lightning protection - Page 8 - SailNet Community

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > On Board > Gear & Maintenance
 Not a Member? 
  #71  
Old 09-16-2008
eherlihy's Avatar
Learning the HARD way...
 
Join Date: Jan 2007
Location: Boston Area
Posts: 3,327
Thanks: 85
Thanked 50 Times in 49 Posts
Rep Power: 8
eherlihy will become famous soon enough
I just stumbled across this thread and tought that I'dd add a couple more cents...

Faraday cages block static electrical fields. Like lightning. In Hartley's pic above, the fuselage acts as a faraday cage. Faraday cages can not block EMF or Electro Magnetic Feilds.

A good Faraday cage on your boat is the oven (if you have one), or the pressure cooker mentioned above. the CDI box probably IS a Faraday cage, but this one in meant to keep the static IN the box, rather than out, as part of a noise supression circuit (so people with nearby radios don't hear your sparkplugs every time they are fired).

Also - WOW! Waltsn's pic is unreal!
Reply With Quote Share with Facebook
  #72  
Old 09-16-2008
Member
 
Join Date: Jul 2008
Posts: 45
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0
waltsn is on a distinguished road
Yup, the designers of the outboard probably didn't worry about it working in a lightning storm. I think if I added a shield around the two wires going to the pickup coil and terminated the shield at the CDI control box, I would have not had any problem with the outboard - but also wouldn't have got any pictures.

Here is a pretty crazy idea (good that its burried way back in this thread) regarding how a lightning upwards leader is formed and possibly has some influence on a sailboats chances of getting hit.

A premise here is that in a lightning event, only air is ionized and water is not because it is at least 80 times as difficult to ionize than air (maybe even thousands of times more difficult to ionize). We could discuss this but I put all this mumbo jumbo in this web site trying to back this idea up ( analogengineering.com/lightning/surface.html - cut and paste again )

You can also read this paper wwwq.lightningsafety.com/nlsi_lhm/Radials.pdf (remove the q after the www) but the bottom line of this paper is that the water surface will charge up during a lightning event. The water surface charge is typically positive because electrons have been driven into the bulk of the water. The speed of this charging depends on the salt content in the water. The charging takes nano seconds for salt water and tens of microseconds for very fresh water. The charging of the water surface also completely "encloses" the electric field between the water surface and the charge in the sky.

So the negative charge in the sky or the lightning streamer coming down from the sky creates an electric field which "charges" the water surface.

The crazy theory is that the water at the surface is very difficult to ionize but the air just above the water is much easier to ionize so the air above the charged water surface gets ionized and creates channels for charge to flow. These channels meet up at some place (like under the downward coming streamer) and then head upwards to meet the downward coming streamer. Sort of just the opposite process of the "radial pictures" you can see in the referenced links. Generating a upwards streamer is of course the best way to receive a downwards comming streamer.

The interesting thing about this theory is that it is pretty much only dependent on structures above the water surface which can influence the electric field such as a mast. Or the indentation in the water surface left by a boat hull. Anything under water (such as a grounded keel) has no influence.

Cant back this up with references.. just throwing out something for discussion.
Reply With Quote Share with Facebook
  #73  
Old 09-16-2008
Senior Member
 
Join Date: Sep 2006
Location: On The Hurricane Bowl
Posts: 407
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 8
Insails is on a distinguished road
from my blog here awhile back:

Lightning is a series of several strokes the first being a stepped leader stroke which develops a path from the cloud to the ground sometimes the channels lead nowhere which explains why lightning forks eventually a path is developed from the cloud to the ground and lightning happens..
Once the stepped leader stroke develops a conducting channel to the ground or you get the "Return Streamer" which is a discharge of electricity from the ground to the cloud. This stroke is more brilliant than the leader stroke and is the observed stroke we see..an average of three leader strokes and three return stokes occur in one conducting channel..

SO

the better the channel the more chance of the strike
I'd say not grounded would be safer and moving is even safer than being at dock or anchor.....
Reply With Quote Share with Facebook
  #74  
Old 09-16-2008
Member
 
Join Date: Jul 2008
Posts: 45
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0
waltsn is on a distinguished road
Quote from this link edis.ifas.ufl.edu/pdffiles/SG/SG07100.pdf (some day I may be able to post a link)

As the negatively- charged stepped leader moves downwards, it induces a positive charge on the ground below. When the top of the leader is about 30-100 yards above ground level, the induced positive charge becomes so concentrated that a new spark forms, as shown in Figure 2. This positively charged spark is the crucial process as far as the attachment to a boat is concerned. If it starts at the top of a boat mast, then lightning strikes the mast. Unfortunately, there is no scientifically accepted technique to prevent this spark from forming. Even if a device were effective in diverting the attachment spark, it would not be a good idea to mount it on the masthead as the attachment spark may start elsewhere on the boat or crew. The likelihood of lightning attaching to the masthead is a safety feature as far as crew is concerned.
Reply With Quote Share with Facebook
  #75  
Old 09-17-2008
anchorsaweigh's Avatar
1980 Nonsuch 30C #77
 
Join Date: Aug 2000
Location: New England
Posts: 39
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0
anchorsaweigh is on a distinguished road
Bottom line is that you could have the most sophisticated grounding/bonding system available. But, if the system isn't independent of the boat (that is, insulated FROM the boat as it is on a house), or if the strike exceeds the capacity of the conductor...you're screwed.

Cheers,

Bob
__________________
You can polish a turd, but you just end up with a shiny turd.
Reply With Quote Share with Facebook
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Lightning Strike! Kathy Barron Seamanship Articles 0 04-07-2005 08:00 PM
Lightning Strike! Kathy Barron Gear and Maintenance Articles 0 04-07-2005 08:00 PM
Lightning Strike! Kathy Barron Cruising Articles 0 04-07-2005 08:00 PM
Lightning Examined Kevin Hughes Seamanship Articles 0 05-10-2004 08:00 PM
Lightning Protection 101 Sue & Larry Seamanship Articles 0 11-22-1999 07:00 PM


All times are GMT -4. The time now is 12:31 PM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012