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Old 28-06-2008, 19:58   #1
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Aluminium Mast - Lightning Rod, Benefits?

My question is: what benefit is there in fitting a lightning rod to the masthead of a aluminium mast?

I am NOT wanting to debate the various merits of lightning protection methods in general (like size of grounding wiring, types of ground point etc) as these are already well done here and elsewhere (IMO).

I currently have an SS inverted brush rod bolted to my masthead. My rigger sold it to me when I asked him for lightning rod. He assured me it was the lastest in protection and IIRC, his reasoning was "it is what all the latest Eurpoean boats are using". I probably had "wood duck" tattooed on my forehead at the time.

Finally after starting to think about the issue, researching the internet, reading every post of CF on lightning - hundreds on them - and applying my electronic knowledge and applying some logic, I am fast coming to the conclusion that only effects I will get from this item will be:

1. Extra weight aloft - bad.
2. Disruption of the radition pattern of the masthead VHF antenna - bad.
3. Extra windage, possibily minor - bad.
4. Talking point - probably good.
5. General disruption to seagull roosting real estate on the mast head - very good.

The only possible benefit would have been if the rod actually reduced the chance of the strike and so far my reading indicates this is not so. That theory seems to be discredited.

However before I take it down, I would like to hear the considered opinion of CF family in case I am missing something.

By the way, I can't decide if it LOOKS good or bad although some have asked me why I have a toilet brush on my masthead - I guess I know what they think of its appearence
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Old 28-06-2008, 20:15   #2
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I currently have an SS inverted brush rod bolted to my masthead....

However before I take it down, I would like to hear the considered opinion of CF family in
G'day Wotname,

Have wondered some of the same things myself. I'm not a qualified sparky (like yourself). Having said that, the research I've come across tends to suggest using a copper rod with a spike (vs an SS brush) is more likely to attract the lightning strike (is copper more conductive than SS???). Like you say, we can't prevent a strike, but anything we can do to keep it away from our electrics has to be good.

As with your points (1-3), I could probably live with that (if I knew that the rod would take the strike). hmmm... will be watching this thread.
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Old 28-06-2008, 20:34   #3
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Well yes, cooper is much more conductive than SS but it is immaterial when it is just a foot or so bolted onto a say 40 ft Al mast. The current will travel down the mast, the rigging, any "lightning conducter" if fitted and it will be inversely porportional to the resistance of each path. So the rod is not taking the strike as such rather its an entry point of the strike. IMO.
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Old 28-06-2008, 21:27   #4
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it is immaterial when it is just a foot or so bolted onto a say 40 ft Al mast.
Thanks for the clarification. I'd seen some yacht designs that have 2 foot copper spiked rods above the masts with separate straight heavy AWG to separate large surface plates. They claimed it would take the brunt of the strike & help against electrics failure. hmmm... I now seem to wonder about their claims

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the rod is not taking the strike as such rather its an entry point of the strike. IMO.
If that's the case, then can the rod be below the VHF antenna on your mast? (or is that a bad idea?)
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Old 28-06-2008, 21:36   #5
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Wotmane, I think you answered your question pretty well with point 1 -4.

exfish, the conductivity means nothing in the end. You do not get struck because there is a better conductor connected nor are you safe because you are a poor conductor. The only reason a strike occurs is a "leader" is "sent up" in the first place. It is the path of the leader the actual bolt follows to ground. The point of those "toilet brush" things was that some thought if you could negate one long leader reaching up by causing it to become many shorter ones, then you may not get hit. But it has since proven to be wrong.
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Old 28-06-2008, 21:42   #6
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I am of the opinion that trying to lure a lightening bolt to your boat is not a good Idea. A bit like poking a stick in a hornet's nest. I have real serious doubts about the ability of a wire conducting that amount of voltage to ground without acting like a giant fuse and disintergrating. I prefer to try and disconnect everything from ground that is sticking up in the air when there is lightening, attempting to eliminate the path it seeks. I remember working in an aluminum smelter many years ago and saw a demonstration of what, only 300 volts DC with a 180k amp capacity can do to a 3/4 in diameter steel rod. It was like a flash bulb from an old camera. Better not to tempt lightening.
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Old 28-06-2008, 21:46   #7
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exfish, the conductivity means nothing in the end..... But it has since proven to be wrong.
Thanks for the info Alan. I was actually considering carrying 2 radome's & 2 head units[1] (due to lightning strike). But reading this thread, I don't think it would do any good, as the circuit board in the 2nd radome would still get fried.

