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Old 18-07-2021, 02:25   #1
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Electric propulsion - lightning protection ?

I'm going to convert my sailboat to electric propulsion.
The 29' boat (Maxi 87) has a cast iron keel and an alu mast.
An air terminal on top of the mast, and cabling with alu cables between the mast foot and the keel will provide a good path for discharging a lightning strike.

The existing 1976 Volvo Penta MD6A does not have any sophisticated electronics so it should be happily running without power in a strike case, exept for the electric cooling water pump.

However, when installing the new system there will be plenty of electronics.

As the motor is just connected to the shaft by a toothbelt, there are no electrical connection to the sea water.

My thoughts are to place all sensitive electronics as far astern as possible, away from the (connected) mast/keel, that will act as the main conductor of a strike.
The shroud and forestay chainplates will be bonded by 4AWG cables to the keel. I will NOT bond the backstay, to keep it insulated.
(Probably I'll change the backstay to Dyneema.)

The motor will be placed right on top (or further astern) of the converted Volvo 100S saildrive.
I intend NOT to bond the motor in any way.

The idea by this is to let the main strike go through the mast, shrouds and forestays, leaving the sensible stern area away from lightning spikes.

So, I ask you experts out there - does this sounds reasonable to you ?
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Old 18-07-2021, 08:29   #2
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Re: Electric propulsion - lightning protection ?

First, the power in a lightning strike is so huge that it could overwhelm pretty much any kind of ground or safety you could afford to install in a boat.

Second, due to the power in a strike what it does is very random. There is no way to accurately predict exactly what path it might follow.

One example, a friends boat was struck twice about a year apart. Both followed very different paths. One jumped from the mast to the 12V wiring at a light in the overhead, followed the wiring to a junction below the waterline and then right through the fiberglass into the water. Melted a 1/2" round hole in the hull, fortunately leaving the melted glass behind like a window that prevented the boat from sinking. The other strike followed a different path to a bronze through hull.

There are systems used to protect very expensive and very sensitive equipment like big computer systems that run banks and stock markets. But the protective equipment is also very expensive, probably many times the cost of your boat.
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Old 18-07-2021, 09:43   #3
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Re: Electric propulsion - lightning protection ?

Skipmac said it clearly and with wisdom. The average lightning bolt is 3,000,000 volts and 300,000 amps. That is beyond our imagination and beyond any grounding conductor we can incorporate into a boat. Parts of that bolt will follow multiple and unpredictable paths to ground (or the other way up). Placing the electronics in a Faraday box is no good, because there are wires coming in and out. Yes, it is worthwhile to ground your rigging, with the hope that you'll get just part of a strike and it will find your open path convenient, but beyond that, redundancy and insurance are your only protections.
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Old 18-07-2021, 18:13   #4
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Re: Electric propulsion - lightning protection ?

i know that a lightning is uncontrollable.
But it can be directed in a better path, than spitting out spikes everywhere on its way to the water, if ungrounded.
As we do on land by using lightning protection systems.

The idea is to let the main strike go through the mast, shrouds and forestays, thus (hopefully) leaving the electronic equipped sensible stern area away from lightning spikes.

So, does it sound reasonable to you, to heavily "fortify" the bow and midship with conductors (making a good path for the lightning bolt, and keep the stern isolated ?
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Old 18-07-2021, 18:36   #5
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Re: Electric propulsion - lightning protection ?

I’d just forget about it. Your current system and pretty much every other system on every other boat is vulnerable in a lightning strike.

Certainly almost every single modern propulsion system is vulnerable.
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Old 18-07-2021, 18:52   #6
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Re: Electric propulsion - lightning protection ?

Well, Chotu, I am mostly sailing singlehanded, and I like to come home to my little daughter after sailing.
In the summer of Denmark, the thunderstorms can build up in just a quarter of an hour. Running home is not an option.

If I don't return, she will never learn to sail when she gets older.

So, protection (as far as possible) is my goal.
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Old 18-07-2021, 19:01   #7
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Re: Electric propulsion - lightning protection ?

Quote:
Originally Posted by carstendenmark View Post
Well, Chotu, I am mostly sailing singlehanded, and I like to come home to my little daughter after sailing.
In the summer of Denmark, the thunderstorms can build up in just a quarter of an hour. Running home is not an option.

If I don't return, she will never learn to sail when she gets older.

So, protection (as far as possible) is my goal.


My point is you aren’t going to be able to protect it.

