Bonding masts to keel bolts

[srah1953]

Hi folks
There was a letter in YM a while back recommending that masts be bonded to keel bolts in case of lightning strike. I'm looking for suggestions for the best way to attach a copper cable to the mast.
Many thanks
Srah

 

[Roberto]

a simple bolt, the cable with a ring terminal and with a steel washer in between, you can use tefgel/duralac to avoid corrosion, a lightning won't even notice it

 

[sarabande]

sadly the OP does not give sufficient info.

If you bond the mast to the keel bolts, lighting will crawl down the mast, reach the keel bolts - and then where ? Depends on the structure of the keel, and whether it is direct contact with the sea.

To dissipate lightning effectively you need a large conductive plate bonded to the mast, and then the plate on the outside of the hull, so that the lightning does not blast a hole through the hull. Some people have a cable attachable to the mast, and then chuck it plus the plate over the side when there is a storm around.

Any connection to the mast must be substantial, otherwise the current will melt the bolt and the surrounding aluminium - which is not a good idea.

 

[srah1953]

sadly the OP does not give sufficient info.

If you bond the mast to the keel bolts, lighting will crawl down the mast, reach the keel bolts - and then where ? Depends on the structure of the keel, and whether it is direct contact with the sea.

To dissipate lightning effectively you need a large conductive plate bonded to the mast, and then the plate on the outside of the hull, so that the lightning does not blast a hole through the hull. Some people have a cable attachable to the mast, and then chuck it plus the plate over the side when there is a storm around.

Any connection to the mast must be substantial, otherwise the current will melt the bolt and the surrounding aluminium - which is not a good idea.

The question arose because the letter I mentioned had said that if the mast wasn't connected to the keel, lightning could blow a big hole in the boat.
The keel bolts are set into a cast iron keel. There isn't a grounding plate outside the hull (no such fancy things on an AWB).
You say that “Any connection to the mast must be substantial, otherwise the current will melt the bolt and the surrounding aluminium” but what will happen if the mast is unconnected ?
I had thought that you possibly couldn't use a stainless bolt to the mast but Roberto has answered that using Duralac wouldn't be a problem. So the question then is – what size cable and how big a connection to the mast.
Thanks

 

[Roberto]

So the question then is – what size cable and how big a connection to the mast.
Thanks

The IEC standard requires 70sqmm, both ISO and ABYC standards 20sqmm (sic).

Per ABYC, it should not make an angle narrower than 90°, and its radius of curvature should not be less than 20cm (which might suggest a suitable height of the bolt position on the mast). ISO requires the cable not to be run horizontally anywhere.

 

[danielefua]

The IEC standard requires 70sqmm, both ISO and ABYC standards 20sqmm (sic).

Hi Roberto, every once in a while we meet!

I like to give my personal interpretation of the apparent inconsistency of the ABYC directive: once the charge starts to flow, the 20 mmq cable will melt but the metal vapour and possibly the ionization will keep the arc on until the full charge will indeed dissipate into the water.
Regarding a previous remark on the keel being in direct contact to the water: no layer of whatever matter will be able to prevent such a charge to make its own path through. In the first milliseconds the keel may act as a giant condenser but then any kind of insulator will reach its breakdown voltage and... ZOT!!!

Daniel

 

[Roberto]

Hi Roberto, every once in a while we meet!

I like to give my personal interpretation of the apparent inconsistency of the ABYC directive: once the charge starts to flow, the 20 mmq cable will melt but the metal vapour and possibly the ionization will keep the arc on until the full charge will indeed dissipate into the water.
Regarding a previous remark on the keel being in direct contact to the water: no layer of whatever matter will be able to prevent such a charge to make its own path through. In the first milliseconds the keel may act as a giant condenser but then any kind of insulator will reach its breakdown voltage and... ZOT!!!

Daniel

Hi Daniel

indeed, looking at 20sqmm cable it seems very, very small for such a destructive current

regards

 

[duncan99210]

The cable on Rampage would appear to be about 50sq mm and is bonded to the forward keel bolt. It looks as if it is attached to the plate the mast stands on rather than the mast itself.

