Lightning strikes

[William_H]

I did wonder if I should have wrapped the anchor chain a couple of turns round the bottom of the mast and dropped it into the water to give a better ground path. Would that have improved the chance of less catastrophic damage? Would that have increased or reduced the chance of a strike?

Yes I think it would reduce the damage if the boat were struck. Providing a better current path mast to water than the path mast to water via bilge, wiring, stays and wet hull. (all of which would get hot (vaporise) with current.
Re your own safety holding the tiller that is an interesting question. I think I would stay at the tiller assuming you did not have an autopilot. Certainly don't touch any metal like stern rail or backstay. Perhaps hold the tiller with something insulating like rags or plastic. The thing is that the huge current, mast to water, can produce volt drop across almost anything that current is flowing through and you need to make sure you are not part of that volt drop and an alternative current path. Have you heard the theory that you should stand on one leg while standing on earth because current flow in a strike can produce a volt drop in the earth between your feet? Is this a real way to avert harm? Is there a parallel on a boat? I dunno!
ol'will






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[Laminar Flow]

I have been in a lightening storm at sea off the African coast. I have never been more terrified in my life, especially since we were by far the tallest thing about. All around us the strikes were hitting the water, lighting up the sea when it struck and you could see it spread out, like cracks in ice. I hastily attached some heavy cables to trail from the upper shrouds and no doubt I would have lit up just as prettily in the process of doing so, if I had been less unlucky.

When we were in Panama, a yacht was hit every day. On fellow had just received a shipment of electronics, still boxed and sealed. When his boat was hit, it fried every single device on board, including all the new boxed stuff.

 

[ProDave]

Re standing on one leg. It is known that cattle who of course have 4 legs have died standing close to a tree that was struck. the current passing through the earth generated enough voltage between their legs to electrocute them.

They say if you want any electronics to survive a strike, put the handheld VHF, your phones and a handhelg GPS if you have it in the oven (Faraday cage)

 

[Plymsea]

They say if you want any electronics to survive a strike, put the handheld VHF, your phones and a handhelg GPS if you have it in the oven (Faraday cage)

Is it not the EMP that fried electronics? While electrical systems need to be passed through, so they would be protected by a faraday cage assuming they and the cage are not grounded.

 

[Dan Tribe]

I had a lightning strike or a near miss a few years ago.
It fried the VHF antenna, and receiver and the Navtex but other electronics survived.
I'm surprised it doesn't happen more often. A big lump of lead or iron sitting in salt water with an aluminium spike pointing to the sky. Perfect lightning conductor.

 

[William_H]

"I hastily attached some heavy cables to trail from the upper shrouds and no doubt I would have lit up just as prettily in the process of doing so, if I had been less unlucky."

Not such a good idea if you have stainless steel shrouds. The resistance of ss is such that the high current will melt or at least soften the ss. Far better to attach the cables to the ali mast base then to the water. Ali having low resistance can carry a lot of current without damage. ol'will

 

[Graham376]

"I hastily attached some heavy cables to trail from the upper shrouds and no doubt I would have lit up just as prettily in the process of doing so, if I had been less unlucky."

Not such a good idea if you have stainless steel shrouds. The resistance of ss is such that the high current will melt or at least soften the ss. Far better to attach the cables to the ali mast base then to the water. Ali having low resistance can carry a lot of current without damage. ol'will

On many boats, if the anchor is down there will be a direct route down the forestay, through the stainless stem fitting and down the chain.

 

[Laminar Flow]

"I hastily attached some heavy cables to trail from the upper shrouds and no doubt I would have lit up just as prettily in the process of doing so, if I had been less unlucky."

Not such a good idea if you have stainless steel shrouds. The resistance of ss is such that the high current will melt or at least soften the ss. Far better to attach the cables to the ali mast base then to the water. Ali having low resistance can carry a lot of current without damage. ol'will

Wooden mast and galvanized shrouds.

 

[Laminar Flow]

On many boats, if the anchor is down there will be a direct route down the forestay, through the stainless stem fitting and down the chain.

Rarely anchor mid ocean due to lack of rode.

 

[coopec]

On many boats, if the anchor is down there will be a direct route down the forestay, through the stainless stem fitting and down the chain.

Maybe a good idea to lower the anchor into the water?

Rarely anchor mid ocean due to lack of rode.

Why not dangle a cable (chain?) into the ocean from the forestay?

