Motorboats and Lightning

[dpb]

So Friday night we were anchored about 150m from the shore just out side the Wareham Channel (Poole Harbour) where we enjoyed a lovely blue sky evening and sunset.
Then 2:30 in the morning woke up to find ourselves in the middle of the most spectacular thunder and lightning show that I have seen. For maybe 20 minutes the lightning flashes were every second and in all directions and the thunder a constant noise.
Neither of us are scared of storms so we just enjoyed the show and the lightning was all in the sky bouncing between the clouds.
Then we saw a couple of bolts come to the ground at which point I wondered a bit more about our situation and decided it might be better to go back into the cabin rather than stood in the cockpit, allbeit under the canopy.
Eventually storm passed and we woke up next morning all ok.
I have since done a bit of googling and would probably be less happy about the situation if it occurs again (ignorance was bliss!). It seems the advice is to have some form of metal conductor from high point of boat to a ground plate in the water. It isn't a subject that gets discussed much it seems.
So were we in any real danger?
Does anyone have any knowledge or experiences to share on this?

 

[ari]

Nothing useful to add - just that I read the title and immediately sang (in my head) 'Very very frightening, Meee...!'

I'll get my coat.

 

[superheat6k]

I was once mid channel on a sailing boat in a heavy electrical storm. The sound of eggs frying on top of the mast is the enduring memory, then there would be an almighty crash and flash and it would go silent then the sound of frying eggs would return. This happened several times during the storm, but none of the boats electrical systems or electronics were affected, apart from complete loss of radio comms during the storm, which then was still Decca, pre GPS. I could not be sure if the boat actually had been hit, and I am not sure the GRP companion way hatch was providing too much protection.

On this boat there was a heavy steel lifting keel directly under the mast so providing a direct ground path for the lightning and we were unaffected, apart from having to change our underwear when the storm abated.

I later learnt the sound was St Elmo's fire, and is literally the trillions of electrons boiling off the top of the mast forming the ground feeder for a strike.

I did learn that whereas better to be avoided, in future to strap the anchor chain around the high metal work then drape this into the water to provide a decent ground path so a strike takes the path of least resistance to the water. I also learnt that a high level conductor suitably grounded provides a cone of protection beneath and around it, but would rather wished I had taken up golf instead of sailing at that point.

 

[Bouba]

I was once mid channel on a sailing boat in a heavy electrical storm. The sound of eggs frying on top of the mast is the enduring memory, then there would be an almighty crash and flash and it would go silent then the sound of frying eggs would return. This happened several times during the storm, but none of the boats electrical systems or electronics were affected, apart from complete loss of radio comms during the storm, which then was still Decca, pre GPS. I could not be sure if the boat actually had been hit, and I am not sure the GRP companion way hatch was providing too much protection.

On this boat there was a heavy steel lifting keel directly under the mast so providing a direct ground path for the lightning and we were unaffected, apart from having to change our underwear when the storm abated.

I later learnt the sound was St Elmo's fire, and is literally the trillions of electrons boiling off the top of the mast forming the ground feeder for a strike.

I did learn that whereas better to be avoided, in future to strap the anchor chain around the high metal work then drape this into the water to provide a decent ground path so a strike takes the path of least resistance to the water. I also learnt that a high level conductor suitably grounded provides a cone of protection beneath and around it, but would rather wished I had taken up golf instead of sailing at that point.

Do you mean put the anchor chain in the water and drape the chain over the railings? If so how do you know when to do it safely?

 

[superheat6k]

Do you mean put the anchor chain in the water and drape the chain over the railings? If so how do you know when to do it safely?

Never had to do it on a mobo, but the metallic high point needs to be in direct contact via metal to the water. Obviously you don't want to be fouling a draping chain on a prop, so I would drape it well forward, e.g. a loop from the normal fairlead, and only metre or so immersed with a wrap around a cleat, and then on to physical contact with the radar arch. Bearing in mind you don't want the hull to be thrashed with a wild chain under the water either.

The alternative here is assuming you have a decent bonding circuit is a decent piece of cable from the arch to the bonding circuit.

I would think about it as soon as I saw an electrical storm approaching.

I have been boating for almost 40 years and have experienced this just once in that time. I am not sure I would want to experience it twice though.

 

[Bouba]

Never had to do it on a mobo, but the metallic high point needs to be in direct contact via metal to the water. Obviously you don't want to be fouling a draping chain on a prop, so I would drape it well forward, e.g. a loop from the normal fairlead, and only metre or so immersed with a wrap around a cleat, and then on to physical contact with the radar arch. Bearing in mind you don't want the hull to be thrashed with a wild chain under the water either.

The alternative here is assuming you have a decent bonding circuit is a decent piece of cable from the arch to the bonding circuit.

I would think about it as soon as I saw an electrical storm approaching.

I have been boating for almost 40 years and have experienced this just once in that time. I am not sure I would want to experience it twice though.

Thanks for that. For a more practical approach, how about carrying a length of large diameter insulated copper wire that could be connected to the radar arch and draped in the water at the bow. What is your opinion?

 

[kashurst]

All the exterior metal work should be connected with fat wires back to either a ground plate or the underwater gear and anodes. As others have said hopefully creating a cone of protection and acting as a lightening conductor. The concept of why lightening conductors works gas lots of theories. No one is 100% sure why they mostly help. The cone of protection is also unproven but seems to help.

However if your boat does actually get struck by lightening it will destroy almost every electrical item on board. If you are lucky that's all it will do. So if you are in an electrical storm stick you hand held VHF and GPS etc in the oven or microwave. These will act as a Faraday cage and prevents any electrical discharge damaging them. Best solution if possible - move away and keep you head below the metal work.

 

[Croftie]

Probably safe... My Dutch steel will make a good Faraday cage, mind you the highest points are the Glomex TV antenna and the VHF aerial so guess they will be toast, still I'll be alive to replace them.

 

[Piers]

We're cruising the west coast of France at the moment. In Port La Foret we had to moor amongst the Vendee Globe yachts. Mind you, when the thunder and lightning started, I was only too pleased - their masts were far taller than our aerials.

 

[benchmark]

I have been out fishing with a friend not far from shore when we got caught in an unexpected drizzle . Nothing unsusal so we sailed on. On arrival at the fishing grounds we stepped out of the pilothouse to rig the rods which in all this time were stored away on the 'rocket launcher ' roof stainless steel rod holder rack . As we looked at the rods we got the shock of our lives, we could see and hear blue sparks hopping from one rod tip to the other. :ambivalence:
Needless to say , his rods were made of carbon which is conductive !