Lightning protection

[geem]

Several years ago we went through a very nasty lightning storm on route from Bonaire in the ABC Islands to Grenada. Lightning all around us. We didn't get hit. At the time, it was the worse lightning we had ever experienced. Recently, whilst on passage from Antigua to the Azores we went through by far the worst lightning event we have ever experienced. For 2 hours we endured hundreds and hundreds of strikes all around us. It was relentless. This probably rates as the second most scary event at sea for me ever. We dropped the genoa and turned on the motor with the idea of motoring away from the storm as quickly as possible. On the radar the storm was 25 miles wide. It simply engulfed us far faster than we could motor away.
So, why didn't we get hit? Both masts on our boat are connected via 50mm wide copper strip to a sintered bronze grounding plate. The theory being that the top of the masts have no more potential than the surrounding sea. Twice now we have been in conditions that you would believe we would certainly get hit but we haven't. Is this grounding doing the trick?

 

[Roberto]

FWIW, I got hit on a boat with grounded mast, it was a through-deck mast and short thick strap from the base to a nearby keel bolt. It burnt all the plastic insulation of the copper cable. (Apart from antenna, vhf radio etc). On my present boat I did not bother to ground the mast.
I'd love to tweak probability laws and think once one has been struck there will not be a second time

 

[RunAgroundHard]

The sea is more conductive than your boat hence the discharge between sea and cloud is far more probable than sea, to boat, to cloud. It’s one of the arguments for not fitting lighting protection and making the boat more conductive.

That’s what I have read, no idea how factual that is.

 

[geem]

The sea is more conductive than your boat hence the discharge between sea and cloud is far more probable than sea, to boat, to cloud. It’s one of the arguments for not fitting lighting protection and making the boat more conductive.

That’s what I have read, no idea how factual that is.

How does the boat become more conductive than the sea if it is at the same potential?

 

[geem]

FWIW, I got hit on a boat with grounded mast, it was a through-deck mast and short thick strap from the base to a nearby keel bolt. It burnt all the plastic insulation of the copper cable. (Apart from antenna, vhf radio etc). On my present boat I did not bother to ground the mast.
I'd love to tweak probability laws and think once one has been struck there will not be a second time

The theory is that the lightning is trying to get to the sea surface not to a depth lower than the sea surface. Theory says you install grounding plates a couple of feet below the waterline and connect to these. There are two schools of thought. Provide a route for the lightning strike. These involves huge diameter cables to carry the very large current or earth the top of the mast to the sea such that the top of the mast has the same potential as the sea.

 

[Baggywrinkle]

Several years ago we went through a very nasty lightning storm on route from Bonaire in the ABC Islands to Grenada. Lightning all around us. We didn't get hit. At the time, it was the worse lightning we had ever experienced. Recently, whilst on passage from Antigua to the Azores we went through by far the worst lightning event we have ever experienced. For 2 hours we endured hundreds and hundreds of strikes all around us. It was relentless. This probably rates as the second most scary event at sea for me ever. We dropped the genoa and turned on the motor with the idea of motoring away from the storm as quickly as possible. On the radar the storm was 25 miles wide. It simply engulfed us far faster than we could motor away.
So, why didn't we get hit? Both masts on our boat are connected via 50mm wide copper strip to a sintered bronze grounding plate. The theory being that the top of the masts have no more potential than the surrounding sea. Twice now we have been in conditions that you would believe we would certainly get hit but we haven't. Is this grounding doing the trick?

The attachment of a grounded lightning rod to a building is a protective measure that is taken to protect the building in the event of a lightning strike. The concept of a lightning rod was originally developed by Ben Franklin. Franklin proposed that lightning rods should consist of a pointed metal pole that extends upward above the building that it is intended to protect. Franklin suggested that a lightning rod protects a building by one of two methods. First, the rod serves to prevent a charged cloud from releasing a bolt of lightning. And second, the lightning rod serves to safely divert the lightning to the ground in event that the cloud does discharge its lightning via a bolt. Franklin's theories on the operation of lightning rods have endured for a couple of centuries. And not until the most recent decades have scientific studies provided evidence to confirm the manner in which they operate to protect buildings from lightning damage.

Physics Tutorial: Lightning

I guess that the first protection mechanism may be what stopped you getting hit.

 

[Roberto]

The theory is

I must have read > a hundred pages of studies, papers etc related to the matter, I am really not convinced by any of the two solutions (grounded or not), at least not to make any significant modifications on a boat, both seem to have their respective merits and boats with the two systems have been hit with various levels of damage. The only certainty I have is a strong lightning direct hit can destroy a boat, whatever system it has or hasn't, maybe one of the two solutions might shift the risk of being hit/damage from 0,0....1% to 0,0.......1%, whatever order of magnitude, I guess I'll live with that.

