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Old 02-01-2016, 15:52   #1
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Should mast be grounded or not?

I tried a search here, but apparently I can search Mast or Grounded but not both at the same time.

ANYhow...I'm replacing a VHF antenna blown off the top of our mast by lightning. I'm also planning to install a new Wilson marine antenna up there. I'd like to do any simple things I could to lessen the attraction of the setup to another strike.

I can't find any strong indication that this mast is grounded to seawater. It is bolted through the cabin top to a compression post, that continues down to the bridge deck. I have not seen any kind of ground strap from anywhere on the mast to anything below the waterline.

Should the mast be grounded to the sea? That last lightning bolt got into the boats 12 volt system and did a lot of damage. I'm wondering if a direct path to the sea would have kept it out of the small wire stuff.
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Old 02-01-2016, 16:12   #2
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Re: Should mast be grounded or not?

I can't answer as too grounding a mast to seawater. I've never heard of it quite honestly, but there's lots I don't know, or pretend to.

I know that my mast is keel stepped, and it's grounded to the encapsulated lead ballast. I haven't been able to find a whole lot on the subject and will be interested in hearing from some people who know more than me.
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Old 02-01-2016, 16:20   #3
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Should mast be grounded or not?

The answer to your last question about whether bonding the mast will keep your electronics safe is no. There really is no foolproof way to keep a lightning strike out of your DC system. At best, you can disconnect any mast head wiring and install surge clamp devices on your panel and even individual circuits. A gas discharge device in the VHF coax might help, but this isn't really proven, and doesn't help with the grounded shield part of the cable, which is the real path into your electronics.

I doubt your mast is bonded to the sea, because it sits over open water. On ours, we have a thick cable on the outside that is connected to the mast and goes into the water with a high edge surface electrode on the end. The purpose of this is to guide the main part of a strike to water - it will not help with electronics. We have it in the water whenever we are anchored, and can put it in under way if needed.

Bonding your mast to water will not increase your chances of being hit. People will pour into this thread telling you that it will, but they are wrong. All statistical evidence suggests that bonding slightly decreases your chance of being hit. I personally think it does neither.

Bonding your mast to water will decrease the chances that the lightning will blow holes through your hulls looking for an escape path to water. It will not help with your electronics, though.

There have been a lot of threads on this in the past that may help. I hesitate to recommend you search for them because there is a lot of drek to sift through in them.

Some of us have taken a hit and have methodically chased the paths and thought hard about their nature and what can be done to minimize damage. Some have made a scientific career doing so, and are regarded as experts in this area.

Others have not been struck and have confused this extremely common and high probability occurrence with dead certainty that every thing they do and believe is correct.

Beware.

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Old 02-01-2016, 17:02   #4
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Re: Should mast be grounded or not?

The consensus that I've heard is that a deck-stepped mast should have a heavy direct cable run from the mast or mast step, through the deck and down to the keel, or to a grounding plate. This is to "encourage" the lightning to go STRAIGHT DOWN and not dilly-dally in your other equipment.


Of course a VHF antenna all the way at the top is practically a lightning rod, minor problem. But there are also lightning protectors from companies like PolyPhaser that will help divert a direct strike from the antenna to a ground path, instead of down the antenna cable (which you presumably also need to replace) and the electronics.


It is also not paranoid for a radio operator to physically unplug the antenna cable, close to where it penetrates the deck, and plug it directly into a grounded coax as well. Ensuring that if you are off the boat and it gets hit, again, the radio isn't in the path of the strike.


If your mast is not grounded, the strike will move down the wiring and rigging and that pretty much can mean anyplace and everyplace in the boat along whatever path it finds best.


There was some research published earlier this year (sorry, no cites) that indicates the charge being carried in the moist salty air (highly conductive) in the foot or two directly ABOVE sea level, is where the charge is most likely to pass. And for that reason, using four grounding plates (two on each side, two forward, two aft) but all located about one foot above the waterline, outside the hull, may actually be more effective than "grounding" into the water, because this moist salty air is where the charge really "wants" to go, to and from, first and fastest.
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Old 03-01-2016, 06:35   #5
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Re: Should mast be grounded or not?

