hedgehog antenna

[youen]

At the top of my mast there is a sort of antenna which looks like a bottle drainer .What is it for ?Thanks

 

[sailorman]

At the top of my mast there is a sort of antenna which looks like a bottle drainer .What is it for ?Thanks

to dissipate a lightning strike ???

 

[Bilgediver]

At the top of my mast there is a sort of antenna which looks like a bottle drainer .What is it for ?Thanks

There are lightening arrestors that look like that. Is there a wire to the keel or somewhere below waterline?

 

[Searush]

Seemples, it stops the seagulls landing & pooing all over your boat.






Seriously, such things are sold commercially & unless you can identify a cable & follow it to some devive, I doubt that it is an aerial. If you don't want it, send it to me & I will fit it on my mizzen.

 

[KellysEye]

It is a lightning dissipator i.e. meant to stop strikes, we have one fitted by the previous owner. There is no evidence, that I can find, that they work.

 

[Searush]

It is a lightning dissipator i.e. meant to stop strikes, we have one fitted by the previous owner. There is no evidence, that I can find, that they work.

But is there any evidence that they DON'T work? How often have you been struck?

 

[john_morris_uk]

The theory is that lightening strike starts with a 'leader' that ionises the air and turns it into an actual strike; the hedgehog thing works on the same principle that provides lightening protection on buildings. There is no way that much survives an actual strike, the hedgehog thing is supposed to stop it starting by dissipating the 'leader'. Does it work? Who knows, but that was the theory as explained to me...

People vastly underestimate the energy contained in a lightening strike if they think the little wire brush and the cable to the keel will survive an actual strike. The cable will be instantly atomised!

 

[KellysEye]

>But is there any evidence that they DON'T work? How often have you been struck?

No we haven't but so few boats have the wire brush that it might never have been tested. The view that it won't work came from a lightning expert.

> if they think the little wire brush and the cable to the keel will survive an actual strike. The cable will be instantly atomised!

There is no cable to the keel it's supposed to diffuse the lightning not conduct it.

We've seen a number of boats that have been hit by lighting in the tropics. It's difficult to tell unless the owner says the boat has been hit. One boat was obvious though the flag on the backstay was seriously scorched.

We had lightning hit the water about 30 yards in front of the boat, the Elmos fire in the rigging is awesome. You should try it sometime ;-)

 

[scoty]

spiked antena

spiked antena, over here, they are fitted on top of the mast,,,,to stop seaguls perching
and then not having to clean up their mess, when you arrive at the weekend.

 

[William_H]

Spikes on mast top

It is most likely that the spikes are deter gulls and other birds.
However the theory of spikes brushes etc to dissipate lightning is that the pointy ends concentrate any static build up so actual discharge occurs at a lower energy level low being more easily dissipated without damage. You find them on trailing edges of aircraft to dissipate static build up. Without the brushes or needles the static can rise a lot higher voltage so when it discharges it causes radio interference. olewill

 

[whipper_snapper]

> if they think the little wire brush and the cable to the keel will survive an actual strike. The cable will be instantly atomised!

There is no cable to the keel it's supposed to diffuse the lightning not conduct it.

There should be! As JM says, it needs to be grounded to 'diffuse the lightning'. THe theory is that as charge builds up above it, it sprays oposite charge thus reducing the attractiveness of the mast to a discharge. To do that it needs connection to ground potential. It is a bit like pulling the blankets over your head and hoping you won't be seen. You lift the ground potential to your mast head. The theory makes some sense but there is debate over its value in reality. Obviously, if it actually gets hit by lightening the spiky thing has failed to do its job and has paid the price by being vapourised.

 

[Searush]

S'funny that, I was always told in Physics that pointy bits attract lightning exactly because of the concentration of opposite charge. Like water choosing the path of least resistance to flow, electrons seek the nearest point of high potential.

That's why your mast is at risk, and nice pointy aerials on top with connection to earth do offer an easy path. However the energy dissipated will far exceed the ability of a wire to carry it, so it will diverge into all sorts of nearby paths. That's how lightning strikes take out hull fittings, speed & depth sensors etc leaving a number of nice tidy black holes in your hull - oh & no radio capability to ak for help with.

 

[Salty John]

At the top of my mast there is a sort of antenna which looks like a bottle drainer .What is it for ?Thanks

If it looks like a bottle brush with the rows of bristles extending horizontally from the centre shaft and has a wire connecting it to earth then it's a dissipator and completely useless at keeping seagulls off the masthead. It's also, in my opinion, completely useless at dissipating lightning strikes.

If the bristles point upward, with no wire connection to earth, it's probably a seagull repellent.