 

[William_H]

The question has been asked "What happens if the mast is not bonded to the keel. Well most pople will have some sort if wiring to the mast. Lights etc. The negative will be connected to the engine hence to the sea. That sort of wiring will encourage a strike but not support the current very well.
If there is no bonding then any path to the sea from the mast might via the chain plates and wet GRP or wood. Even with relatively high resistance of shrouds the conductivity will be enough to invite strike. The shrouds will be at least overheated if not evaporated. The moisture in the GRP will boil (explode as in a tree) Hence the idea of blowing a hole in the hull. Or the mast itself might be connected via wet GRP to the keel or sea.
So the idea of bonding the mast to the keel or another plate is to try to get the current away to the sea with as little resistance as possible. Every bit that goes via the bonding cable is diverted from other paths. Now if you look at an old church steeple you might notice copper strap from the top coming down the walls. Typically 50mm wide 3mm thick or 150sqmm. Now that on the one hand might be an overkill but on the other hand seems to have been successful over many years. Ultimately it depends on whether you think you will be struck and just how critical the damage might be. good luck olewill

 

[pvb]

My boat has a cable to the keel from the stainless steel mast support pillar. The cable looks about 50 sq mm.

 

[Scotty_Tradewind]

My boat has a cable to the keel from the stainless steel mast support pillar. The cable looks about 50 sq mm.

should blow the keel off nicely then!

S.

 

[pvb]

should blow the keel off nicely then!

Yes! It's a patented process - lightning strikes, the keel blows off, boat inverts automatically to put the fire out.

 

[bedouin]

should blow the keel off nicely then!

S.

That is what worries me about this advice

Would I rather have my mast vapourised, or my mast vapourised and the keel drop off

 

[danielefua]

That is what worries me about this advice

Would I rather have my mast vapourised, or my mast vapourised and the keel drop off

You forgot to mention your hair: in which case will it be vapourised?


Daniel

 

[William_H]

should blow the keel off nicely then!

S.

Where did you ge this idea that the keel will be blown off? This arrangement would be excellent, The cable should present a reasonably low resistance to high currents. The bolt should take the current into the keel and hence to the water. Yes at worst the bolt may be vaporised as with the cable and lug but hopefully the whole will conduct current until the strike has diminished.
What could be far more likely to blow the keel off and or hole the boat is to have no cable. Such that conduction of current would be through the wet GRP from mast support through the GRP to sea and or keel. The moisture in the GRP would quickly vaporise and explode taking the GRPO with it. (like a tree explodes with high current through the moisture. My only comment re the shown arrangement is that the stainless steel post might exhibit too much resistance so get very hot (melt or sag) a cable to the mast ali would be far better. Similarly the bolts may be SS so connection to several bolts would be better. Ali of the mast itself has very low electrical resistance so mast should survive well but not so stainless steel especially shrouds. good luck (as in I hope it is never tested) olewill

 

[pvb]

Where did you ge this idea that the keel will be blown off?

I think Scotty was perhaps making a little joke, as my boat is a Bavaria and, as everyone knows, Bavaria keels drop off if anybody onboard sneezes!

 

[srah1953]

Any suggestions as to best online place to buy 3-4m of 50 sq mm cable?

 

[paul-essex]

you could bond every chain plate to a keel bolt




I think as it is raining most times a lightning strike happens the current runs down the mast and wire rigging to the deck area
and the water from rain then takes the current over the side to the sea

 

[danielefua]

you could bond every chain plate to a keel bolt




I think as it is raining most times a lightning strike happens the current runs down the mast and wire rigging to the deck area
and the water from rain then takes the current over the side to the sea

Sorry but I wouldn't be very happy looking powerless at a lightning zig-zagging over the deck in order to find the lowest resistance path!

Daniel

 

[William_H]

Sorry but I wouldn't be very happy looking powerless at a lightning zig-zagging over the deck in order to find the lowest resistance path!

Daniel

Unless you have galvanised iron shrouds you should try to divert the current away from the SS shrouds. I am not sure you would see the current zig zagging across the deck more likely you would see the aftermath of burnt GRP both deck and topsides down to the water. Both superficial and possibly internal to GRP.