lightening protection

CAPTAIN FANTASTIC

CAPTAIN FANTASTIC

Well-Known Member
Joined
15 Jul 2009
Messages
3,311
Location
Bristol Channel
Visit site
Do you protect your boat against lightening and how?. When I feel that we would be expecting lightening, I use a length of chain connected to the lower stroud rigging, dangling in the water. I also disconnect the vhf and the radar.

I do not know whether in practice my solution will be adequate or effective. A permanent protection would be more suited perhaps in the in the Caribbean but not in the Solent
 
GrahamM376 said:
Earthing the mast without fitting a dissipater such as this - http://www.forespar.com/products/boat-lightning-static-dissipater.shtml - is more likely to attract a strike.
Click to expand...

I don't agree with that bold statement. A dissipator actually atracts a discharge and by so doing will dissipate the charge at an earlier so low current point than without. I reckon that by the time a lightning bolt can stretch possibly thousands of feet down to your boat it won't make much difference. Dissipators do make a difference to static charge that can build up under the same conditions. (like St Elmo's fire).
A Lightning stroke is initiated by a tiny discharge from earth up to the cloud. The ionisation in this discharge provides a current path back down. What that means is that the initail discharge can be via quite high resistance conductors. (A tree is a classic example) In the case of a tree the floowing high current and high voltage from the bolt overheats the resistance of the tree. This overheat boils the moisture causing the tree to explode.
Now in the case of your boat the mast is the obvious strike point being closer to the source. A wooden mast may be sufficently moist to end up like a tree or dry enough that not so much current can flow. Almost always there will be enogh conduction to initiate the leader upwards.
An aluminium mast has very low ressitance and might be capable of carrying the high current through it's length. If the mast is connect to the water by a similarly low ressitance (high current carrying capability wire) then we might anticipate minimal damage. if as is likely the wire connection or the keel itself exhibits resistance it can not carry the current without overheating and causing a volt drop (causing an arc). Both of these are inclined to cause a fire to some extent. Fortunately the high current is usually for a short period so heavy wire can cope with a large current.
If as OP suggests you use shrouds as part of the current path you will be in trouble. Stainless steel does not have a low resistance. at currents around 50 amps 3/16 rigging wire will glow red. Imediately loosing its heat treatment. Probably around 150 amps it will fuse and part. So mnake the current go through the mast not the stays.
If you do not earth the mast then the current will go down both the stays and the mast then makes do with wet decks bilge water etc to complete it's path. All with lots of burning.
Now things like a VHF antenna on mast top will take a big hit of current. The voltage will jump the antenna to mast fairly easily. You would not want your radio connected to this wire at that time. Other wiring will take a hit if it is connected to sea via engine/gearbox or even transducers. Again you would not want the electronics connected to this affected wiring. Similarly you would not want your body touching any equipment, shrouds or anything else in primary or alternative paths from amst top to water.

Next problem is that this huge pulse of current flowing down through conductors or even just air can induce a voltage and current in any other conductors. It is like a big transformer. The longer any wiring in the boat is the more voltage it will have induced into it. Thus any electronics connected to any wiring even a microphone cable can get a jolt. One way of minimising this induction is to place elctronics in the metal oven. The metal of the oven can act like a transformer winding and soak up the magnetic pulse. Steel is even better at soaking up (bypassing) the magnetism.

As you can see things are bad if your boat is struck by a proper bolt of lightning. (not to mention the shock to yourself of the sound of thunder from the rapid heating of the air around the ionisation.) So I reckon earth the mast with the best biggest conductor you can fit. If it is to keel boilts use multiple bolts. Church steeples have relied on this approach for centuries. If a storm is nearby disconnect all electronics you can and if possible put in a metal box. Don't touch anything and hope like heck. olewill
 
William_H said:
I don't agree with that bold statement.
Click to expand...

Quite an essay and I don't disagree with what you say. However, I stand by my statement that something which is earthed, as in the case stated by the OP is more likely to attract a strike than something which isn't. A "properly" earthed mast with a dissipator will minimise the chance of a strike and cause less damage if one occurs.
 
When I researched the subject a few years ago it seemed that there were two almost totally conflicting theories according to the experts. This thread has already highlighted them: either full insulation or full conductivity. Unfortunately lightning obeys no logic whatsoever, witness strikes on masts with higher ones all around, in enclosed ports with nearby churches fully equipped with lightning conductors, etc.
 