[1] the 2nd radome would be disconnected from the head unit, the head unit would be disconnected from the power/ground.
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Old 28-06-2008, 23:17   #8
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the conductivity means nothing in the end....
I should qualify that . Because the lightning has just traveled several miles through a virtual insulator (Air) the last few meters via the mast means nothing to the strike. Conductors of varying resistance will still take all current to ground but the ability to conduct or more importantly not conduct, is seen as heat. So a poor conductor has a greater chance of being destroyed.
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[1] the 2nd radome would be disconnected from the head unit, the head unit would be disconnected from the power/ground.
Probably won't be safe even then. The safest measure is to place it inside say the oven. Metal casing all around (faraday case) stops the magnetic field from destroying the electronics. I can just see you now trying to stuff a radome in the oven ;-)
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Old 28-06-2008, 23:53   #9
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The safest measure is to place it inside say the oven. Metal casing all around (faraday case) stops the magnetic field from destroying the electronics.
Am going steel hull & house, not 100% sure whether that would help.

On the subject of "Faraday cage principle", wouldn't tinfoil wrapped around electronics be just as good? (have read about sailors putting small electronics in aluminum pots (with the lid sealed)).

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I can just see you now trying to stuff a radome in the oven ;-)
She'd have to be a big oven
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Old 29-06-2008, 01:53   #10
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Yes tinfoil would be fine.
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She'd have to be a big oven
Or a big boot or hammer;-)
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Old 29-06-2008, 02:10   #11
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Lightning Rods (Air Terminals) are placed on top of sailboat masts, not to dissipate any accumulated static charge, but instead to be the guaranteed harmless target point for a lightning strike.

Lightning dissipaters, such as the “Bottle Brush” Air Terminals, have been widely discredited and criticized by lightning researchers and professionals over the last 30 years.

Blunt or rounded Air Terminals have been shown to be slightly more effective than sharp Lightning Rods
, because they have lower breakdown voltages, and longer time to breakdown compared, to other air terminals*

The basic principles of lightning protection are:

1) provide preferential strikes point for lightning
(an array of conductors higher than the objects being protected), a good grounding system, and conductors between the two to conduct the damaging current from a lightning discharge away from the boat.

2) provide appropriate transient protection on power and signal wires entering the boat to protect equipment and personnel from the effects of induced lightning currents.

An Air Terminal (Lightning Rod) is a passive device, which serves as a sacrificial device when the lightning strikes it, rather than the occupants, boat, mast, or antenna.
The other components of the standard Lightning Protection System are the down conductor and the earth terminal. The function of the down conductor is to channel the lightning current safely from the Lightning Rod to the ground terminal. The function of the ground terminal is to safely dissipate the large lightning current into the water (ground) effectively. Together, they form a low impedance grounding system, which dissipates the lightning strike energy, with a minimal rise in ground potential .

The results of studies suggest that moderately blunt metal rods (with tip height–to–tip radius of curvature ratios of about 680:1) are better lightning strike receptors than are sharper rods or very blunt ones.

Dr. Charles B. Moore et al, reported* that the electric fields above the blunter rods were as much as two times stronger over greater distances than those above the sharp rods. This, he said, ''can be significant in the possible interception of an approaching lightning streamer.''
Moreover, Dr. Moore said, “the sharp rods create around their tips a dense sheath of electrified, or ionized, particles, which reduce the probability of lightning's striking the rod. In so protecting itself from lightning, instead of drawing it, the rod is not as likely to fulfill its intended function of diverting lightning from other exposed objects in the vicinity. “

Goto: Lightning Rod Improvement Studies
By C. B. Moore, William Rison, James Mathis, and Graydon Aulich
Langmuir Laboratory for Atmospheric Research (New Mexico Tech)

Abstract:
AMS Online Journals - Lightning Rod Improvement Studies

Full Text (from Journal of Applied Meteorology, May 2000):
http://ams.allenpress.com/perlserv/?...t-document&doi
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Old 29-06-2008, 05:22   #12
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I should qualify that . Because the lightning has just traveled several miles through a virtual insulator (Air) the last few meters via the mast means nothing to the strike...
Wheels, just want to correct a minor point here, as I understand it, the lightning doesn't really travel through the air, rather it travels along a path of ionized gas (ionized air in this instance). The ionized air has a much much lower resistance than air. The air is ionized when a sufficent voltage exists to cause the ionization procress, this occurs whenever any spark (big or small) is produced in air.

Off the topic, but have you ever noticed that you can't create a spark from a 12 volt battery by shorting out the terminals as the conductor is brought up to the battery post. The spark only occurs after the conductor is brought in contact with the other terminal and then removed. It is the back EMF generated by the collasping of the current flow in the conductor that raises the voltage high enough to ionize the air grap and allow current to continue to flow (as a spark). I don't remember the exact figure but IIRC, one needs more than 10,000 volts to ionize air at sea level. Of course we all know that shorting out a 12 volt battery is very dangerous and should never be done at home .

