lightning deflector and VHF aerials

catmandoo

catmandoo

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There is a device which looks like an upside down SS lavatory brush which when fitted to the top of ones mast tells big bad lightning flashes to go away .

Can any one advise if its effect is reduced or negated if the VHF antenna / possibly whip aerial projects above it /forums/images/graemlins/shocked.gif
 
Yes i believe there is a device, but if it work's or not i dont know.

I wonder what other people think?
 
There was an old woman hauled up in a basket
Nineteen times as high as the moon;
Where she was going I couldn't but ask it,
For in her hand she carried a broom.
Old woman, old woman, old woman, quoth I,
O whither, O whither, O whither, so high?
To sweep the static right from the sky!
 
I've only seen one on a carbon mast - where there is a definite earth strip ...

There needs to be continuity between the device and the sea - the sea being the Earth!! if you see what I'm saying.
 
There was ae winsome wench and walie,
That night enlisted in the core ,
( lang after kenn'd on Carrick shore)
For mony a beast to dead she shot ,
And perished mony a bonnie boat
-------------

Her Cutty Sark o' Paisley harn ,
That while a lassie she had worn ,
In longitude tho' sorely scanty ,
it was her best and she was vauntie -
Ah little kenned the reverend grannie
That sark she coft for her wee Nannie
Wi twa pund Scots ( 'twas a' her riches )
Wad ever grac'd a dance of witches

/forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif
 
Your question assumes that the bottle brush dissipater is actually effective. As one who has been struck by lightning on passage I have followed the development of these dissipators with great interest. Unfortunately they seem to be scientifically discredited by some serious organisations. Having abandoned hope of protecting my boat from further lightning strikes by using the bottle brush I have disposed of much of the research I did. However, the following summarises the position; it comes from a commercial ligthning protection firm, the name of which now escapes me:
"Lightning dissipaters: The idea that a device might be able to prevent a lightning strike is very appealing to the average sailor. Devices that attempt to eliminate or reduce the incidence of lightning strikes generally have a bristly appearance caused by multiple conducting points. Under the influence of an electric field under a thunderstorm, it is undisputed that these multiple points release charge into the air in a similar manner to the phenomenon of St. Elmo's fire. How effective these charge flows are at eliminating or reducing lightning has been the subject of investigations by NASA, FAA, the Departments of Army and Air Force, NFPA, and FDOT. None of these agencies consequently supported their use. Scientific papers by scientists in reputable journals have also been negative. In 1994 Donald Zipse, IEEE Fellow, compared the conventional Franklin air terminal with, amongst other systems, multipoint discharge systems and concluded "The claims of being able to dissipate any and all lightning strokes have been shown to be untrue." In a subsequent study that considered only lightning elimination devices employing the point discharge phenomenon, (that is, lightning dissipaters) Abdul Mousa, also an IEEE Fellow, documents many instances when lightning struck towers at Kennedy Space Center and Eglin AirForce Base, and one case of a strike to an FAA control tower in Tampa. In his paper Dr. Mousa concludes that "Natural downward lightning flashes cannot be prevented." His phraseology "natural downward lightning" is carefully chosen, but does cover the case of a sailboat mast. Perhaps the reason why these devices are still popular is the rave reviews they get from the telecommunications industry. When a dissipater is at the top of a telecommunications tower on a mountain it does appear to lower the incidence of lightning strikes that originate at the tower, that is, upward lightning. Unfortunately, this is only applicable to this type of lightning, which is not the type that predominantly strikes boats, or other structures on a flat earth".
At least they don't seem to do any harm. Sorry about the long post.
 
I think js48 is probably right.
Multiple sharp pointed discharge points do dissipate accumulated static. They are standard on aircraft wing and tail trailing edges where they encourage earlier (lower voltage) discharge into the atmosphere. They are usually in the form of a pair of needle points of s steel or a floppy fabric kind of brush of presumably conductive strands. They are primarily fitted to reduce radio interference from large static discharges produced by the aircraft flying at high speed through the dry atmosphere picking up a charge.

Now I understand that under some conditions a mast top can exhibit St Elmos fire being a visible continuous discharge.( as do parts of aircraft) I don't know if this would be eliminated by the mast top brush discharger or perhaps more likely would simply concentrate it to the brush tips.
I would imagine a discharge from a mast top would actually encourage a lightning strike but then as the discharge is only visible for a few inches it would not significantly change the chances of a strike from a cloud thousands of feet up.

It is interesting how lightning strikes are described as an initial leader rising from the ground to facilitate the strike from cloud down to ground. This presumably is what js48 source describes as natural downward lightning.

In answer to the original question I think a VHF antenna would negate the effect of any dissipator if it extends above the dissipator when we are talking lightning. i would expect however the dissipator would still (or perhaps) dissipate static.
I seem to be raving on when I don't really know much about the subject. I would go for robust Al mast earthing as the best protection from lightning damage. olewill
 
I'm following this thread with interest.

Any suggestions as to the best practice for bonding in an encapsulated keel to the mast? I understand that to use either a dyanaplate or a skin fitting risks the destruction of the fitting and its ability to keep the water out!
 
We in South Africa have one of the highest lightning strike density in the world and we also have a lot of thatch roof houses. The houses do have high lightning poles but I have never seen auch a device mounted on top.

Also having had a strike on my house some years back the power in the strike destroyed my TV antenna and melted a 6 sqmm earth cable, so I dont think this device would last long
 
200 yards from strike, depressed water surface enough to see it,ALL equipment had no power, the next day the Decca failed along with alt main diode faulty, the Decca suffered fron the Varta battery blown off the ciruit board and the vhf went to its maker. A direct hit makes me sweat as to what would have been the end result.
 