William_H said:
I don't agree with that bold statement. A dissipator actually atracts a discharge and by so doing will dissipate the charge at an earlier so low current point than without. I reckon that by the time a lightning bolt can stretch possibly thousands of feet down to your boat it won't make much difference. Dissipators do make a difference to static charge that can build up under the same conditions. (like St Elmo's fire).
A Lightning stroke is initiated by a tiny discharge from earth up to the cloud. The ionisation in this discharge provides a current path back down. What that means is that the initail discharge can be via quite high resistance conductors. (A tree is a classic example) In the case of a tree the floowing high current and high voltage from the bolt overheats the resistance of the tree. This overheat boils the moisture causing the tree to explode.
Now in the case of your boat the mast is the obvious strike point being closer to the source. A wooden mast may be sufficently moist to end up like a tree or dry enough that not so much current can flow. Almost always there will be enogh conduction to initiate the leader upwards.
An aluminium mast has very low ressitance and might be capable of carrying the high current through it's length. If the mast is connect to the water by a similarly low ressitance (high current carrying capability wire) then we might anticipate minimal damage. if as is likely the wire connection or the keel itself exhibits resistance it can not carry the current without overheating and causing a volt drop (causing an arc). Both of these are inclined to cause a fire to some extent. Fortunately the high current is usually for a short period so heavy wire can cope with a large current.
If as OP suggests you use shrouds as part of the current path you will be in trouble. Stainless steel does not have a low resistance. at currents around 50 amps 3/16 rigging wire will glow red. Imediately loosing its heat treatment. Probably around 150 amps it will fuse and part. So mnake the current go through the mast not the stays.
If you do not earth the mast then the current will go down both the stays and the mast then makes do with wet decks bilge water etc to complete it's path. All with lots of burning.
Now things like a VHF antenna on mast top will take a big hit of current. The voltage will jump the antenna to mast fairly easily. You would not want your radio connected to this wire at that time. Other wiring will take a hit if it is connected to sea via engine/gearbox or even transducers. Again you would not want the electronics connected to this affected wiring. Similarly you would not want your body touching any equipment, shrouds or anything else in primary or alternative paths from amst top to water.

Next problem is that this huge pulse of current flowing down through conductors or even just air can induce a voltage and current in any other conductors. It is like a big transformer. The longer any wiring in the boat is the more voltage it will have induced into it. Thus any electronics connected to any wiring even a microphone cable can get a jolt. One way of minimising this induction is to place elctronics in the metal oven. The metal of the oven can act like a transformer winding and soak up the magnetic pulse. Steel is even better at soaking up (bypassing) the magnetism.

As you can see things are bad if your boat is struck by a proper bolt of lightning. (not to mention the shock to yourself of the sound of thunder from the rapid heating of the air around the ionisation.) So I reckon earth the mast with the best biggest conductor you can fit. If it is to keel boilts use multiple bolts. Church steeples have relied on this approach for centuries. If a storm is nearby disconnect all electronics you can and if possible put in a metal box. Don't touch anything and hope like heck. olewill
Click to expand...

This is a very comprehensive explanation, thank you. I agree that earthing the mast would be much more effective than providing a path for the current from the shrouds only. I will get a longer piece of chain, pass it around the mast and drop the other end in the water.
 
What I don't understand is that every year we all rub down the brightwork and compound the GRP - both of which remove material - so the boat should get lighter, yet an old boat always gets heavier...

Lightning does seem to behave in strange ways. My father was struck whilst playing bowls. Strange as he was the tallest person around, yet he was actually crouched down in mid-delivery. In the UK where lightning is less frequent than in other parts of the world, your chain to earth a strike may be sufficient, though electronic equipment would need to completely disconnected from any wiring to ensure its safety - some suggest putting them in the oven as it will act as a Faraday Cage. Just thinking about the possibility of a strike to the mast getting into the lights wiring and thence to the main circuits, where a switch would not provide a realistic break in the circuit.

Rob.
 
The boat of some friends sailing in USA was struck. Every single piece of wire on board was burnt out, some explosively. Lightning has jumped several miles to the boat so it seems unlikely that a couple of millimetres gap in an isolator would do much.
 
Lightning hits the best conductor not the highest object.

As I've said before - lightning does whatever it wants. Our puny efforts with chains, cables and bottle brushes may, or may not, have some mitigating effect, but when you've been aboard during a strike you realise just how overwhelmingly powerful a lightning bolt is.
 
Some good stuff here http://www.frugal-mariner.com/Lightning_and_your_boat.html

Note that hanging a chain from the shrouds in a FG boat increases the risk of a strike but reduces damage.

The spikey ball protectors seem to make no difference.

I suffered a direct masthead hit which zapped some electronics even though they were disconnected and sitting on a wooden table. Perceived wisdom is that I should have put them in the oven. One unexpected consequence of the strike was that my steel hull was strongly magnetized and all compasses rendered useless.
 
You must log in or register to reply here.

Other threads that may be of interest

T
Notices to Mariners - Week 50
Replies
2
Views
725
tillergirl
T

Share this page

Top