Thunderstorms?

[SteveGorst]

This week I sailed through a couple of thunderstorms on passage from Holyhead. Does anyone have any theories on the dangers and precautions that can be taken as from down in the cockpit the mast looked like an awfully good lightning conductor.

One person has suggested tying a rope around the mast and trailing it in the water. I'm not sure as I think this may just make the boat more attractive to the lightning by providing a better earth.

Any thoughts?

Steve

<hr width=100% size=1><A target="_blank" HREF=http://"www.nwcc.info">North Wales Cruising Club</A>

 

[doug748]

I was just re-reading an old PBO (No 443). Alan Watts said, amongst other things:
"... these storms are not specifically dangerous to small craft. Lightning strikes very rarely hits yachts at sea.......Yachts are more likely to be hit in the marina... I wonder if the reason is that when sailing the masthead is in continual motion, so a pre strike leader cannot develop from it.
He continues to suggest you think about a lightening conductor system......but with no great enthusiasm.

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[Keith]

You don't have to take a direct hit for it to ruin your day, my boat had a strike close by (while sailing) i think at about fifty metres but i was going to check as i was diving through the door!! the upshot was it knocked out my radar, depthsounder both temporarily but it fried the vhf aerial but not the radio.........keith

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[AuntyRinum]

The advice used to be to unplug all the electrics and trail a chain attached to the backstay. Never heard of anybody actually doing it. I did once meet somebody who got a lightning strike in Chichester Harbour. The boat was seriously damaged and his wife had serious back injuries.

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[RPC]

If you read all the information on the following link you should have most, if not all, your questions answered for lightning strikes on yachts and how to reduce the chance of a strike.

Have you visited the Blue Water Cruising Site <A target="_blank" HREF=http://www.onpassage.com>http://www.onpassage.com ?

 

[SteveGorst]

Thanks, this site was very interesting. For yachts in salt water they suggested a 4 guage copper wire connecting the mast to a one foot square piece of metal in the water.

It occurs to me that I have a lifting keel which is wound up and down by a mechanism attached to the foot of the mast. As long as the grease on the bearings and gears doesn't have a high resistance I have probably got a circuit from my mast directly down to the cast iron keel, which vastly exceeds one foot square. I'll just have to find a way of testing the connection without arranging a lightning strike.

It would be interesting to see what the statistics are for lightning strikes on yachts compared to all the other safety features we worry about.


<hr width=100% size=1><A target="_blank" HREF=http://"www.nwcc.info">North Wales Cruising Club</A>

 

[Samphire]

Simple answer is to sail a steel yacht,it gives you a perfect earth to dissapate strike. The risk with grp/wood etc is even trailing wires/chains etc from stays or backstay is that you will literally risk frying the rigging due to mega electrical charge involved, with obvious consequences .Samphire.

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[Dominic]

Re: Thunderstorms = Wind and Lightning

About a hundred miles east of Charleston (on the east coast of the US) you get these wonderful thunderstorms, perfectly formed and probably fuelling up from the warm water of the Gulf Stream.

In the centre of the thunderstorm is a strong downdraft. The trick is to try to sail to the windward side of the thunderhead. On the lee side the downdraft induced gusts are added to the ambient wind while conditions are a much milder ( ! ) on the windward side.

Having been unable to dodge one in the dark and ending up with strikes onto the sea on all sides ( A "buttock clenching" experience that had me pulling all the circuit breakers and hiding down below - in a steel hull) I am suprised that yachts get lightning strikes so rarely.

It´s the gust from the downdraft that get you - strikes on yacht occur but are strangely rare.

I suggest you should be more worried by the wind gusts and downdraft and ignore the lightning.


(A deduction from a non-statisticaly valid sample)


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[MainlySteam]

Re: Thunderstorms = Wind and Lightning

<<<strikes on yacht occur but are strangely rare>>>

Because the strike generally takes the shortest path and if one looks at the geometry from the points of origin above of strikes to the ground, then ones mast either has to be very high or the origin of the strike pretty much exactly overhead for the boat to have much chance of being struck. Lightning does not bother to go sideways looking for yachtmen to fry and any specific point on earth has a very low probability of being struck.