Again for my humble opinion
Encapsualted keel. I think the dynaplate or earthing of prop skin fittings etc is still your best hope. Yes the bolts to the dynaplate and bearings etc in the drive train will take a beating possibly to the extent of melting bolts etc. I can only suggest large electrical connections. If you have real concerns at the time you could drop an additional conductive plate in the water connected by heavy wire to the mast. How heavy? Proably any size might be inadequate but starter or welding cable size would be a start. Have a look at the conductors on old churches etc copper bar down the wall from the steeple seem to be adequate.
The current path from the base of the Al mast to the sea in a boat with encapsulated keel is speculative but would probably find a way through the wiring to the engine and prop. Or worse still via damp fibreglass. (which would explode like a tree) The problem is that it will almost certainly find a path which it will destroy before finding another path. So you are trying to give it the lowest resistance from the begining. If that path gives small holes in the hull from dynaplate bolts melting then that may be a preferred small damage.

With regards to electronics. A nearby strike carries a huge current which tends to radiate like a radio transmitter or the winding of a transformer. Any electronic equipment with wirng going into or out of the electronics will receive a large voltage spike from the wiring. ie the longer the attached wire the greater the induced voltage spike. So a stand alone HH GPS or VHF may survive OK but a VHF attached to a cable up the mast and 20ft of power wire in the boat will recieve a jolt enough to kill it. Like wise wind insttruments etc.
I thnk the best bet is to disconnect all plugs possible if a strike is imminent. A further prrotectioon is to put equipment into the metal oven box. Obviously a steel boat has more protection inside but the external wiring will stil get fired up.

So that is my humble opinion. Your boat is probably safer in a marinna or protected anchorage because of proximity of taller masts buildings or hills. Most boats however spend most of their time on a mooring alone and you don't get much lightning damage so you must assess the risk yourself but I reckon for me the chances of a strike damage are pretty low. olewill
 
Funny how everything gets mixed up here. Though none of the experts have the answer (NASA and such /forums/images/graemlins/cool.gif /forums/images/graemlins/cool.gif What do you expect, they cant even figure out a styrofoam strike let alone a lightning strike).

Not even the manufacturer of the "upside down broom" claims that they prevent lightning strikes 100%.
Here are some thoughts which may be closer to facts than fiction, hearsay or assumption.

There are three parts of lightning protection.

1. Preventive
any device which discourages the buildup of ionization (which otherwise makes the air more conductive than good for comfort) helps to reduce the risk. There comes the dreaded broom

2. Passive

A solid as possible path to ground starting from the mast tip.
Now the mast itself comes in handy (as long as it is metal /forums/images/graemlins/grin.gif) from there make sure that some substantial means connect it to the water (forget copper, it vaporizes) better galvanized steel (inspect one of your favorite down town office buildings lighting protection system to get an idea on the wire errr rod size required).

This makes sure that if you get hit, most of the energy takes the preferred path and spare your seacocks, "moisturized" GRP and other little items you treasure.


3. Active

All your electronics and electrics may suffer even from a strike a few 100m away from the electromagnetic pulse. The only way to offer them some protection is the installation of so called transient arrestors. Little electronic devices which get installed into your DC, VHF, GPS, RADAR cabling with the purpose to establish a short circuit to ground in case the voltage exceeds levels harmful to the connected device.
Those prevent the secondary effect of lightning but NOT the consequences of a direct hit.

Having all three components sorted out your risk is dramatically minimized and in the rare event of strike even your favourite insurance company runs out of the typical arguments.

Now let me also clear up a few common misconceptions:

1. Lightning strikes always the highest mast
Wrong. Lighting takes the path of least resistance (or multiples thereof [by branching out]
2. I am safe in the marina because there are many other larger boats
Wrong. More masts increase the chance of a strike IN the marina and all adjacent boats suffer secondary damage or get a good measure of juice from a branch off.

3. Out at sea I am more vulnerable on a sailboat due to my high mast.
Negligible

4. By grounding I attract lightning
Negligible. However if you do not ground and get struck you may loose the boat.


I hope that sheds some light on the discussion.

Happy sailing /forums/images/graemlins/cool.gif
 
[ QUOTE ]
I would imagine a discharge from a mast top would actually encourage a lightning strike but then as the discharge is only visible for a few inches it would not significantly change the chances of a strike from a cloud thousands of feet up.

[/ QUOTE ] Hi Will,

My understanding of the latest research is that although your statement would seem to be intuitively true, the reverse is actually true.

Lightening strikes start with 'leaders' forming to the object to be struck. The theory is that the lightening conductor dissapates the leaders and reduces the risk of a strike.

Someone else made a comment on the size of the conductor on church towers etc. Even this (large) conductor is not designed to take a direct hit. It is there to reduce the possibility of 'leaders' forming as above.

You are quite right in saying that the induced emf from a local stike is usually what causes the electronic failure and that a direct strike is very rare.

If you do get a direct strike the forces are HUGE. Things just explode....
 
Following on this vein, as I understand it lightning occurs when the particles (water droplets, dust etc) build up a charge, say a negative charge. Since like charges repel each other, this huge negatively-charged cloud forces free electrons out of everything around it - trees, masts, hair, the ground, etc. thus causing a positive charge in all of these things. The massive electric potential in the clouds cause the air around to become ionized, which leads to the formation of leaders. A similar thing happens in the positively-charged articles, which then give off "feelers". When a "leader" connects with a "feeler", the circuit is connected and a lightning bolt is the result. My personal theory is the stainless toilet brush devices don't dissipate the positive charge, as that is not really possible, but they can disrupt the formation of the "feelers" or reduce their length, so that the "leaders" and therefore lightning will likely connect with a more 'attractive' target than your mast.
 
I think that's where most people start, then they start reading around the subject, and realise how complicated it is, and the forces involved with direct hits.
 
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