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Old 24-09-2009, 12:22   #31
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Originally Posted by sandy daugherty View Post
Its like radar reflectors. Bogus.
I have a bit of trouble dealing with this.
I have watched, on my radar, sailboats go by with the old cardboard radar reflectors working like a charm, while sailboats with the skinny tube tipe goes by with hardly blip in the screen.
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Old 24-09-2009, 13:02   #32
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Seriously, and it may be related -somehow- to all this; I recently found that my mast (& of course the rigging) has DC- on it... I coincidentally ran into this while replacing my anchor light assembly and have been quite concerned about it. Now, I realize that DC- is connected to the engine block and it "may" be somehow connected to the rigging and/or keel bolts. I'll be digging deeply to see where the source is.

Does anyone know whether this is normal and I am chasing ghosts - or is it something that needs to be corrected (my current position)?


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DC + Rigging= electrolysis. Not a good thing- the rig should be at 0V to ground.
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Old 24-09-2009, 13:16   #33
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DC + Rigging= electrolysis. Not a good thing- the rig should be at 0V to ground.
Thanks for your response - it is consistent with my thought - I think I have a cracked wire insulator on the mast or some sort of short I need to chase after.

Sorry about the thread drift - - Now, for more about that sparky thing that comes from the Gods above.... ....

Sailndive
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Old 24-09-2009, 13:17   #34
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I'm sorry to add to the thread drift towards radar reflectors. But some have apparently not read the accident report and radar reflector tests from the sinking of the yacht Ouzu by a ferry (with the loss of all three crew) I believe it's been mentioned several times on this board but maybe not for sometime. It should be required reading for cruisers.

The radar reflector tests showed particularly poor performance from the thin Plastimo tubes reflectors - as also noted by Vic de Beers. Here's the link:

Marine Accident Investigation: Ouzo

It's unsettling reading. Not for bedtime!

Carl
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Old 24-09-2009, 14:04   #35
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Swedish boats twice as likely to get hit by lightning ...

Swedish boats twice as likely to be hit by a bolt,
Thus - Swedish CATS four as likely to get hit !!!

Proof 1 - in 6 years of cruising I have never seen a Swedish flagged cat,
Proof 2 - the only first hand accounts of getting hit I heard were from Peter Mikkelsen and from Thomas Swan, both Swedish (monohull) sailors.

Now, the accounts being from monohull sailors, no wonder wise Swedes stay away from cats !

b.
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Old 24-09-2009, 14:56   #36
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Factors like the larger spacing and fact catamarans are often further out on the dock certainly makes sense to me as reasons they would see more lighting strikes. Another factor I did not see addressed in the articles, is that his data is likely based on insurance claims. On average, one sees many more less expensive, more lightly outfitted monohulls than catamarans. It could be that more of the lighting strikes to monohulls go unreported for this reason. - The damage isn't worth filing a claim on.
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Old 24-09-2009, 14:59   #37
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The radar reflector tests showed particularly poor performance from the thin Plastimo tubes reflectors - as also noted by Vic de Beers. Here's the link:

Marine Accident Investigation: Ouzo

It's unsettling reading. Not for bedtime!

Carl
Plastimo is horrible and the Firnell blipper isn't much better. The IMO spec is for .8 square meter equivalent and IIRC only the Luneberg lens models came close, and only the large versions of those. None tested as well as the active repeater (by a large margin).
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Old 24-09-2009, 15:06   #38
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Barnakiel you are right, not many cats in Sweden. Sailors are very conservative here, when it comes to hull, other stuff we adapt quick,. And there is not so much room for cats here in our archipelago could be a factor.
We still like full keel, we can get some weather here./ harry
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Old 24-09-2009, 17:22   #39
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Harry: I know - my own boat an Allegro 27, S-299 ...

Depends on which archipelago you are talking about! Because there is plenty of space in 'my' part of Goteborg Archipelago ...

;-)))
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Old 24-09-2009, 22:26   #40
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If I would sail a cat, my worry would not be the statistical chance of being hit more often than a mono hull.... my worry would be about what would happen to me and the boat when lightning strikes. For some reason, only few in this thread share that point of view.