On my yacht all the chain plates are connected to the earthing strips (but not the forestay)

 

[AndrewB]

About 20 years ago my steel yacht was hit by lightning while anchored in the Intra-Coastal Waterway, Florida. It was probably a side-strike. It took out the VHF and GPS aerials, the radar antenna, the autohelm fluxgate compass: all of these fixed to the mast but as a precaution disconnected from their devices. The steering compass developed 40 degree deviation. Surprisingly the mast navigation lights survived.

There was no damage to anything inside the yacht, including me. However, there was a 5cm wide stripe outside of the yacht from the port shroud chainplate vertically down to the bottom of the keel beneath the water, where all the paint had been taken off leaving a shiny metal surface. I assumed that this marked the route the lightning had taken to earth, not though the anchor chain.

 

[Graham376]

Rarely anchor mid ocean due to lack of rode.

I wasn't answering you, I was answering William's comment about damage to stainless rig.

 

[coopec]

Another thing I've read is that lightning doesn't like going around corners.

(Apparently even the experts don't fully understand lightning)

 

[Poey50]

Another thing I've read is that lightning doesn't like going around corners.

I've noticed from what research there is that the destructive nature of a strike varies widely - it is not all or nothing. The biggest variable seems to be whether the boat is on fresh water (worse) or salt water. Anyway reducing the odds of having a majorly destructive strike seems worthwhile.

Accordingly, I'm planning a simple grounding method to mitigate damage that takes into account the need to avoid bends as far as possible, that is temporary, has a sufficiently large grounding plate in strip form, and takes into account that positive charge is found on the surface of the water.

It is a length of 50mm2 cable temporarily bolted to a pre-tapped hole high enough up the mast (somewhere from waist to head height from deck) to allow a smooth curve of cable (well above 90 degrees) over the port side of the boat (electrical runs being on the starboard side of my boat). The lower end of the cable will attach, just below the surface, to one end of a strip of copper 60mm x 4mm x 1500mm giving a total surface area for a grounding plate of approx x 2 of the minimum size of a grounding plate recommended by ABYC with plenty of edge length. The far end of the strip to be supported also just below the surface with a float which itself is attached by line to the boat such that it can either be stationary or towed. The mast will also be permanenlty topped with an aluminium rod (air terminal) just higher than the VHF to attempt to protect the mast-head instruments when the grounding is deployed. (I'll experiment with that to see how much it affects the VHF.)

Lightning doesn't generally come as a bolt from the blue so there will be time to set up the system when it threatens. I reckon it could be deployed in 5-10 minutes. Most of the time this will be at anchor but it is also a system that could be deployed when underway (with some additional support for the cable). There is of course no bonding system so side flashes to other metal are still possible but presumably a good grounding system would reduce the probability / extent of their destructiveness. Being temporary resolves the problem of a permanent connection of copper terminal to aluminium mast. Not being cross-bonded and permanent removes concerns about galvanic corrosion.

Less bang for my buck - I hope.

 

[ProDave]

How about just thick wires from the mast base down to the keel bolts (Iron keel)?

 

[Poey50]

How about just thick wires from the mast base down to the keel bolts (Iron keel)?

A few reasons: my keel is epoxied, the keel is anyway less suitable for discharge (apparently), I couldn't get a straight run to my keel with a cable, a permanent connection of cable to mast would need regular maintenance because of the copper - aluminium connection and corrosion issues, and lightning led down the centre of the boat might be more prone to side flashes than when led down outside.

 

[oldharry]

Another thing I've read is that lightning doesn't like going around corners.

(Apparently even the experts don't fully understand lightning)

Yes, Ive heard accounts of lghtning going down a deck mounted mast and puching straight from the mast through the hull. A keel mounted mast might actually do better if it is mounted directly above a long ballast keel. Or not.

 

[coopec]

How about just thick wires from the mast base down to the keel bolts (Iron keel)?

"A long strip is more effective at dissipating the electrical flow than a square plate of the same area. "
Electricity prefers to exit the plate via an edge, so the plate should not be faired into the hull."

When lightning strikes

 

[Poey50]

"A long strip is more effective at dissipating the electrical flow than a square plate of the same area. "
Electricity prefers to exit the plate via an edge, so the plate should not be faired into the hull."

When lightning strikes

I hadn't read that one, thanks. On the basis of that I'll amend my plan (above) to grind some grooves into the edges of my submerged / towed copper strip (stopping short of weakening it) in order to increase the edge area.

 

[vyv_cox]

I hadn't read that one, thanks. On the basis of that I'll amend my plan (above) to grind some grooves into the edges of my submerged / towed copper strip (stopping short of weakening it) in order to increase the edge area.

That is one of the articles that came up when I was researching my PBO article. Sensible stuff. What is a little difficult to follow is how this research that they mention was carried out. Or is it all theoretical?