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Go Back   SailNet Community > On Board > Gear & Maintenance > lightning protection
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Topic Review (Newest First)
09-17-2008 08:07 AM
anchorsaweigh 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
09-16-2008 08:20 PM
waltsn 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.
09-16-2008 08:00 PM
Insails 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.....
09-16-2008 06:48 PM
waltsn 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.
09-16-2008 05:12 PM
eherlihy 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!
09-16-2008 04:36 PM
Jafo During an electrical storm, we had best observe Bre'r Fox' advice closely: "Lay low, an' don't say nuffin!"
09-16-2008 02:38 PM
Freesail99 Holy crap, what a picture. So did you have to change your shorts ?
09-16-2008 02:14 PM
Cruisingdad
Quote:
Originally Posted by waltsn View Post
I got to expererience another aspect of lightning about three weeks ago. I have about a 1990 Honda BF8 outboard on an old Macgregor 26S sailboat and was trying to start it during a lightning event at 8600 feet in elevation in Colorado. The motor is typically difficult to start at that elevation but during this event, I could hear "little zaps" and felt some tingling when I touched metal. The outboard would just sputter a little bit and then die. I was under some stress and maybe not doing things right but it seemed to me that the sparking was effecting the engine spark timing and keeping the outboard from running correctly. During the lightning event, I could not get it started. FYI - why did I keep messing with the outboard at a very dangerous time to get high by lightning - because a big fricken tornado was heading straight my way - here is a picture I took at a particulary nasty time:
analogengineering.com/elevenmile/tornado/tornado_1.jpg (not sure when I can post a link so cut and paste).

In a lightning event such as this, there is a big changing electric field present and any metal structure will take on the single voltage potential of its geometric mid point. So insulated peices of metal at different heights take on different voltage potentials and when the potenitals build high enough, there can be a discharge between the peices of metal (looks like a build up of static charge - but really is an effect of the strong electric field). Same effect that causes you to get shocked on shrouds.

Assuming that the outboard actually did have some problem running and it just wasnt me freaking out (which is possible), I looked at the inside of the outboard and looked at the schematics. This motor has a "CDI" electronic igniition and the electronics are all in a nice metal box (ie, faraday cage) so probably not affected by the elecric field and sparking. Signals to the sparking coil are all very low impedance and not likely to have picked up enough noise to spark. However, the spark timing is taken from a sensor coil and on my outboard, this coil is at the end of a fairly long (ie, maybe 8 inches) of not twisted and not shielded wire. This is also likely a fairly high impedance circuit which makes it good a picking up electromagnetic noise. I think this circuit could have been sensitive enough to be picking up the noise and causing the false spark timing.

As I mentioned, there is some chance that I just had trouble starting the motor as this has also been the case on nice sunny days but it seemed at the time that something about the way the motor spark timing was unusual and it took a LONG time to get it started.
That is one of the most amazing pics I have seen. I wouldn't have needed the outboard. I would have walked (run) on water all the way to safety - at which point I would find a secluded place to clean myself up.

- CD
09-16-2008 01:49 PM
sailingdog IF Jody's got the right photo...freaking out is acceptable...

BTW, even if the ignition components are in a metal box, they're not protected, since the EM field can induce a voltage across the wires that enter the box and fry the electronics by the induced transient voltage.


Quote:
Originally Posted by waltsn View Post
I got to expererience another aspect of lightning about three weeks ago. I have about a 1990 Honda BF8 outboard on an old Macgregor 26S sailboat and was trying to start it during a lightning event at 8600 feet in elevation in Colorado. The motor is typically difficult to start at that elevation but during this event, I could hear "little zaps" and felt some tingling when I touched metal. The outboard would just sputter a little bit and then die. I was under some stress and maybe not doing things right but it seemed to me that the sparking was effecting the engine spark timing and keeping the outboard from running correctly. During the lightning event, I could not get it started. FYI - why did I keep messing with the outboard at a very dangerous time to get high by lightning - because a big fricken tornado was heading straight my way - here is a picture I took at a particulary nasty time:
analogengineering.com/elevenmile/tornado/tornado_1.jpg (not sure when I can post a link so cut and paste).

In a lightning event such as this, there is a big changing electric field present and any metal structure will take on the single voltage potential of its geometric mid point. So insulated peices of metal at different heights take on different voltage potentials and when the potenitals build high enough, there can be a discharge between the peices of metal (looks like a build up of static charge - but really is an effect of the strong electric field). Same effect that causes you to get shocked on shrouds.

Assuming that the outboard actually did have some problem running and it just wasnt me freaking out (which is possible), I looked at the inside of the outboard and looked at the schematics. This motor has a "CDI" electronic igniition and the electronics are all in a nice metal box (ie, faraday cage) so probably not affected by the elecric field and sparking. Signals to the sparking coil are all very low impedance and not likely to have picked up enough noise to spark. However, the spark timing is taken from a sensor coil and on my outboard, this coil is at the end of a fairly long (ie, maybe 8 inches) of not twisted and not shielded wire. This is also likely a fairly high impedance circuit which makes it good a picking up electromagnetic noise. I think this circuit could have been sensitive enough to be picking up the noise and causing the false spark timing.

As I mentioned, there is some chance that I just had trouble starting the motor as this has also been the case on nice sunny days but it seemed at the time that something about the way the motor spark timing was unusual and it took a LONG time to get it started.
09-16-2008 01:48 PM
smackdaddy Waltsn - All I can say is "You are the man." Giu can't even touch that. You oughtta hang out in FightClub, dude. No one would understand any of your "fancy" talk about electric stuff - but it would be fun.
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