The same geometry shows why even comparatively low antennas on steep hill tops have a high risk of being struck.

If one also considers the number of times most of us have sailed with the strikes immediately around us ie flash and thunder clap are coincident within a small fraction of a second, then the risk can be seen to be low just from that.

Of course, some think lightning goes looking for mirrors, forks, hairpins, boat electronics and other assorted metal to zap on purpose, but anyone who believes that is obviously a lost cause and will worry themselves to death long before the lightning finds them.

John

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[Richard_Woods]

Re: Thunderstorms = Wind and Lightning

Even though lightning strikes are rare in Europe that is not the case elsewhere.

I was hit by lightning on the east coast of the USA last year when sailing my 32ft catamaran Eclipse.

There is planned to be a full report and suggestions for safeguarding your boat and equipment in the October issue of PBO.

The bottom line is, if you plan to sail outside Europe then fit some form of lightning protection!!

<hr width=100% size=1>Richard Woods of Woods Designs for quality sailing catamaran designs
www.sailingcatamarans.com

 

[MainlySteam]

Re: Thunderstorms = Wind and Lightning

I do not live in Europe and was talking from a worldwide perspective. However, despite what you say, parts of Europe, being southern Europe/Mediterranean, in fact have a comparatively high lightning strike rate for a sea area with parts little less than that for Florida seas (no doubt because of its encirclement by land).

I have endured some very violent thunderstorms around the world (especially in USA) and I think that you will find that the explanation I give is relevant everywhere - the area of sea that ones mast "protects" and so draws the strike to it is very very small.

Even in the highest strike areas of the USA being Florida, East Coast and inland to mid west (which have as high a frequency of strikes as anywhere) there are only around 20 strikes per sq.km per year but that is only over land as the strike frequency is very much less over the sea. The rate reduces to 10 or less off the coast of Florida/East Coast USA which is one of the few places in the world where strike rates are other than minimal over the sea . Most everywhere else at sea the strike rate is generally less than 2-3 per sq.km per year for the few "high" strike coastal areas (the Mediterranean gets up to around 8 in some areas) and less than 1 (almost always very much less, so as to be very rare) elsewhere over the sea.

If one is in one of the few very high strike rate areas of 10 strikes per sq km per year and ones mast "protects" a radius of 2 boat lengths about a 12m boat (an approx 50 m dia circle), then the probabilty of a strike on the mast is approximately 0.002 per year. For most sea areas it would be less than 0.0001. The risk is obviously less with shorter masts in smaller vessels.

The strike rates quoted are from NASA satellite surveys of lightning strike rates.

Obviously the probability of a strike increases with the frequency of storms but the risk is still low. As with being eaten by sharks, there will always be those that have suffered a strike but that does not mean that the risk is high and worth losing sleep over. In any event, even with protection, I suspect that the results of a full strike (rather than leaders, side strikes, or induced emf) on a small yacht are always going to be severe and unavoidable.

In any event, after all that, we sail a metal boat /forums/images/icons/cool.gif.

John

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[charles_reed]

The downdraft

is far more dangerous than the lightning - though I've had the horrible experience of having my hair rise and heard the sound of "singing" in the rigging.

This was just outside Douarnenez, I'm glad to say I wasn't the final earth, an unfortunate cow in the field above Pointerailing de Leydé sacrificed herself, but the flash and explosion were nearly sufficient to persuade me I'd actually been struck.

The only electric failure (I'd turned all electrics off) was the fuse in the vhf power supply.

I researched the subject - lots of reports of lightening strikes, usually with entirely random results, from the odd fuse blown to holes burnt in decks.
It seems being at anchor, or trailing a bight of chain, is considered the appropriate response, but taking that course of action didn't seem to reduce the damage from lightening strike.

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