paint prop strut and lightning?

[scherzoja]

If the prop strut is the ground for the boat (all bonding cables are terminated to the strut bolts) and one paints the strut, will that isolate it from the water, causing it to no longer conduct electricity (lightning) to the water?

I know grounding and bonding are different things, but they are all tied to the underwater parts.

Also, if the underwater parts are bonded together and they get painted (or epoxied, then painted), won't that prevent current flow?

So, if the parts are painted (or epoxied, then copper bottom paint)...
1. How does this affect bonding?
2. How does this affect grounding?
3. How does this affect lightning dissipation?

Strut and prop are bronze.

 

[Chris Patterson]

My opinion? If lightning decides to leave the boat via strut, that 1/8" of paint ain't gonna slow it down..

 

[Bill Roosa]

Easy to test. I believe that paint is not that great an insulator and only a good resister, so it would reduce the current flow not stop it. To test get an extension wire for the multi-meter and drop one end over the side of the boat. test the resistance between the wire overboard and the bonded part.
Also I would tent to agree with Chris 1x10^9 volts trying to jump a mega-ohm resistance is still thousands of amps. You would be hard pressed to get a paint layer have a resistance in the gega-ohm range due to the thickness. Lightning will not even see the paint (electrically speaking)
The low voltage corrosion currents will only be slowed down by the resistance (that is a good thing as it will also slow the erosion of your zincs) of the paint. You could get localized corrosion concentrations where you don't have paint that will corrode faster due to the lack of a resistive paint so make sure all the surfaces are covered

 

[walt]

You can’t think about lightning in terms of "volts" or the resistance of material that you can measure with an ohm meter.

Lightning flow involves dielectric breakdown and “resitance” becomes very non linear. http://en.wikipedia.org/wiki/Dielectric_strength

Example, if you measure air with an ohm meter.. it’s about infinite resistance. But in breakdown, the "resistance" of air dramatically changes and becomes a very good conductor - which is pretty obvious as a lot of charge came down through a mile of air.. you need a good conductor for this to happen.

Paint might have a different dielectric breakdown threshold compared to air but it also has the same extremely non linear resistance.. the resistance drops dramatically in breakdown.

Its not really just "volts" that causes dielectric breakdown, it’s the volts over some distance that maters. Volts per distance is called "electric field". Example, say the charge up in the cloud created a "million volts" between ground. Lets say the distance is 1600 meters (about one mile). In this case, the electric field would be 1million volts / 1600 meters = 625 volts per meter. This is WAY under the field required for air to break down (about 3 million volts per meter). No current flows because the air is not in dielectric breakdown.

However, take the same charge that created the 1 million volts and now make the distance the thickness of paint - lets say 2 millimeter. Now the electric field is 1 million volts / .002 meters = 500 million volts per meter (remember the field was 635 volts/meter when the distance was near one mile).

Everything - paint, air becomes a very good conductor at 500 million volts per meter... Actually air becomes a very good conductor at 150 times smaller fields but hopefully the point comes across... that paint might look like a good resistor normally but add in the charge involved with a lightning strike and it goes into dielectric breakdown and becomes a very good conductor.

My opinion.. I don’t think paint makes the slightest difference to what happens in a strike..

 

[Bill Roosa]

I think what you may be missing in your analysis (which is right on BTW) is that voltage causes the breakdown and ionization of the material in question.
And
if you don't think about lightning in the terms of electricity (volts moving amps through a resistance) aka the lowest resistance path is the one that is always taken, you would never come up with the idea of a lightning rod and down conductor (neither get ionized BTW) to safe building from lightning. And before you say it, it is not always possible to determine the lowest resistance path before the lightning strikes due to the fact that it can ionize materials (and lower their resistance). it is possible to make a very low resistance path to ground in most cases that will be used however.

 

[Sailm8]

My opinion after the boat next to me has been hit twice, a little paint isn't going to matter to a gazillion watt lightening bolt.

 

[RichH]

There's two parts of any lightning discussion, some have discussed the 'strike' portion of the lightning strike.
The parts that are seemingly missing from such discussion is the 'neutralization' of the lightning strike potential by the emission of ionization from 'sharp' air terminals and the effect that painting (or not) those close to the surface of the water conductors have on the attempt of ...... 'lightning strike' PREVENTION'. ;-)

 

[chuckwayne]

don't paint the strut (bronze) with a copper based paint - you will de-zincify the strut and end up with a very weak strut that can break with catastrophic results

 

[scherzoja]

Thanks ya'll for the advice. I'll go with unpainted for now and just scrape the barnacles off periodically when I dive to wipe the bottom.

I understand the overpowering nature of lightning. I guess, in retrospect, my underlying question is how painting (ok not with copper-based) the strut and prop will affect DC grounding, if at all. I don't really understand these depths of DC (I know I need to), except for the pos to neg loop that DC travels and, since the neg is terminated on the engine block, will painting the strut/prop have any negative affect on DC grounding? This question really stems from my ignorance of the details of DC grounding. DC neg is connected to engine block and engine block is bonded to the strut bolts.

I guess, since cars have DC systems and they are grounded to earth, not having a DC ground to earth (the water) would be equally unimportant on a boat (unlike AC, which is grounded back to the shore power, then to earth).


Thanks