Lighting dissipator

[southchinasea]

Does anyone know where I can obtain one of those chimney sweep brush lookalike
lightning dissipators (Mail order preferred). I think they are made by Forespar.
No need for the debate on whether or not they are effective thanks!
Cheers
Jonathan

 

[tonyleigh]

Interested to know how this works and whether it is worthwhile? (sorry - know you don't want discussion!!!) Question prompted because of lightening strike this summer in Iles Chausey which appeared to be very efficiently earthed but caused rather a lot of damage to electrics and wiring (not to mention the sleep of the 2 occupants.

 

[halcyon]

Also interested in comments, our mast has been connected to a large grounding plate by the previous owner, partly for the SSB he had fitted for trans-atlantic.
Understand lighting stricking mast forms a Faraday cone ?, clearing the boat. If the mast is earthed all the energy passes throught the conductor to the ground plate, what happens to conductor/mast, ground plate is between the heads input and output skin fittings, were's the energy go ?, see hole blown in hull!.


Brian

 

[Chris_Stannard]

Advice from Najad's electrical guy was to have a pair of jump leads on board. These serve the dual purpose of corrs connecting batteries if required and acting as lightning conductors in a storm, you attach one end to the shrouds and the other dangles in the water. Apparently this is the practice in sweden where I understand summer thunder storms are quite common.

As regards the mast forming a Faraday cone, I have my doubts. As I understand it lightning from the sea to clouds is caused by the build up of a positive charge in the cloud. However the electron flow is from positive to negative, therefore from the sea to the cloud and not the other way round. The appearance of the lightning coming down from above is an optical illusion, because it all happens so quickly. The boat , with its mast, shortens the path to the cloud and the electron flow takes the line of least resistance. Your boat would need a very thick copper wire to take a lightning strike from the grounding plate to the mast.

Of course I could be talking b***s and, no doubt if I am, will be corrected.

One other thing to be aware of is that a friend of mine,who was sailing when his boat suffered a strike, ssaid that in addition to blowing the electrics and electronics, the strike also knocked them out for a few seconds. He put this down to the high induced magnetic field that accompanies a high current flow which affected their brain patterns.

Chris Stannard

 

[Guest]

Try either http://www.sailnet.com or www.defenderus.com Both are mailorder places in the US. If you know someone who has an address in CA or elsewhere, have it delivered to them and then on one of your planes to you.

I know you didn't want to hear about effective or not, so I won't tell you about the three boats that got hit by lightning here in our marina in SW FL and all three of them had those things on their masts.....but I won't say a word about that, or the fact that they were surrounded by masts that didn't have them.

Or are you going to do something else with that thing???

In reality, the battle pro and con rages back and forth.....and yet despite all the evidence, confusing at best....no-one knows which is the best way to go......

Reality is the cause of all stress!!

 

[southchinasea]

No I had no intention of doing anything else with it other than stick in on the
top of the mast!!!

Actually I am fitting a Guest dynarod and ground plate but a friend also has the dissipator and argues that the more 'protection' the merrier! Incidentally he has also fitted an AC power surge protector which tripped twice during severe electrical storms. This may give credence to those that think shore power can act as a conduit?

In response to Tony Leigh's post, I'm really no expert but I quote (Forespar)...
"...it reduces the build up of static ground charge and retards the formation of the
ion 'streamers' which complete the path for a lightning stike. The dissipator employs the point-discharge principal to continually leak off the lightning causing ground potential in low amperage over a long period of time instead of the short duration, high amperage, discharge occuring with a lightning strike".

Frankly I have no idea whether or not it works, but like you, we have had 3 strikes this year in the marina I will be using when my boat arrives next week. One liveaboard couple lost everything electrical/electronic and the underwriters are
forking out over U$40K! They had a nasty deductable/excess to find themselves.

Cheers
Jonathan
Hong Kong

 

[HaraldS]

I think the Forespar brush you can order from West Marine, but it is a rather small one.

I have mounted a Dual-Tech SP-1000 dissipator on my masttop. Got it from Jatco Marine, 643 East Avenue, Warwick, RI 02886; tel.: +1-401-828-5590.

The SP-1000 has 2700 stainless steel needle points, in a bottlebrush like arrangement, covered by a slotted plastic sleeve. And on top it has a regular pointed lightning terminal. It mounts on a standard VHF antenna bracket.