Sounds like I really don't need to go through the hassle of grounding the mast on a catamaran, then. I have backstays terminating in chain plates within a foot of sea level, and plenty of heavy copper strap grounding inside both hulls.

We've already experienced, first hand, what happens when lightning hits the VHF antenna. Yes, as stated, the primary path into the boat electronics was definitely the coax shield on the VHF antenna. I am still chasing down problems from that strike, three years later.
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Old 03-01-2016, 07:13   #6
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Re: Should mast be grounded or not?

There was discussion of grounding and lightning strikes in this thread, but no clear conclusions:

Offshore Sailing in Thunderstorm far from land
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Old 03-01-2016, 07:26   #7
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Re: Should mast be grounded or not?

For better or worse my mast is grounded. A large cable runs from the mast base to a bolt on the transmission case, grounding the mast to seawater through the prop shaft.

Only change I have made is to remove a loop in that cable by shortening it to make a straight run.

My forty year old original through hulls are bonded as well.

And the bonding bus is jumped to the dc negative bus.

All this original Pearson wiring, I believe.
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Old 03-01-2016, 07:33   #8
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Re: Should mast be grounded or not?

The Catalac was originally wired pretty much like that, with the exception of (possibly) the mast. But bottom line, sounds like unless I build a Faraday cage around the entire boat there's really not much I can do to prevent lightning strikes.

Most sensible, and cost effective, thing I am seeing here is to try to remember to disconnect the antennas on the mast head when there's a storm going on, or when leaving the boat.

I see some nice arguments here for putting everything of value on a radar arch and just living with some reduced range to line-of-sight systems. Or mounting antennas on the spreaders, perhaps?
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Old 03-01-2016, 07:41   #9
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Re: Should mast be grounded or not?

See Dr. Ewen Thomson’s excellent website, for LOTS of lightning information, including :

Catamarans

PDQ 36 Retrofit ➥ http://www.marinelightning.com/syste...Q/PDQ36LPS.jpg

Lagoon 50 Retrofit ➥ http://www.marinelightning.com/syste...es/XelaLPS.jpg

Grounding concepts
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Old 03-01-2016, 08:59   #10
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Re: Should mast be grounded or not?

In our boat it is not.

Then again, if the fore stay runs to a stay fitting that reaches water level ... it is!

So, in our boat there is no grounding system BUT the mast is in fact "grounded" to the sea water.

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Old 03-01-2016, 09:24   #11
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Re: Should mast be grounded or not?

Our mast is bonded to a lightning plate on the hull. I haven't installed the protection on the VHF coax because I'm still weighing the trade-off between the slight loss of signal strength and the slight risk of lightning strike.
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Old 03-01-2016, 10:02   #12
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Re: Should mast be grounded or not?

I'm not going to try to pretend I know anything much about lightning - but can it possibly be OK to say "My rig is grounded because the stay is in the water"?

You've got lightning blowing up trees when it's really severe - won't that fry your stays or shrouds instantly?

Trying to do some very rough math - if a mast is 1/8 inch aluminum and 4x6 inches rectangular cross section - that would be (4 + 6 ) x 2 = 20 inches perimeter x .75 (because it's ovoid not rectangular and therefore less) = 15 x 1/8 = just about 2 square inches cross sectional area for the current to flow.

That is of aluminum - truthfully I don't know if the alloys used in masts are as conductive as pure aluminum.

But your stay will be around 3/8 inches - cross sectional area = 3/16 x 3/16 x pi = about 30/256 square inches - or 1/8 square inch -

In other words, if they were the same conductivity, the mast can carry twenty times the current of the Forestay - if you ground it with a thick wire at the bottom.

My point is, a lightning strike that can fry your forestay instantly will not fry your mast - so why leave the stay as a weak point when you don't need to?

