Lightning protection...any thoughts?

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robmdknapp

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Hi team - I am well into my Cutlass 27 refit for offshore, but before sorting the re-wire I am trying to sort lightning protection.

The research turns up that masthead dissipators may or may not actually do anything, but can't hurt... but that a copper wire running from masthead to a grounding plate on the hull is a good idea. Dynaplate make one, but for grounding radios/SSBs etc, rather than for a strike.

Does anyone have some authority on this? Do I fit a plate (considering I want to be able to sail in lightning heavy places)? If so, who supplies such things?

Ta muchly.
 
My boat's manual says if there is a thundestorm then everyone should go below and wait for it to pass. This was very helpful in my last storm when I was sailing through the Farnes in a wind gusting over F7. I saw a strike hit the sea and nearly soiled my trousers.
Following some folksy advice I carry a set of jump leads and if a storm is brewing these are clipped onto the stays at one end and chucked into the oggin at the other. The folklore is that this will dissipate some of the charge in the mast area and make a strike much less likely. No idea if this really works but was told this is what they do in the US where strikes are a lot more common.
Suggest a bit of reseach on US websites may be best source of ideas
 
A dissipator dosent stand a chace of working, the principal on land is to gently discharge the local area, not going to happen with sea water and precious little good with fresh water, it just isnt pure enough to not conduct.
Lightning conductors are a different matter, the jump leads idea has merits but the junction points (the clips) are going to explode, along with what they are clipped to.... damage will also occur at the point it strikes, so a big chunck of something to sacrifice at the top will save some quids. After that you would need a very large cable to significantly reduce what goes down the mast, so probably not worth it. From the mast to the water might be worth looking at though, at least it will give a good hint to the bolt as to which way to exit the deck, and a place for you to avoid, but remember that if it is a much thinner conductor it will exoplode, so the next hit puts you back to square one, hiding inside.
 
I seem to remember reading somewhere that, even though your metal mast is sticking up in the air by 30ft (in my case), by dint of your boat sitting in salt water (conductive liquid) any charge is dissipated and so your mast has no more potential than the sea around it and so has the same chance of being struck as the sea around you. (or something along those lines). Any physics boffs out there know about this?
 
Peter Berman says in Outfitting the Offshore Sailboat (& I know you can't believe everything you read...):

"Lightning loves sailboat masts & can do quite impressive damage...Even high end builders usually short-change lightning protection." He goes on to explain that he has had direct hits & his strategies seem to have worked.

He says: Connect number 4 Copper insulated cables from each chain plate to the mast step (below deck). Connect the mast-step to a large heavy-duty external grounding plate.

It is this grounding plate that I am trying to pin down. I have heard a few ship-wrights mention this.

He doesn't mention masthead dissipators but I see them in every marina. I believe the principal is that the fibres on the dissipator allow small amounts of charge to leave the boat (friction of any kind - ie between water & fibreglass - will cause electrons to build up, hence static charge) & move into the atmosphere. This would indeed only be of use if the boat had significantly more charge than the surrounding seas in the case of a lightning storm... maybe the dissipator would take the heat, so to speak, in the event of a strike, but it is likely that masthead & electrics would be kaput regardless.
 
Trouble is there are so many opposing views on the subject. An ex Prof of physics gave me a long lecture on the subject, most of which wasn't absorbed but the main point (in his opinion) was that an earthed mast is more likely to attract a strike but will do less damage if you have one. He though the keel was the best ground plate. OTOH, an ungrounded mast is thought less likely to be struck.

Our mast isn't bonded and we've had strikes into the sea between moored boats and onto the salt marshes around us but no direct hits. Must admit my bum was twitching. The stay away from metal down below amuses me as we have chain plates behind the settee berths, fore and aft stays at the end of the fore and aft cabin berths. radio, radar etc over the chart table and a steel mast support post down the middle.
 
I am appreciating the input...

I guess when it comes down to it, it is cheaper in the short-term to not bother...insurance will pay out if you are struck...

