bonding and lightning

[walt]

Mainesail,

I hope I am not saying something incorrect here.. but I believe I have seen you post recently that "bonded" sailboats are slightly LESS likely to get struck by lightning compared to boats that are not bonded.

I know of a technical paper that sort of gives a reason why this may also be true (more related to "grounding") but I wonder if your source is from the insurance industry.. or???

 

[Maine Sail]

Mainesail,

I hope I am not saying something incorrect here.. but I believe I have seen you post recently that "bonded" sailboats are slightly LESS likely to get struck by lightning compared to boats that are not bonded.

I know of a technical paper that sort of gives a reason why this may also be true (more related to "grounding") but I wonder if your source is from the insurance industry.. or???

Surveyors, insurance and I do lightning strike repairs, estimates and damages assessments for clients before the insurance adjuster gets there so they don't get hosed...

Dr. Ewen Thompsons Florida Sea Grant paper also found the same thing.

What I really see is less hull damage on WELL bonded boats for lightning.

 

[Piratenorm]

MS, not sure what is meant by a Well Bonded Boat for lightning! Please give examples. Thanks

 

[Maine Sail]

MS, not sure what is meant by a Well Bonded Boat for lightning! Please give examples. Thanks

Sufficient wire gauge from mast to keel or mast to underwater lightning grounding strip.

As straight a wire as possible to main lightning grounding point.

Sufficient wire gauge from shrouds to keel with minimal bends.

CLEAN non corroded connections to keel or main lightning grounding point.

Most boat owners never look at the lightning bonding wires and they are so corroded as to be essentially useless.

 

[Sailvayu]

The term "bonding" should not really be used when talking lightning protection. Lightning protection requires a good "grounding" I know this sounds a bit like nit picking but the bonding system is to prevent galvanic corrosion while a ground is to allow a low resistance path to earth ground. The only reason I point this out is to avoid misunderstanding of the systems and note that the bonding system only uses a #8 wire which is really too small to carry a lightning strike. Also for a lightning ground you would not want to be tied into the bonding system as this could (and has) damaged thru hulls in the event of a strike by allowing high voltage to exit out the underwater fittings. A good lightning ground should be separate and use as large a wire as practical, think battery cable or heavy solid copper strap. MS is right the run should be straight with few or no turns in the wire and the connections should be tight and well sealed. The larger the ground plate the better but never use the sintered bronze type such as the Wonder Bar.
From what I have read in Boat US and my own experience doing lightning surveys there is really no proof a good ground helps prevent a strike. In fact it is pretty much 50/50 for boats with a ground and those without that get hit. On the other hand damage can be less with a good ground but it truly is amazing how lightning reacts when striking a boat. I have seen a boat hit at the mast right next to the VHF antenna, (within 2") it blew the bilge pump to pieces yet the VHF still worked. So to be clear think of the bonding system and a lightning ground system as 2 separate systems and not connected to each other. Other than that keep your fingers crossed and head low in a storm!

 

[Stu Jackson]

http://www.westmarine.com/WestAdvisor/Marine-Grounding-Systems

 

[walt]

thank you very much for responding to that initial post..

FYI, the technical paper that I referenced earlier which is “sort of” related to the sailboat ground or not ground is here
http://onlinelibrary.wiley.com/doi/10.1029/1999GL011053/pdf

These guys basically did various experiments over a number of years on a mountain top using grounded lightning rod but with different sharpness on the tips. They had both sharp and blunt tipped lightning rods and over an eight year period, they had 12 blunt rod strikes but zero strikes to sharp rods.

It’s an interesting paper with descriptions of the Corona current mechanism that causes a lot of the things we see on sailboats such as getting shocked on shrouds, buzzing, etc.

One thing of interest is that the fields for the Corona effects we see (and how the wire brush dissipaters work) is on the order of 2KV/m for a sharp tip. However, for a leader to propagate and form a leader that is traveling upwards, the fields have to be a huge 200 times stronger (440KV/m) and those fields don’t occur until the lightning strike gets fairly close.

The researchers attributed the sharp points not getting struck partly due to being better at generating Corona current which leaves a “field reducing” positive ion charge around the tip. The blunt tips don’t start generating Corona currents until the field is over six times stronger than a sharp tip does. The other reason “This is because the field enhancement for a sharp rod decreases much faster than for a blunt rod as the distance from the tip increases”.