Gord,

Thanks for the information but my question remains is how does an "Air Terminal" bolted to the masthead of an Al mast really help. I figure the masthead itself would effectively be an "air terrminal" with the mast being the conductor. Of course it maybe not as perfect as a purpose built one.

If one considers the sacrificial aspects of a say a 3/8 copper rod as the air terminal, then I thinking the said copper air terminal could disappear very fast during a strike leaving the mast head becoming the "new" air terminal while there is still a considerable charge to be dissipated.

But perhaps I have the wrong end of the cat here and missing something basic.
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Old 29-06-2008, 13:12   #13
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To ionize air, you have to have immense heat. The bolt leaves a path of ionized air after the "spark". A leader is something slightly different and is not true ionizing as the correct use of the term exists. Science knows that leaders exist and they reaches up high into the air, but there is still a substantial jump of Bolt to leader through non-ionized air. And so far as science understands it, there is no other path. The same result has been proven in the lab with giant lightning machines. A enormous charge is raised and when the Pd reaches a high enough potential to jump the gap, it does so with out the help of leaders.
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you can't create a spark from a 12 volt battery by shorting out the terminals as the conductor is brought up to the battery post. The spark only occurs after the conductor is brought in contact with the other terminal and then removed. It is the back EMF generated by the collasping of the current flow in the conductor
Only if the conductor has a coil. The back EMF is a current produced by a collapsing mag field in the coil. But if you place a plain wire across the terminals, you will still get sparks when removed. That is due to a plasma being set up between the terminal and the wire. This plasma is the sole reason why we can arc weld. If the plasma and current can be maintained, the electrons will happily cross the gap via the highly charged bridge and create more heat. A soon as oxygen gets into the gap, the transfer stops and the arc ceases.
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"Air Terminal"
I did say this earlier, the brush was theorized as creating many smaller leaders instead of one leader reaching high up. But the theory is one thing, the reality seems to be another.
So another theory is, "well if you are going to get hit anyway", why not make something that the bolt will happily go to and lesson the excess going else where as much as possible. So copper being an excellent (and cheaper than other higher conductors), conductor, a Copper rod is used to "attract" the bolt to it instead of the mast. However, once again in reality, we are talking several million volts and several hundred thousands of amps. That takes a lot of conductor diameter to be effective. Remember that you are trying to tens or even hundreds of millions of watts of energy to ground at the speed of light. Any conductor not up to that job means the excess still waiting to go to ground will take the next available path = Mast.
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Old 30-06-2008, 00:41   #14
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To ionize air, you have to have immense heat. The bolt leaves a path of ionized air after the "spark". A leader is something slightly different and is not true ionizing as the correct use of the term exists. Science knows that leaders exist and they reaches up high into the air, but there is still a substantial jump of Bolt to leader through non-ionized air. And so far as science understands it, there is no other path. The same result has been proven in the lab with giant lightning machines. A enormous charge is raised and when the Pd reaches a high enough potential to jump the gap, it does so with out the help of leaders.

Only if the conductor has a coil. The back EMF is a current produced by a collapsing mag field in the coil. But if you place a plain wire across the terminals, you will still get sparks when removed. That is due to a plasma being set up between the terminal and the wire. This plasma is the sole reason why we can arc weld. If the plasma and current can be maintained, the electrons will happily cross the gap via the highly charged bridge and create more heat. A soon as oxygen gets into the gap, the transfer stops and the arc ceases.

I did say this earlier, the brush was theorized as creating many smaller leaders instead of one leader reaching high up. But the theory is one thing, the reality seems to be another.
So another theory is, "well if you are going to get hit anyway", why not make something that the bolt will happily go to and lesson the excess going else where as much as possible. So copper being an excellent (and cheaper than other higher conductors), conductor, a Copper rod is used to "attract" the bolt to it instead of the mast. However, once again in reality, we are talking several million volts and several hundred thousands of amps. That takes a lot of conductor diameter to be effective. Remember that you are trying to tens or even hundreds of millions of watts of energy to ground at the speed of light. Any conductor not up to that job means the excess still waiting to go to ground will take the next available path = Mast.
Air Terminal.

Perhaps I have not been clear in my posts regarding the “air terminal”. I am using the term in general way to include any sort of rod (copper, SS, round ended, sharp pointed, inverted brush etc) bolted to the Al. mast head to act as an entry point for a lightning strike. In my case, the body of the mast is the down conductor which is connected to the ground point accordingly.

