why your hair stands on end before a strike. Sure it can turn once on the ground but not when its coming down. It follows the least resistance once on the ground until it dissipates. BILL

I didn't hear the part about ground-up first, but the rest of this is very consistent with what my electrician told me: no unnecessary bends because the lightning spike will jump out of the cable and take its shortest route to ground, which in our case is the water.

 

[SKYCHENEY]

I once saw a sailboat on the hard being repaired after a lightning strike. It had numerous pinholes through the bottom of the hull where the electricity shot out. It was before the age of blisters but now that I think back, it was almost like that. It was all over the bottom, except these were not superficial. These were actually holes going straight through the boat.

I have a big aluminum arch up there and I always think about it when I get caught out in a storm and see some lightning. I try not to get anywhere near that, but sometimes you have no choice.

 

[Paul45c]

At least the arch isn't mounted to something that, if holed, would sink the boat. It might make a heck of a mess out of the fiberglass around the bridge but you should be out of the problem area. Just keep the crew from hanging onto it if the weather starts getting surly!

 

[Beckytek]

A friend of mine had his sailboat docked on the canal behind his house and it got hit by lightning. He didn't even know it until the next morning when he saw the boat had sunk. When they got it up it too had all the little pinholes throughout the bottom. After the insurance co paid the claim he repaired it and he's good to go.

 

[Genesis]

Actually that's not quite right.

Lightning is initiated with what is called a "step leader" from the cloud. This is a "small" (in relative terms) spark that forks and takes different paths on the way down. It continues until it gets to a grounded item (e.g. your boat.)

The actual stroke itself is from the ground upward. The step leader's "purpose" is to ionize the air so that it becomes conductive - once that path is established then the stroke returns up along the ionized path to the cloud. The effective frequency of a lightning hit is in the gigahertz range and high-frequency energy travels along the outside of a conductor only, which is why microwave transmitters use what looks like pipe instead of wire to conduct the signal to the antenna.

However, the rest is definitely right. Bends in the wriing are bad news because the stroke has, effectively, "momentum" and if there is a bend in the conductor the energy will tend to keep going straight instead and travel right through whatever is in the way.

This, by the way, makes outriggers (pipes!) great lightning rods. That helps if you properly ground them, but is extremely dangerous if not, because the energy will go right through the side of the hull (or cabin!) and get you if there's no good, straight, low-resistance path from them to the water on the outside of the boat.

If you are caught out with riggers on the boat one "last ditch" thing you can do is to clamp some battery jumpers on the outriggers at the base and drag them in the water (straight line down from the base, of course!) This gives you a decent shot at conducting the charge into the water safely. Even better is a nice big plate of metal on the clamp that's dragging in the water so there's a lot of surface area for the charge on that end.

Its not perfect but it beats nothing if your grounding system on board is substandard.

 

[Trojan]

I would lay the out riggers flat out the stern and not ground them. Lightning on the water acts a little different than on land. I keep my 2 iron on the bridge for protection from lightning.




BILL