I have read about everything on lightning research I could get my hands on, and the theory of how they work sounds plausible. However what won me over was a statistic from a Florida cell phone tower operator, which showed a over 100 : 1 reduction in lightning strikes to the towers, once they were equipped with dissipators. Though quite big ones. I have also seen them on many drilling towers. In general it seems they are more popular outside the boating scene.

I wanted to make sure I did what I could to minimize the risk of a fatal lightning strike. So for prevention in the first place I mounted the dissipator, the SP-1000 is for boats under 80 feet and Jatco recommended not to choose the next size down, called the SP-500 with 1250 needle points. (there is also an even bigger one with three of the 1000 style brushes mounted together). That sizing is probably a very approximate art, but certainly more points ionize more atoms, hence a wider area of inverse charge around your boat.

I also looked what I could do to minimize the damage if a lightning strike hits and asked my yard to bond the foot of the mast directly to the keel, (I have a non encapsulated lead keel), and also to bond all chainplates to the ground system. (Of course that is no issue on a steel or aluminum boat)

To make the dissipator work, you need a well grounded mast anyway, so doing this with the right wire size and underwater surface is only one extra step. Then there is a lot you can do to reduce the chance of frying your electronics, and I did what seemed sensible. But I'm sure, you'll never be perfectly safe.

 

[kgi]

I don't have one on my boat, but i have taken a lightning strike close astern of the boat, that fried the vhf and left the radar with a speckly screen

 

[kgi]

I don't have one on my boat, but i have taken a lightning strike close astern of the boat, that fried the vhf and left the radar with a speckly screen,oh it also deafened the depth sounder which didn't read right for days

 

[Gunfleet]

also it made all your messages appear twice

 

[Fabian1956]

How to Ground

I also looked what I could do to minimize the damage if a lightning strike hits and asked my yard to bond the foot of the mast directly to the keel, (I have a non encapsulated lead keel), and also to bond all chainplates to the ground system.

Hi

I'm at that ponit of doing of the grounding to the keel. I’m running a AWG 1 wire from the foot of the keel to a buzz bar (1”x1/4” tinned copper) which has two smallish bolts connecting the bar to 1” naval bronze keel bolts into the lead keel. I feel it necessary to eclectically connect the keel to the water. Have you done this and how?

Good post thanks

Fabian

 

[WindermereColvic]

The appearance of the lightning coming down from above is an optical illusion, because it all happens so quickly.

Almost, but not quite. There are in fact two strikes, One Ground-Sky, the other simultaneously Sky - Ground.

One up, one Down.

Oh and re the blacking out/ EM pulse thing, not my area of expertise, but i doubt it. More likely he was just s*iting himself.

 

[William_H]

Lightning Strikes

The little brush dissipators are fitted to the trailing edge of aircraft surfaces to help dissipate static electricity build up. The static build up will reach a large amount and discharge causing interference on radio particularly HF. The dissipators are also often in the form of a needle point. They certainly work to dissipate static build up.
In a lightning prone atmosphere they would presumably produce from the points a discharge some what earlier than you would get from from the flat top of your mast to producing a discharge which is less powerful. At least that is what I imagine would happen. Certainly churches and tall buildings often have a pointed tip to the lightning conductor which would attract the lightning to that point and perhaps at an earlier time so less energy discharged.
I am convinced however that providing a good low resistance path to ground (sea) is the best protection against the damage of a high current discharge of lightning. I would not use the stay wires because the high resistance of stainless steel would mean they just overheat and fuse very quickly. The aluminium mast is the best low resistance conductor and a lead or steel exposed keel the best connection to the sea. They need to be connected by very heavy cable.
When the only path to the sea is via wet fibreglass then just like lightning hitting a tree the moisture will conduct the current and boil instantly. This production of steam is what will blow holes in the fibreglass or wood. So for encapsulated keels you need to provide an earth plate or use the prop etc but ensure that there is a low resistance path to water. Small gaps will be breached by the current with arc damage. Resistive parts of the circuitwill be overheated by current flow.
As i see it your best insurance is provide a low resistance path that will bypass current flowing through any electronics. A VHF antenna on top of the mast will probably become the first conductor. The gap at the insulation of antenna to mast will be jumped by an arc but the voltage drop across the arc will probably take out the VHF set with smoke. This connection of the coax to the antenna will probably carry current via the radio to the -ve power system then via the stern gear top water demolishing all the wiring as it goes. So an antenna with DC connection to ground (mast) may help to reduce the current through he radio. Best remove the antenna connection at the radio if you expect a strike.
Or like me just hope it won't happen good luck olewill

 

[peterb]

Inductance

The aluminium mast is the best low resistance conductor and a lead or steel exposed keel the best connection to the sea. They need to be connected by very heavy cable.