I still think the mast head itself is effectively an entry point regardless of the “air terminal” and if struck, the fitted air terminal could quickly melt, disappear, explode (or whatever term one wants to use) leaving the mast head still the entry point.

I can see that if the air terminal is insulated from the mast and has its own down conductor (inside or outside the mast, the circuit is slightly different in so far the strike now has at least 2 options. One is enter at the air terminal, travel down the independent down conductor to the ground point with the other being to enter at the mast head, travel down the mast and then flashes over to find a path to the water. Not convinced how effective that really is. I am still thinking of just grounding the bottom end of the mast to an earth point - undecided at this time.

OK the following is not really part of the original post is a dreadfull example of thread drift so please ignore it if any of you are not interested.

Wheels, I beg to differ however please do not take offence as you may well know more about this than me – just consider the following as the rambling of a fellow boatman.

Heat is not required to ionize a gas (including air), it may be a by product on a large current flow through ionized gas. For example, a large quantity of heat is not required for neon signs, fluro tubes or vacuum tubes (CRT, triodes etc). All of these rely on the gas being ionized without input of heat.

As for plasma, wikipedia tells me it is just ionized gas so we are really talking the same thing. Plasma (physics - Wikipedia, the free encyclopedia)

My explanation of how it occurs given basic electrical principles are as follows and I can't say fully understand all the mechanics in the process but:

I reckon it requires lots of volts to initiate it and is dependant of the pressure of the gas and the type of gas. Dry air at a standard atmosphere of pressure is rather difficult to ionize compared to say neon gas at low pressure.

Perhaps my terminology was not 100% correct when I referred to back EMF which is a term normally associated with coils and relays etc. I was using it in the sense in induced EMF as occurs in all conductors whenever there is a change in value (or direction) of current flow.

All conductors (and I am referring to practical conductors that one might find in the average boat, garage etc) have some inductance. When there is change in current flow, an induced EMF exists due to the changing magnitude of the magnetic field (generated by the current flow) “cutting” the conductor – normally this is so small at DC (and low frequency AC) as not to be noticed hence we coil the wire to use the effect some whatever reasons we want – motors, relays etc.

Faraday tells us the value of the induced voltage is determined by the 3 factors; the inductance, the amount of flux and the time rate of change.

So back to the 12 volt battery, when one brings the + terminal very very close to the – term with a conductor, nothing happens as long as there is a tiny air gap between the terminals, the resistance of the tiny air gap is extremely high and no current flows. This tells me that 12 volts is not enough PD to ionize the air.

However when the circuit is made, current flows and a magnetic field (flux) around the conductor is established. Assuming the battery is well charged, has a reasonable capacity and the conductor is very low resistance, the current will be very high thus the magnetic flux will be large. Now when the conductor is removed from one terminal, the current is interrupted and begins to collapse to zero.

The induced EMF will become very high very quickly given Faraday’s law of induced voltages because although the inductance of a single wire conductor is very small, the collapsing flux “cutting” it is very large and the rate of time change is very very large. Therefore there is very large induced voltage in the end of the conductor as soon as it is removed from the battery terminal. This is large enough to ionize the very small air gap and once ionized, the resistance drops rapidly and current begins to flow again though the ionized air gap.

This is the spark we see. The current flow is now determined by the terminal voltage (12V), the resistance of the ionized air gap, conductor and internal resistance of the battery. The duration and intensity of the spark is determined by the charge available in the battery, size of conductor and the size of the air gap. Lenz’s law tells us the direction of the current flow remains in the same direction as the original direction of current flow.

We both know that experimenting like this with a charged battery is extremely dangerous but for those who don’t know the risks or dangers – just don’t do it or put your idiot cap before you attempt it.

If you want to see what happens with smaller arcs in water, have a look here. http://tesladownunder.com/TeslaPoolTinHat.jpg
Note the mesh glove on the right hand acting as a ground point in the water and the conductor from the "helmet".

This guy does some interesting stuff with ionized air and lives down the road a bit - that means a couple of hundred K's in west Oz
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Old 30-06-2008, 00:55   #15
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Well does the fact that you wrote all that and I read all that make us geeks or Nerds or what ever you guys would call...err, ones that would write and read that ;-) :-)
Quote:
I still think the mast head itself is effectively an entry point regardless of the “air terminal” and if struck, the fitted air terminal could quickly melt, disappear, explode (or whatever term one wants to use) leaving the mast head still the entry point
Or you may end up with a new form of alloy. Copper and Aluminium fused together. Hmmm, aluopper, coppuminium, ?? :-)
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