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Old 10-07-2008, 16:39   #16
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Can a big enough diode be put in the DC power system to use keel bolts as both the Ground for the power... and for lightening without it back feeding and burning out everything else?

Its either that or installing another big lump of metal outside the hull, or using my rudder shaft as a ground... which just feels wrong.

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Old 10-07-2008, 16:59   #17

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In a word, NO. You don't have enough money to buy one that big, if they exist.

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Old 10-07-2008, 18:41   #18
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Originally Posted by tomrobalo1 View Post
He claims that he simply attaches a heavey cable or chain to his backstay and puts the cable/chain overboard as a "path of least resistance".

Imagine electricity as water and the conductor, usually metal, as a pipe.

The mast is a pretty big pipe. The cables from the battery to the starter are sorta big. The backstay is kinda small although it is multi-stranded with improves load carrying capability. The wire going to a light bulb is real small. The chain is kinda big but what about the "joint" between the chain and the backstay - it has to be a very small contact patch area. This is going to be like a huge restriction in the pipe to ground.

Once the energy gets in the boat - it's gonna get to ground. Voltage is like pressure and a lightning strike has all the pressure it needs.

Even a copper braided cable has limited cross section, the lugs have to be large area to make a low resistance joint and even if all that is bonded to the keel there will be energy released via other paths.

I think the chain off the backstay is pretty much a waste of time.

I think hellosaior has the best advice - absorb what you read, teach yourself and then decide what your personal strategy is going to be.
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Old 10-07-2008, 21:20   #19
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Originally Posted by hellosailor View Post
In a word, NO. You don't have enough money to buy one that big, if they exist.
Generally speaking... I don't have the money to buy much of anything! Seems to be what gets me thinking over the top in the first place... (Grin!)

Did some searching around earlier today on the hunt for switches that take lightening and send it to ground in order to protect a circuit. Closest thing I've found to boat strikes, is a "Direct strike" to fixed land based expensive stuff that earns its keep. (Power grid, telephone lines, radio towers...)

Thought this was an interesting read: LIGHTNING AND SURGE PROTECTION

The oldest being a carbon rod with small gap to ground, so that it arcs above a certain voltage. (Doesn't do anything to protect solidstate devices... as it takes time for the charge to build, and then jump the gap.) This is what protects copper wire telephone lines.

On the power grid side... Gas tubes. Wired in parallel, so that when sufficient voltage is applied to charge the Neon inside, it transfers the current to ground. (I am thinking its about the same as wiring in a capacitor to smooth out voltage, except on steroids... and intended to blow up... grin.) Edit: Just read that a gas tubes trade name is a "Crow bar." Grounds out a bus bar the same as tossing a crowbar across them... weird visual.

Next: Selenium diodes. Two metal plates with a dollop of selenium between them. I've only played with these in voltage rectifiers... but evidently they make pretty decent lightning shunts too, and good forever... don't degrade. They get HOT normally... so who knows what the mounting looks like if its intended to be smitten by Thors hammer. (I'll have to do some more reading about these... seems like a dead technology.)

Then there are MOV's or Metal Oxide Varistors. Variable resistors, that resistance changes when voltage changes. The greater the voltage, the greater the resistance. Solid state devices, and no lag time like the ones above. These are what are in surge protectors for computers... Arrays wired in parallel so they all take an equal load. (Limited life expectancy, time decay... and only good a few times.)

Then... Silicon Avalanche Diodes. I'm having a hard time finding anything out about these. Probably because the technology is above my paygrade... (Grin)

These folks (ITW Linx | Category 6 Surge Protectors | Power and Lightning Surge Protection Solutions) talk about using fusible links in the power wire, after the circuit breaker trips so the circuit is physically made open... no more wire! They are also used in some of the power strip surge protectors on the market... Cheapest of all possibilities. But... who knows how many amps even a teeny wire can carry at a bajillion volts!

A part of me is wondering about just using the negative terminal of the battery as ground for the house power and only worrying about inductive damage from lightning, or have a massive knife switch somewhere that is open during storms. (Keeping the rig up and a hole free hull are the main goal... but it'd help not to lose all the electrical wingdings too.)

Interesting topic... I'm not so sure I want to know what happens to a carbon fiber mast when it gets hit.

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Old 10-07-2008, 21:22   #20
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The brand spankin new 40.7 Bene in front of us took a direct hit and was heavily damaged. Our spar is 30 foot taller................... go figure
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Not a whole heck of a lot you can do about the lightning...unfortunately. If it hits your boat you are going to be damaged. I think that grounding does help somewhat, but, as was noted above, there is so much energy involved that even attenuated it will still be bad.

I have a big, thick cable from my mast to my keelbolts. If lightning ever does strike me, I am convinced that the fibreglass around the bolts will melt, the keel will drop off and the boat will turn turtle and sink.... but that's much better than having the enitre boat splinter and burn isn't it ????

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