 

[Neeves]

In the days of SSB many yachts had sintered grounding plates. Apart from lightening strikes I can think of no reason to have one now as yachts now would be less likely to have an SSB, there are better options.

We had a sintered grounding plate and SSB on Josepheline as our Bureau of Meteorology broadcasts weather forecasts, local and blue water, twice a day on SSB

Geem - why do you have a sintered grounding plate?

Has the incidence of lightening strikes changed (does anyone collate data)

Jonathan

 

[Roberto]

Has the incidence of lightening strikes changed (does anyone collate data)

Changed over time I do not know, there are lightning strike density charts of the world you might maybe find historic data, Florida, Panama, Equatorial regions etc have very high densities, another area (oddily) is the Adriatic sea, I have sailed a lot there in the past and in say 100 night crossings Italy-Croatia or back we have had maybe 20 with lightnings, a real nightmare

 

[geem]

In the days of SSB many yachts had sintered grounding plates. Apart from lightening strikes I can think of no reason to have one now as yachts now would be less likely to have an SSB, there are better options.

We had a sintered grounding plate and SSB on Josepheline as our Bureau of Meteorology broadcasts weather forecasts, local and blue water, twice a day on SSB

Geem - why do you have a sintered grounding plate?

Has the incidence of lightening strikes changed (does anyone collate data)

Jonathan

We used to use SSB but removed the set as we stopped using it. I wired up the masts to the ground system after discussion with a lightning protection engineer I knew at the time.

 

[noelex]

We were hit by lightning several years ago on our previous yacht. The risks are real.

My understanding is an effective grounding system has a very beneficial effect on reducing the severity of a lightning strike, especially the more severe strikes that can blow large holes in a fibreglass yacht. Grounding has a small beneficial effect in reducing the chance of a strike, but unfortunately the effect is not great.

Catamarans are hit about twice as frequently as monohulls, but unfortunately grounding systems are more difficult to fit on this type of vessel.

 

[Sea Change]

Catamarans are hit about twice as frequently as monohulls

Any idea why this is the case?

 

[Sea Change]

A few years back there was a lightning expert on the 59⁰N podcast. He had previously been in charge of issuing the 'go, no-go' for shuttle launches whilst flying around Florida in a Learjet.

His take was that it was a good idea to fit a spiky brush to the top of the mast, and to drift with the wind. The idea is that you accumulate a sheath of ionised particles which makes a protective barrier around your boat. The lightning will travel through the sheath rather than your own boat.

It could be voodoo but I did think the guy's credentials were pretty solid.

 

[noelex]

Any idea why this is the case?

There have been many theories and few definitive answers.

 

[jamie N]

I believed that lightning travels from the surface upwards, towards the atmosphere/clouds, and not down. However this NOAA doc gives better info on it.

 

[Laysula]

. On my present boat I did not bother to ground the mast.
I'd love to tweak probability laws and think once one has been struck there will not be a second time

Is that a bit like carrying a bomb onto a plane as the odds of there being two bombs on a plane are pretty remote.

 

[franksingleton]

I cannot answer the OP, but the LightningMaster, Home is worth considering. They have been in business for many years now. For a moderate cost, this passive device could save you grief. We have one and survived a strike that was extremely close. The theory is that it prevents a build up of ions at the mast head. I am sure that it does that but not sure that it will always prevent a strike. After our experience, I always recommend it.

 

[Bouba]

I cannot answer the OP, but the LightningMaster, Home is worth considering. They have been in business for many years now. For a moderate cost, this passive device could save you grief. We have one and survived a strike that was extremely close. The theory is that it prevents a build up of ions at the mast head. I am sure that it does that but not sure that it will always prevent a strike. After our experience, I always recommend it.

How much did your system cost ?

 

[franksingleton]

FWIW, I got hit on a boat with grounded mast, it was a through-deck mast and short thick strap from the base to a nearby keel bolt. It burnt all the plastic insulation of the copper cable. (Apart from antenna, vhf radio etc). On my present boat I did not bother to ground the mast.
I'd love to tweak probability laws and think once one has been struck there will not be a second time

That is a bit dodgy. If you throw two dice and get 2 6s, that is a 1 in 36 chance. The chance of your next throw being another 2 6s is still 1 in 36.

 

[franksingleton]

How much did your system cost ?

Back in around 2005, about £50. I think costs now are about £250, but please check. For that kind of cost and the fact that it might well prevent a lot of damage, it is well worth it. Certainly better than doing nothing and just hoping.

Plus costs of bolting or riveting to the mast.