I wonder what Moitessier did on Joshua (which was a metal boat)...I have an 'If it's good enough for him, it's good enough for me' thing going on...
 
Can I give shouts out to saltyjohn for pointing me towards the ABYC TE-4 06 doc, which basically runs through the tech spec of all of the above.
 
I did a bunch of reading on this last year and concluded that during a thunderstorm I should hoist a metal bottlebrush on my burgee halyard (so that it protrudes above the top of the mast) attached to some heavy duty electrical cable that leads over the side and into the sea, where it has a few metres of stripped copper in the water.

I'll see if I can dig up my references.
 
rptb1 said:
I'll see if I can dig up my references.
Click to expand...

Ah here we are, "Practical Junk Rig" chapter 5 section 2 last para:
It is possible to provide a temporary lightning conductor, to be rigged on the approach of an electrical storm, consisting of a metal spike sent up the burgee halyard with the conductor wire bonded to its bottom end. From here it hangs down to a position on the boat's rail where it is lashed in such a way as to leave a long length of bare wire trailing in the sea. The surface area of 1/8in diameter wire is 4.7 sq in per foot of length, and to get the required 1 sq ft of area you would need 31 ft of bare wire actually underwater, which could be awkward. It might be better to have a short length of wire towing a short length of lightweight copper tube giving the required surface area.
Click to expand...
 
Lightning is always a negative charged cloud and positivly charged earth (or sea). The strike downwards is much faster than the upward strike. Being electricity it will take the path of least resistance, which in a rain storm is pretty random but always full of air gaps.
When over land the negativly charged cloud may move quite fast, dagging a positive charge along the ground behing it (Its that charge that gives the distinctive smell and makes dogs bark). If you place a rod with a sharpened top in the ground then coronal discharge* at the tip will dissipate the positive charge on the ground reducing the chance of a strike. if you place a pole of good conductor in the ground then it will be a lower resistance path than the air so increase the chances of a strike. So, if you have a dissipator, keep it sharp, they do go blunt.

When at sea it is a different story, singe salt water is good conductor it is not possible to dissipte the local ground charge, you will not produce the same safe island. On freash water it will depend on water purity, but it ia still likely to be a very good conductor.
On a metal boat you will ust have to take the hit, damage will only occur at the strike point and any current bottlenecks, an even or improving conduction path to the sea will not be damaged.
On a GRP boat it is most likely that the path will be down the mast and rigging, then across the wet deck, that is where it can be dangerous to people. If the ]re is a chnace of conduction through the hull then serious problems may ensue as these paths will be high resistnce and heat dramatically.
For wooden boats then it all depends on paint/varnish thickness and how dry the wood really is.

* coronal discharge. when charge density on the surface of a conductor is high enough the surrounding gas will become ionised, charge density is highest on sharp edges.
 
rptb1 said:
Sorry Dougal, could you rephrase that part? It seems that the scheme proposed in PJR is about improving a conduction path, and I couldn't quite follow what you were saying about it.
Click to expand...

Sorry, a bit clumsy of me. If the path that the ligthning will follow has any restrictions you will get significant heating at that point, possibly causing that part to act as a fuse. So, as thick as possible and with as few joints as possible and all joints must be very well made off.
 
I really am not comfortable with the idea of an earthing plate being firmly attached to the underwater hull
.

It should not be painted or antifouled for obvious reasons, so unless you are meticulous in cleaning it, the plate will acquire a marine garden in no time (unless it is pure copper). Should the time come for the plate to do its designed job, then a very large electrical and magnetic force will zap through the plate into the sea. The resulting discharge will have a physical reaction back against the plate, and unless it is exceptionally firmly braced internally, the hull is likely to be perforated by what is in effect a plasma torpedo.

I've heard of people attaching an anchor chain to big ali cables from the chain plates, and trailing that overboard. With a reasonable size chain, you could get lots of square feet of conducting surface into the water with not much immersed length.
 