As I say, this is all speculation on my part and I may be way off base.
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Old 03-01-2016, 10:28   #13
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Re: Should mast be grounded or not?

Boatbrain is correct - relying on a stay or shroud gives one very little safety. The mast will be the main conductor, and by the time the strike travels through it, it will be looking to exit through the hull. Any travel down a stay or shroud will just be luck, particularly if one is counting on reaching water through mere proximity or a clip on wire. A lightning bond wire needs to be substantial in surface area - either something like 4/0 or strapping.

So far, I have closely assessed the aftermath of 6 boats struck directly (we were one) - 3 bonded and 3 not bonded (3 cats and 3 monos, 1 cat and 2 monos bonded, if that matters). All lost all electronics and many electrics. The unbonded boats had holes punched through the hulls and were severely damaged - two of them were a write off, the third required quite a bit of glass work. The bonded boats had no further damage.

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Old 03-01-2016, 10:43   #14
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Re: Should mast be grounded or not?

Our mast is grounded to the boat's ground strapping and in turn to the short wave ground plane plate on the outside of the hull. Our shrouds are also attached to this network. The purpose is to make the mast, shrouds, lifelines, copper strapping throughout the hull act as a Faraday cage. This will reduce the likelihood of the charge ripping through the main cabin. The main charge is carried as far to the outside as possible and to a good ground.

Most strikes are nearby and not direct. The induced surges are not as great.

As noted, nothing is known to reliably prevent a strike. We have been hit 3 times in 8 years. Once while on the hard. The last hit (direct & witnessed) destroyed VHF radio, Watchmate AIS, Stereo, 3 cabin LED lights, all forward cabin wiring, back-up alternator, 18 glass buss fuses, 60 amp main shore power breaker, all nav lights, 3 of 6 LED deck lights, Windex, 2 VHF antennas, Raymarine ST-60+ suite, checked the Awl Grip on the transom. The Xantrex needed a hard RE-Boot to re-start. Note that the lightening arrestor was not damaged. Four other boats in the marina were also damaged. All DC breakers are OFF when we leave the boat. Nobody had holes in the hull.

Of note, all equipment with air gap double pole switches in the off position survived totally. This included the Simrad NSE, Radar, bathometric scanner. We have since added these switches to everything as close as possible to the device. Most modern electronics have only a software switch. I examined the boards on the stereo, VHF and LED lights. The main power boards, or rectifier circuits were all smoked where the power enters.

There are rare gas filled VHF antenna protection devices available that will ground the center to jacket to vessel ground on a strike. You need one at each end and they are not cheap. They are also supposed to be replaced after a hit. OUCH. I have multi-pin connectors now that isolate the mast electrical circuits from the boat.

Our keel is encapsulated so the path to water is limited to the ground plane and prop. I recall that the PO had clip on cabling on board. I plan to bring those back as well. We are the tallest stick in the marina so a virtual lock on every opportunity. Our mast is 80 feet and 7 X 12 section, shrouds and backstay are 3/4 inch. Ground strapping is 1/16 copper sheet X 4 inch wide widely used throughout the vessel.
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Old 03-01-2016, 10:46   #15
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Re: Should mast be grounded or not?

A couple of points.

Lightning is all frequencys from daylight to DC more or less. High frequency likes to travel on the outside of the conductor. Lightning will see you rig and run down the wires if it can. The mast is on the inside.

The university of Arizona and the University of Alaska did some ground breaking research on this. There is a Natl. Geo program on it. If you can find it and watch it will help you understand. According to their research Lightning starts in the Ionis(sp) sphere. It puts out "leaders" that look like very weak lightning. An object on the ground puts out the same if it is charged with static. They showed the two leaders moving around. When the two found each other lightning traveled down the path instantly. Their assumption is that lightning is not looking for ground. It is looking for an opposite charge. Grounding your rig to the water bleeds off the charge. There are no guarantees but I know it helps. We were in the Bahamas and lightning hit the water about 400 yards behind us. Our mast was grounded by a zinc fish in the water on a stainless cable from the upper stay. We had no damage.
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