I am not a professor of electric engineering, but I listened to them very well when they tried to make me an engineer ;-) The best non-technical way of thinking about what will happen is to imagine yourself being the lightning bolt coming down for your boat. You want to get to ground and you want to do that as easy as possible. You don't need conductors because you have enough power to create them from air (ionizing the air) but conductors make it easier so if available you use them. You reach the masthead and you have the mast itself and the stays/shrouds as easy paths. You split forces and take all of them down. Now you come to the lower end, what do you find? Hopefully copper conductors ending in thick bronze plates under water. If there's no conductor, you will ionize the air again, seeking for the easiest path down to ground. Hopefully your body isn't near because that is a far better conductor than air. For a monohull, it's easy to make a permanent path from the bottom of the mast to ground, for cat's it's much harder and often only a temporary solution, which should be deployed while at anchor or moored, is possible.
Chains or wires hanging overboard from the shrouds and mast provide a path.

The "brush" type dissipaters are a different thing. I do not believe the claims that they can prevent a lightning strike, but I do believe that when properly installed, they do help to make you less attractive so that hopefully the strike seeks another, easier target. Properly installing them means that they must be grounded. No ground means they are useless.

cheers,
Nick.
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Old 25-09-2009, 09:54   #41
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As an Electrical Engineer in a former life with some knowledge of lighting protection for explosive ordinance factories... I can say that lighting is a scary thing and not always well understood.

One poster asked about attaching a copper wire to the chainplates and running it into the water... Probably not a good idea. Lighting protections has two major factors... prevention and damage avoidance/dissipation if hit. The overall best protection from damage from a lightning strike is the development of a Faraday Shield basically an encompassing series of conductors that envelope the item to be protected. Some feel the standard metal rigging on a sail boat would provide a somewhat effective Faraday Shield but it is far from ideal... or even good. With the Shield the lighting would strike it and not the protected item. Problem is the application/ design of the Faraday Shield. Difficult on land and even more difficult on a boat. What a Faraday Shield dose is raise the probability of strike to be on an engineered element where it is attempted to direct it to a discharge path that would less likely harm the protected item.

Two though paths exist on this subject. One is Actually moving the Ground potential to a high point above the protected item which would be like having deep rods attached to heavy copper lines which attach to numerous lighting rods around the protected item... the down side is you would Attract more hits than if none existed... you never hide under the tallest tree on a golf course type thing. However if the bleed-off engineering were good most of the damage potential would hopefully be abated. Some designs actually have active elements in the lightning rods to polarize them so that they will attract at a higher level than simply ground.

The other method is having a similar protection grid that is detached from a ground signal or has an active element that would make the lightning rod appear less desirable until a strong potential is detected by sophisticated detection equipment then and only then would it be switched to attractive potential. These systems use more of a Spark Gap type theory. Where if hit the lightning would jump a small gap to reach ground but the small gap would prevent the elevated lightning rod from appearing to be a more ideal target.

Lots of people have played around with the spark gap approach in boats and it seems to work in test conditions but not as well in real life conditions in ocean environments. The major problem being that the gap effective is defeated in a salt water environment when the gap is expose to it or its spray or just general salty environmental conditions. Some companies have developed a sealed spark gap but the same problem occurs when the exterior of these sealed units are salt encrusted or simply getting spray when a strike event occurs.

Bottom line is that is appears Cats are struck more often due to the damn fur on them than sea dogs in "real boats"....

In real life we evacuated the munitions plant when ever lightning strikes occurred within a 4 mile radious... not very practical in any sail boat.
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Old 25-09-2009, 10:09   #42
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In real life we evacuated the munitions plant when ever lightning strikes occurred within a 4 mile radious... not very practical in any sail boat.
Well theres a few chests we don't need to worry about puttin' medals on

But what we can do on a boat is just the 'normal' stuff of turning off the electricty, unplugging the computer (etc) and putting the handheld GPS away from the Nav switch pannel etc.

The just hope any damage doesnt blow the keel off (etc)

Is that what you would do?

I tried turning off the batteries and the autopilot once and hand steered... but then I noticed I was holding onto a bare SS wheel! I prefer the auto pilot to be zapped than Moi! LOL
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Old 25-09-2009, 12:17   #43
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I tried turning off the batteries and the autopilot once and hand steered... but then I noticed I was holding onto a bare SS wheel! I prefer the auto pilot to be zapped than Moi! LOL
Yea Mark!
I am with you now.

Lightning Strike - Sorta
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Old 25-09-2009, 17:22   #44
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wi-fi lightning strike !!!

Hi,

Today I was adjusting my flat panel wi-fi antena that has a metal base plate. I was holding it when the back of my hand touched the backstay ...

Not quite like a lightning strike rehearsal but not a pleasant experience either ...

b.
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Old 26-09-2009, 09:19   #45
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Barnie,

You experienced a static charge. If your rigging is bonded, it's wise to connect the metal antenna part to ground too. If your rigging isn't bonded there's not much you can do apart from designing a bonding system...

cheers,
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