If you put a lightning conductor on top of a building, then you increase its likelihood of it being struck, but with several heavy copper tapes and grounding rods properly bonded to the conductor, you will significantly reduce the damage of such a strike; it therefore follows (to my mind) that any sort of 'conductor' on a boat will similarly increase the strike likelihood, but you will not be able to achieve the properly bonded grounding elements of such a system on a boat - you're sat in a puddle rather than on solid ground for a start.

The electrical discharge from the lightning wants to 'earth' asap and on a grp or wooden boat (perhaps not so much with a steel hull) the time 'saved' by it hitting the top of your mast, rather than the water 40 - 60' below will be partially, perhaps wholly offset by the time it subsequently loses in passing through the less conductive materials of the hull - is the slim coating of water (assuming it's raining too) enough to offset this balancing factor?

My personal choice is to do nothing - other than make sure the liferaft and grab-bag are very handy when storm clouds approach - on the basis that we must've met 20 - 30 yachts now who've been struck and whilst some had like us done nothing, those that had installed 'protection' none of their many and varied ideas seems to have made a damned bit of difference; shit-happens. The thing I've found particularly weird about lightning struck boats is that of those that we've met, a significant (20-25%) proportion have actually been hit twice; whilst a Dutch yacht in Suriname was repairing after his third strike in eight years!
 
A few important points here.
A lightning strike starts with a fine leader discharge ground up wards. This low current high voltage can be conducted through apparently not good conductors. Wet deck, wet grp wet wood etc. A pointed or mutiple point discharger at the top can dissipate charge at a lower voltage (due to concentration of charge) so possibly avert a full strike.
What happens when a strike does come back from the cloud through the ionised (conductive) air at huge voltage and current is where destruction occurs. Trees have their sap boil so explode. Similarly if current goes through anything wet that water will vaporise. If the water is internal the vapour pressure will open up the structure.
We need to conduct that current with as low resistance as possible.
An ali mast is a brilliant conductor so no need for a wire from the top. Just the base to the sea. Ideally a metal keel however a dynaplate or any copper plate wouold be good. We have to account for huge currents. So heavy cooper or ali wires to multiple connections to keel bolts.
Using a copper plate we have to consider current in the plate. a thin plate might not carry the current as it disperese to the sea. ie if you use foil and a single bolt then copper material around the bolt will just fuse from high current. Multiple connection bolts will help.
If at all possible do not allow current to pass through stay wires. Stainless steel has relatively high resistance and will over heat quickly. either melting completely or losing its hardness temper. So an anchor and chain to the mast not the chainplates.
Any point of resistance in the current path will result in intense heating.This may result in metal melting/vapourisation like an arc welder. The current will continue but with huge destruction. So hopefully connections to the keel bolts and connection keel surface to water will get very hot but hopefully not to damaging. Any other connection mast to water like speed log and wiring (via mast wiring) will disapear in a flash. Don't confuse one poster's comment that it is like a fuse. A fuse is designed to open (at a voltage lower than it's rating). Such that an arc will not sustain when it opens. With lightning type voltages and current mostly arcs will ssutaion if metal conductor fuses. Just causing more heating damage.
So for a direct strike the lower the resistance more current carrying capacity of conductors to the water the better. The heavy copper strip down the side of church spires is an indication of what is needed or has apparently protected the building for a long time. Mostly I think multiple straps of 50mm by 5 mm copper.
The voltage spike on the VHF antenna is likely to melt your vhf radio. likewise any oither antenna. So disconnect all wiring to the radio if you think you will be struck. Any nearby lightning strike produces huge currents which can be induced into any wiring nearby. (transformer like). The only thing you can do is disconnect all electronics from ships wiring. Many advocate putting them into the oven. This metal box can for a shield from electrostatic and magnetic pulse. Especially if metal is magnetic conductor. I think first off just disconnect plugs. A subject of great uncertainty so plenty of room for argument and misconceptions good luck olewill
 
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