I've had a boat hit by lightning and it didn't have a dissipator - evidence to some, perhaps, that they work! Every item of electronics containing a chip or printed circuit was damaged - some items irreparably, some just needed a chip replacing. The masthead wind instrument transducer was vaporised. The internally earthed vhf antenna was unaffected but the radio needed repair.
An interesting experience that allows me to chuckle when people talk about gaining protection from such an awesome force by the deployment of bottle brushes at the masthead or bits of chain dangling from the chainplates..

 

[oldvarnish]

Try this for a read:

http://www.amazon.co.uk/gp/product/0963956604/ref=oh_o06_s00_i00_details

 

[CreakyDecks]

I have noticed that where I live there are spiky attachments on the top of the telegraph poles. The copper strip that is supposed to carry current to ground has long been stolen on most of them. So what the spikes will actually achieve in a storm, if anything, is to attract lightning to people's phone lines

 

[whipper_snapper]

S'funny that, I was always told in Physics that pointy bits attract lightning exactly because of the concentration of opposite charge. Like water choosing the path of least resistance to flow, electrons seek the nearest point of high potential.

That's why your mast is at risk, and nice pointy aerials on top with connection to earth do offer an easy path. However the energy dissipated will far exceed the ability of a wire to carry it, so it will diverge into all sorts of nearby paths. That's how lightning strikes take out hull fittings, speed & depth sensors etc leaving a number of nice tidy black holes in your hull - oh & no radio capability to ak for help with.

You are confusing the thinking behind a single pointy thing which attempts to act as a conductor after the event and a multi pointy thing which attempts to prevent a strike. You can picture a big charge in the sky pulling oppositely charged ions to the surface on the ground below, like a fully charged capacitor. When the charge difference overcomes the resistance of the air, a spark travels between the surfaces. Imagine one capacitor plate covered in spikes which shed charge and locally prevent a build up of charge difference.

That is the theory, like salty john I am sceptical.

And if it is not well grounded it can't do much more than discourage seagulls.

 

[Salty John]

Try this for a read:

http://www.amazon.co.uk/gp/product/0963956604/ref=oh_o06_s00_i00_details

Or you could read the Boat U.S. Marine Insurance and Damage Avoidance Report:

You Can Run, But You Can’t Hide
Volumes have been written about methods to mitigate damage or even avert a lightning strike. Lightning, however, doesn’t seem to read them. As an example, one boat, fitted with a popular “fuzzy” static dissipater at the top of the mast was struck twice in one year; ironically, the second time the bolt hit the dissipater even though the VHF antenna right next to it was higher (claim #0308082). Dewey Ives, a surveyor in Florida and member of the BoatU.S. Catastrophe Team who has seen his share of lightning damaged boats, says that lightning is unpredictable. “I’ve seen a small sailboat docked between two larger ones get hit and sometimes a powerboat in the middle of a marina filled with sailboats gets it. If lightning wants your boat, there’s not much you can do about it.” Ewen Thomson agrees, “Current research shows promise in mitigating damage from a lightning strike, but there is nothing that is effective in preventing a strike.”

Lightning does what it wants.

 

[mobeydick]

And if it is not well grounded it can't do much more than discourage seagulls.

And yet, why would lightning strike a mast rather than anything else around? Because it 'sees' a lower resistance path to ground there. So - possibly - it is enough, in combination with the pointy things, to discharge the potential.

Also, the conductors from the lightning rods on church towers (which DO work) seem way too thin, to me, to take the massive currents of a full lightning strike.

(Interestingly, there was a long debate on if lightening conducts should be spikes or balls when they were first being concidered)

md

 

[William_H]

Lightning

I think there is a little misconception here on static disipators. People have said they must be well earthed.
No the initial build up of charge from the ground end can be slow and quite low current although high voltage such that a high resistance earth connection will do the same as a low resistance earth connection.
It is when this tiny discharge from the mast provides an ionised discharge invitation to the main lightning strike that real current flows. It is then that any resistance results in vapourisation of current carrying conductors.
My belief is that a very stout electrical cable from the (Al) mast base to the iron keel is your best protection. A cable a lot heavier than your starter battery cable and as short as possible must help to divert current from damage to stay wires and underwater gear. Its not going to help your vhf connected to that aerial on top though.
Much depends on where you live. We don't get a lot of lightning here in Swan River colony but some areas of USA are terrible for lightning. good luck olewill

 

[martcouz]

I don't pretend to understand the physics associated with lightning strikes but I certainly have a full understanding of the consequences. We were struck by lightning in the eastern channel a few years back and everything at the masthead was vaporised; moreover, everything electrical throughout the boat was comprehensively destroyed. Of more relevance here, my insurers (whose support was nothing short of excellent) elected to fund the fitting of a dissipator at the main masthead when repairs were carried out. It wasn't my suggestion so I guess they agreed this with the repairer who relayed the proposal to me.

I haven't been struck by lightning since ... mind you, I now keep well away from thunderstorms !