The paper talks about some of the interesting cases they saw.
How does this relate to a sailboat.. maybe it doesn’t.. but If I were to keep a boat in a marina, I would put a sharp “Franklin” tipped point at the top and hope that my neighbor’s boat had more of a blunt tip point at the top. Both of these tops make good lightning rods, you just hope that the neighbors lighting rod works a little better than yours does. You still want a lightning rod (mast) on the boat since if lightning is going to hit you, at least you know where it will go (mast).

The mast grounding part related to the paper is a lot more of a stretch but a grounded mast is going to be better at generating Corona currents at its tip vs. an ungrounded mast. The ungrounded mast would have its potential float up as it was charged by the corona current (wire brush or not.. these all add electrons to the mast, they don’t dissipate charge) which would tend to reduce the field at the top and also the Corona current. One of the reasons in the paper that the sharp tip made a slightly worse lightning rod was that it was better at generating corona current – and a grounded mast would also be better.

Does this mean anything.. maybe not but this paper is the only thing I have ever seen that comes even slightly close resembling the ground or not ground issue..

Personal choice only but Ill go with grounding or not grounding doesnt matter for your odds of getting struck - but bonding or grouding is going to reduce the structure damage.

 

[Maine Sail]

The term "bonding" should not really be used when talking lightning protection. Lightning protection requires a good "grounding" I know this sounds a bit like nit picking but the bonding system is to prevent galvanic corrosion while a ground is to allow a low resistance path to earth ground.

Considering Dr. Ewen Thompson, the ABYC, Nigel Calder, Ed Sherman, Boat US and many other industry experts all refer to lightning bonds or lightning bonding as the wires that interconnect the lightning grounding system I would disagree that it is a term that should not be used.

There are simply two types of bonding lightning bonding and galvanic bonding. It is just importnat to clarify which bonding system you are referring to. In this case we were discussing lightning bonding not galvanic bonding.

I think Nigel Calder sums it up nicely:

Calder:
"Bonding is done not just for protection against stray-current corrosion, but also for protection against lightning strikes. In the case of lightning, it is not just immersed metal, but also all major metal objects that should be wired together....

Without bonding, there may be massive voltage differences between metal objects, with the potential for "side flashes" to leap from one surface to another, injuring or killing people in the process and wreaking havoc on the boat and its equipment."

 

[Jim Seamans]

bonding ??

I have a boating friend that was in the Navy and Navy contracting and very knowledgeable. Many years ago (not sure of time), Navy had built some new minesweepers that were taking lightning strikes which was blowing away very expensive electronics. He told me the engineers discovered by removing all bonding to the the sea the strikes stop. So after a couple of ships had tested this, they retrofitted all the minesweepers to have no bonding. I asked for paper on this and he laughed - "that's not how they (Navy) do things".

Has been a liveaboard for 17yrs, from New England to the Bahamas. No lightning strikes on his boat, no bonding or lightning prevention.

Makes me wonder and shake the head...Have not heard of this until in conversion with him..

 

[mitiempo]

If I were to keep a boat in a marina, I would put a sharp “Franklin” tipped point at the top and hope that my neighbor’s boat had more of a blunt tip point at the top.

It would probably help keep the birds off the masthead as well.

 

[roncynthiaboomer]

pictures of shroud, chainplate,stanchions wire connections a

Having read this with interest and going online to research available pictures of connections the only picture I found was the wire connecting the mast to the keel bolt. Does anyone have any pictures of the other prescribed wire connections cited in these responses? thanks in advance.

 

[walt]

I think you are sailing in fresh water?

If you read the "fine print" on all the bonding stuff related to lightning, it really only works very well in salt water. You may see something llike you need the underwater structures to be 100 or 1000 times larger in fresh water than in salt water.. Hmmm... is your fresh water keel 100 times larger than what would be common in salt water?

There is a law used in analog circuit design called Kirchoffs law http://en.wikipedia.org/wiki/Kirchhoff's_circuit_laws and it basically says the sum of current into a node is equal to the sum of current out of a node. Any path off the node with lower impedance will carry more current, any node path with higher impedance will carry less current. I "think" (as always, I could be wrong) that this also applies to a mast involved in a strike. The current "into the node" is simple.. its the lightning strike that attached to the mast and you have zero control over how much current this is.