Not just low resistance, it needs to be low inductance as well. The very high rate of change of current can induce high voltages at any sharp bends in the conductor. A conductor that goes directly down from the mast to the keel is best, but one that has right-angle bends round doorways, etc., may induce side-flash.

 

[oldharry]

Fascinating subject and some very interesting input this time round. Being one of the original wimps when it comes to being afloat just below a 10m metal pole with lightning around, I too have read quite extensively on the subject.

Accounts from people aboard boats that have been struck suggest that thast close to the discharge the faraday affect is minimal anyway. There are many recorded accounts of people receiving severe shocks from metal objects when there is a strike nearby, so directing the current through an earthing plate might just reduce the discharge slightly, but will still cause mayhem to electronics and personnel on board.

My understanding of the lightning mechanism is that a step leader descends from the cloud, seeking a pathof least resistance. Recent high speed photgraphy (on Tv, cant find it on the net yet) shows that these branch out in all directions to quite a distance from the main descending step leader.

The increasing static charge in the air causes ground leaders to reach up from various points - leaves, grass, twigs, your mast etc. One of these makes contact with the step leader, and completes the path for the main discharges of several 10s of thousands of amps - usually destroying the original object from which the groud leader was generated.

The object of lightning protection is either to prevent the ground leader from developing in the first place, or to provide a conductive path through which the discharge can safely pass with minimum destruction. The latter is the principle involved behind 'lightning conductors' on buildings which provide a safe path from the top to the bottom of the building without damaging the structure or occupants.

Preventing the leader from forming in the first place is more difficult as the science is not properly understood yet because of the random nature of lightning. It is well established however that a pointed object such as a VHF aerial is far more likely to generate a ground leader than a flat or smooth surface. The object of the brushes OP asked about is to provide a multiplicity of points, so that no one of them can generate a strong enough leader to make contact with a descending step leader.

American research of commercially available lightning protection systems tended to confirm the brush as being fairly effective, while some of the more sophisticated appliances actually appeared to attract lightning, not dissipate it!

As to hanging things over the side to provide a path, I rather suspect that this is more likely to allow an ascending leader to develop in practice - but that is only an assumption on my part. Boats on swinging moorings certainly get hit fairly regularly - there was a recent spectacular posting of a moored boat being struck only a month or so ago. But as boats spend most of their time on their mooring, it would be difficult to draw any conclusions one way or t'other.

I concluded that if lightning is going to hit, theres not a lot you can do about it, but that earthing the rig is marginally more likely to assist creating a ground leader - so I don't.

 

[William_H]

The Farady cage

I think that a mast will provide a reasonable faraday shield in an area under and around the mast so that any person typically standing on the deck of a boat under the mast will be shielded form the possibility of being struck. ie the lightning will not hit his head. However this does not mean he is not susceptible to receiving a shock. The magnetic field form a mast carrying 10s of thousands of amps in a rapidly rising waveform can induce currents/ voltage potential difference in other metal objects or even conceivably the body. So don't touch any metal object if lightning is around.
Secondly the low resistance mast will still develop a large voltage drop because of the current so that current may try to flow via other paths ie radio antenna coax and light wiring so that much of the electrics will be at a high voltage to ground (sea) Almost everything you touch could give shock. Some current will flow via wet hull. Hopefully because of the earthing of the mast the current through other parts will not destroy those other parts ie hull and stay wires.

Regarding the leaders rising from earth that start the strike, being low current these do not require a low resistance path. Hence a tree can produce a decent leader with ionisation enough to carry the strike. So it is that the inherent connection of the unearthed mast/stays to the sea via wet fibreglass electrics etc will be enough to instigate the leader. The problem being when the real current comes back the resistive conductor parts fuse causing great damage.
So I still reckon try to appease the current by getting it into the sea with minimum resistance and damage.
All just theories of course olewill

 

[oldharry]

Well, in 50 years sailing, I never got struck by lightning , so perhaps my theory works....or not.