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**David M** · 18-01-2012, 21:43

After a lightning strike, a sextant, clock, paper chart and a compass might end up being all that is left.

jackdale · 18-01-2012, 21:49

Quote:

Originally Posted by David M

After a lightning strike, a sextant, clock, paper chart and a compass might end up being all that is left.

A Davis or an Astra sextant? Is this an argument for the plastic ones?

St. Elsewhere · 18-01-2012, 21:57

Quote:

Originally Posted by Ozbullwinkle

Just some lateral thinking here and to assist in my own knowledge. Why would it not be effective to keep your spare instruments in a solid wooden box? As far as I know wood does not conduct electricity if it is kept completely dry. Yes I know that trees hit by lightening are blown to pieces but I always thought that the reason for this was that apart from their height this was due to the amount of water and resins in the tree which acted as a conductor. Ok, shoot me down, but I thought it was worth asking to those who are more knowledgeable on this subject.

The reason that a faraday cage works is a basic rule of physics: similar charged particles repel each other. That means that electrons will reside only on the exterior surface of a conductive material. A conductive container, a faraday cage, will have zero charge on its interior surfaces. This is what allows aircraft to take a lightning strike, and to conduct it over its exterior without any noticeable effect on the interior of the aircraft. The same applies to a car. You will do better inside one, than being outside it if lighning should hit. Lightning will seek the least resistance to ground. The best strategy is to provide a path that does not include you or your electronics. Insulation, such as a wooden box, is only effective if there is a lower resistance path available.

s/v Holiday · 18-01-2012, 22:29

Would installing a separate fuse panel at the battery bank for all of your grounds keep current from running into your equipment? I am assuming that the equipment blows out because the lightening energizes the ground system of the boat. Just a thought.

CJ

daddle · 19-01-2012, 02:05

Quote:

Originally Posted by afmstm

In practice, you do need to be concerned about gaps in your shield.

The post by afmstm is mostly correct. But I would think for the cage to work in you will need most edges connected along their length. Current needs to be able to flow in all directions around the cage (unless you can predict the orientation of the lightening strike's field. Doubtful.

Also I doubt an oven makes a good faraday cage as the door edges are not connected electrically to the adjacent sides. Same probably true for a microwave.

Probably does not matter much which metal is used for the conductors of the cage. S/s would be fine.

Grounding the cage will not make any difference in it's effectiveness as a cage.

daddle · 19-01-2012, 02:06

I keep my beers in little aluminum faraday cages. Just in case....

**GordMay** · 19-01-2012, 03:54

Greetings and welcome aboard the CF, adellefave.

You might be interested in ➥ http://www.cruisersforum.com/forums/...rts-47798.html

**Pblais** · 19-01-2012, 04:53

The high power worker video shows the situation quite well. Note 3/4 of the suit is for fire protection not electrical and he is not grounded - ever. The Faraday suit works. In a lightning strike you are grounded and the voltages could be 10 or more times higher than the largest power line. The risk of fire is perhaps the greatest risk. A near miss is far more likely and even at that the path of the voltage can come from any direction. Having your spare gear in a dry bag not grounded probably is as safe a thing as you can do. It should be easy to grab as the boat sinks. Most strikes are not of the huge variety and even those are mostly near misses as lightning can induct a big jolt just by getting close. It's still an issue. On land, livestock can be killed at a 100 yards. Theoretically, a large bolt hitting your dry bag directly could vaporize it from just the heat alone (the Faraday cage too).

Of the two strikes we have had at our dock. The electrical wiring was inducted and radios and instruments were the common items damaged though you never saw anything as a far as burning nor were many other things more sensitive or connected similarly. We know one of the strikes hit a tree a 100 yards away as it also trashed the boat owners TV and telephone in his house as it did in his boat instruments at the same exact moment.

In a near miss, not being electrically connected protected all those items in both the house and boat. The same was true the second time though we don't know where it struck. Our boat was not plugged into power and was saved and our house was about an extra 300 yards away. If you add up all the odds, being lucky is most likely, and a near miss is possible yet not common. In a full direct hit your odds are not good given the multiple ways you might be killed either directly or as a result of the damage done to the boat.

Having a ready escape plan would seem the best investment you could make since it would cover MANY other risks that could take your life that have nothing to do with lightning. A single emergency plan that is well planned and practiced will be the best insurance you can have when time is very critical. Consider the odds of being hit with a meteor? The same escape plan works. The number one disaster to be very afraid about is fire! 80% of those are electrical.

teneicm · 19-01-2012, 05:11

Quote:

Originally Posted by Tashtego

Why not just get an Aluminum brief case. Make sure the two sides are electrically connected through the hinges and latches (use your DVM).

I thought about this, but I was concerned that the lid and base would only be electrically connected via the hinges. I was (and still am) uncertain if, to be effective, a faraday cage requires more or less total continuity around all edges of the cage.

**GordMay** · 19-01-2012, 06:36

Quote:

Originally Posted by teneicm

I thought about this, but I was concerned that the lid and base would only be electrically connected via the hinges. I was (and still am) uncertain if, to be effective, a faraday cage requires more or less total continuity around all edges of the cage.

Yes - I believe that the shielding must be more or less continuous & contiguous, within the limits of the pulse’s wave length, to achieve omnidirectional protection.

stevensuf · 20-01-2012, 13:05

an old biscuit tin will do the job!

Nicholson58 · 20-01-2012, 14:23

I am not sure I agree regarding the need for total continuity at the door of the oven or microwave. I am sure that its better than no preparation at all. I have an Oregon model Garmin hand-held. I fugure if it gets whacked in my pocket the hit was too close. I suggest that owning a hand-held GPS and two or three spares stored around is good insurance for serious offshore cruising.

Fuses might help if you are lucky. Its hard to imagine what happens when you are hit with a brazillion volts. Fuse melts and the power arcs the opening anyway. In the house, we unplug the TV and other appliances. I know this has helped in our summer cottage where we are struce often.

I first started racing on a CAL 40 in the olden days. It was struck and the strike exited the hull by the metal through-hulls. These were mounted in Nylon bushing that melted. The boat sunk in its slip. (Lake Huron)

I helped transport a Heritage One-Ton Cedar Point to Cleveland (Lake Erie) and watched a picket fence of bolts strike around us at about 100 yards for almost 20 minutes. What a show but no hits.

The boat we are restoring was apparently hit before we bought. Radar, alternator, charge controler, GPS, mast instruments - all trash. We were hit on the hard in the fall two years ago and our new Raymarine wind transducers were burned.

By the way, the alternator was re-built at a truck alternator & starter shop in Grand Rapids for 60 bucks. 14 of 16 diodes blown. What a deal! I hope to get another rebuilt for spare from the same shop.

mikereed100 · 20-01-2012, 14:51

Quote:

Originally Posted by Jesse

Perhaps a crab pot with the sides,top and bottom bonded would do the trick?

I experimented with this and it protected my electronics from lightning very well, but when I pullled the trap up the salt water had ruined everything.