At the bottom of the mast, you will have a sum of currents going out that is equal to the current going in. I dont think anyone would argue that ionized air is a very competitive conductor and this is one way that the charge goes from the bottom of the mast to the water. But.. this "side flash" means a bolt of lightning inside the boat through the hull - which is not a good thing. So the goal of bonding is to compete with ionized air and route the current in a known path. A competitive conductor (compared to ionized air) has already been discussed, it must be large enough wire, low enough inductance, connections which are low impedance and wont get blown apart when a lot of energy flows through them. And.. it must have a low impedance "sink" to dump the energy such as a keel in salt water. Kirchoffs law also says that multiple conductors will also share the current and the better job you do of getting the charge to ground, the less chance of field build up that creates a side flash.

I really cant prove this at all - just an idea but I think there is a fair chance that the discharge electrode placed at the water surface that Dr. Ewen Thomson pioneered is the most effective way to "ground" in fresh water. This site http://analogengineering.com/lightning/surface.html (disclosure, this is mine) shows actual discharges from an electrode placed just above the water surface. Near the bottom there are pictures of this discharge in both fresh and very salty water (more salty than the ocean). This discharge at the water surface forms "fingers" of charge likely using ionized air at the water surface and the fingers are a LOT longer for fresh water, and very short for salt water. Sort of looks to me like in fresh water, the fingers just spread out over a larger area on the water surface. There are two way to find an eqivalent number of charge carries - in a small area with high concentration or over a large area with lower concentration.

 

[Sailvayu]

Hi Walt I am sure you have looked at this as well http://www.marinelightning.com/Siedarc.htm

I will have to review your site when I get more time but I have been interested in what you talk about with the water surface effect as I have read about this before. The biggest problem I see is this may work well at the dock but not so sure how it would work on a vessel in a seaway where the electrodes would be dipping below the water.

The problem is the general lack of empirical data to support any one theory. (Hard to get a boat repeatedly stuck to test different things.) Thanks for sharing your info and I will be sure to take the time to study it in more detail. I will likely have more questions when I do.

 

[walt]

Yes.. those guys have an aditional patent now for putting a "siedarc" type of discharge electode at the water surface by using a flexible down conductor (ie, a large diameter wire) and either floatation or water planning. I dont know what they will come out with - or if they will but here is an example

And, you pointed out the problem with beleiving any of this works.. you really need controlled experiments where just one thing is varied and that will NEVER happen. Lightning stikes are just too rare and who the heck or how would they participate in a controlled experiment.

I dont think we will ever really know... I think the only thing that is really known is that a good bonding system on a boat used in salt water does mechanically protect the boat if it is struck.

 

[mitiempo]

If I lived in an area with lightning - or traveled to one - I would run a large cable from the base of my deck stepped mast to the nearest keel bolt in as direct a route as possible. I would also run cables from the chain plate bolts to the same keel bolt. A good cable size would be 1/0 or 2/0. And a sharp spike atop the mast. The keel is iron. There are no electrical connections to the keel, mast, or chain plates making this a totally independent system.

Beyond that I wouldn't worry about it.

 

[walt]

That would still only work in salt water.. in fresh water you are trying to put all the charge underwater and there is a lot of evidence that fresh water is just not conductive enough (unless you are also going to make your keel 100 times larger). So the very competitive side flash steps in to take care of the "job" and discharges from the mast to the water surface (using the water surface fingers as these out compete the underwater structure for dissipating the charge).

 

[mitiempo]

That would still only work in salt water..

I only sail in salt water.

 

[walt]

It turns out the link I provided earlier to an interesting paper was a cut down version of the one I wanted to show.. here is the one with the more detail http://analogengineering.com/lightning/lightning_rod_study_1.pdf

This is just playing with numbers which may not really mean anything but here is another way to look at ungrounded vs grounded masts and also what do the wire brush things actually do.

When you place a metal conductor such as a mast in an electric field, the conductor has only a single voltage over its length so the ends of it end up with enhanced electric field and if the fields are high enough, this creates corona current http://en.wikipedia.org/wiki/Corona_discharge In a few words, the high field at the end of the conductor (mast in this case) causes electrons to be ripped from an air atom and the electron accelerates to the mast and the left over positive air ion drifts up and away. If you read the paper above, its talks about the corona current in a little more detail but the corona current pulses and the rate is audible - explains the buzzing people hear on sailboat rigging during a lightning storm.

The wire brush things have lots of sharp points at the end of the mast so are likely good at creating corona current. So these strip off electrons from air atoms, the positive air ion floats away and the electron goes to the mast.

What the electron going to the mast does is if the mast is not grounded, it looks like a capacitor. So the electron that was added to the mast by corona current charges the mast capacitor according to coulombs law.

As electons are added and the mast capacitor is charging, this raises the mast voltage potential - which then decreases the potential to ambient at the top of the mast but now starts to increase the potential to ambient at the bottom of the mast. At some point, a corona current will begin to also flow at the bottom of the mast and when the corona currents at the top and bottom of the mast are equal, the mast potential stops changing. If we assume equal corona mechanism at the top and the bottom (probably not exactly true), the mast will charge to the ambient electric field at its mid point.

Now.. using some numbers from the link.. Before a strike and when the wire brush is working to charge the mast by corona current (actually any sharp point on the mast will do the same thing), the ambient electric field is around 5 KV/meter. So for either a grounded mast or an initially uncharged not grounded mast, the potential between the top and ambieint at 10 meters high is 50 KV (10 meters @ 5KV/ meter).

Now we let the ungrounded mast charge up to the potential at its mid point which would be 25KV (5 meters @ 5KV/meter). Now the top of the mast is only 25KV different than the ambiet field potential.

So the grounded mast has a higher field at its tip - so has higher corona current - which according to the paper, can delay the formation of an upwards streamer.

Now the interesting thing is that when the lightning gets close to the ground, the fields required for a streamer to propogate are 440KV/meter. Ie, the field went from 5KV/m before the strike to 440 KV/m when ground leaders can propogate - a huge difference.

At 440KV/meter, the ambient potential at the top of the mast at 10 meters is 4400 KV.

This potentail at 10 meters of 4400KV compared to the really small 25KV or 50KV of the grounded or ungrounded mast before the strike makes what happened with the mast before the strike came near the mast pretty much insignificant. The differene in the mast potential for grounded or ungrounded conditions is only 0.56% of the potentail when things really matter - just before the strike.

This is another completely unprovable argument that grounding or not grounding doesnt really make much difference.

It also shows why the wire brush deals also really dont make any difference in your chances of getting struck although since that paper showed a difference between a sharp or blunt tip in formation of a leader, maybe the wire brush makes some difference here. The argument that it "discharges" the mast is not at all correct.. these are actually adding charge to the mast but corona current will occur with our without the wire brush.

 

[Sailvayu]

Its some interesting information but I think the real weak point is the variable of the ground coupling of the mast (or other strike point) to the water. THese tests were all conducted on land where a good ground is a given. Many make the assumption that the simply connecting the the down lead to a keel bolt will accomplish this but unless the keel metal is directly exposed to the water there will be resistance. Most keels these days are either encapsulated or will have an epoxy type coating. This of course will not stop the high voltage of a direct strike but I think will effect the potential of the mast. This is where I think the results of grounded verses ungrounded get confused. This of course is the problem with trying to pull any real data from real world strikes. 2 years ago i was in a marina where a boat about 50 yards from me got hit. The interesting thing was our 2 boats were almost identical in mast height (My mast came from his boat so they were the same size) we had similar grounds. Why was he hit and not me? We likely will never know. The point is a low resistance coupling to the earth ground is difficult to achieve at best and on a boat very difficult. We have a lot of lightning here in the SE, I see many sport fish boats (out riggers) and sailboats get hit and the one consistency I have observed is there is no consistency in strikes or damage. I suppose in the end the best we can do try to provide a path for any strike away from crew and equipment. This is a fascinating subject and certainly open for more understanding which makes it fun. It is important to the mariner to try to understand how best to protect our crew and vessels. This type of exchange of information is helpful and hopefully one day lead to a better understanding of this subject.

 

[Stu Jackson]

Wow, another long topic about lightning. Nice that it's been spelled right.

Reinventing the wheel is always such good fun.