So invest in safety equipment for getting home instead. (Small kicker petrol outboard, sails, life raft, epirb, etc)

There is no way to mitigate the current that would develop in your boat’s wiring. The lightning doesn’t even have to hit your boat to destroy nearly every wire onboard. Just the induced current from a bolt passing nearby the boat can fry every single wire.

Forget about this idea. It is a waste of time, money and effort.

Lightning (and especially the currents induced in your wiring) will do whatever it wants. You can’t stop it. There is literally nothing you can do to prevent it from blowing out any or every wire onboard.

So plan for that. Be ready for that situation.

Some of survival is luck. This is a case where luck is 90% of the outcome. 10% is planning for what you’ll do after every wire is melted.

There is no defense
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Old 18-07-2021, 19:06   #8
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Re: Electric propulsion - lightning protection ?

The motor itself will probably survive, especially if it isn't grounded and has belt drive. The motor control electronics are another matter, but motor controllers are not too expensive so it would be sensible to have a spare motor controller and keep it inside a metal box, then if your motor controller fails (for any reason, not just lightning) then you can swap it out.

Same applies to battery management systems, battery chargers and solar controllers. Keep a spare unit in a metal box, placed in a dry compartment near the stern for anything which is mission critical. You should also do that with autopilots, spare VHF, spare GPS and other critical electronics.

BMS boards for lithium batteries are a tricky one, not so easy to replace. If you're going offshore then I would consider using good old fashioned lead acid batteries to eliminate any battery management electronics. Position them low down as useful ballast and the weight is not so much of an issue.
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Old 18-07-2021, 20:09   #9
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Electric propulsion - lightning protection ?

No point protecting electronics if half your wiring is fried

Carrying around a complete spare control system , AP , other electronics is way too expensive. Your going to have to make a call
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Old 19-07-2021, 06:12   #10
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Re: Electric propulsion - lightning protection ?

I'm going to take the position that there is at least some utility in supplying "easy" routes for lightning, or at least avoiding being part of them. Sure, ground the shrouds and mast. At the same time, don't count on any such arrangement stopping a direct hit from turning everything in the boat to a smoking blob. Electronics take only very small spikes to fry, and the electromagnetic field will contain the entire boat. There is no point to elaborate grounding systems, and the protection will be only from a marginal strike.
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Old 19-07-2021, 06:41   #11
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Re: Electric propulsion - lightning protection ?

Chicago... 4 story building office/warehouse. Metal fire water tank on top of the building to feed sprinkler system. 8" steel pipe feeding out of the tank to sprinklers and as a support for the tank mounted in a concrete bed 8 feet in the ground. Three 2" copper rods drive 21' into the ground at three different point in the building all attached to the sprinkler system in case of a lightening strike.


Lightening hit tank, system sort of worked and main strike went to ground. Side effect, even with no direct hit every phone, computer terminal, mini computer, tv, printer, etc. was smoked. Induced voltage is a "&^%%#%". Shielded cable made zero difference.



Reality, electronics designed to work on 5vdc or 12vdc just doesn't do well with thousands of volts of induced current. Spend all you like, ground or don't, mother nature won't mind.
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Old 19-07-2021, 07:09   #12
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Re: Electric propulsion - lightning protection ?

So far, advices has been :
Ground the mast, shrouds and stays to the keel, so the hull ain't gonna blow holes.
Get a gas outboard for spare use.
Keep my VHF and cellphone in a Kellogg's box during the storm.
And pin up a nice picture of Santa Barbara.

Anything else, please ?
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Old 19-07-2021, 09:51   #13
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Re: Electric propulsion - lightning protection ?

My mum always took the antenna out of the TV when there was a storm. Or you could connect everything with fibre and keep the important stuff in a faraday box. A direct strike is not really the issue. That is an insurance job. The problem is the St Elmo’s fire. Static will wipe out semiconductors.
Someone downed a foxbat. They laughed at the crude valve circuitry in the receivers until they thought about EMP….
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Old 19-07-2021, 11:35   #14
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Re: Electric propulsion - lightning protection ?

I think you are worrying unnecessarily. This FDP hasn't EVER heard of lightning striking a boat catastrophically in Danish inland waters. Should it happen to you, you are not likely to have much to say about it afterwards :-).

As for getting home: It's a sailboat, so if you can still wiggle your extremities, just sail. Nowhere is very far from anywhere else in Danish inland waters. Just as a for instance: You've departed Bogense bound for Kalundborg. You are at about 57º45'N/10º40'E within a coupla miles, and you know that, because having a healthy skepticism about electronic navigational tools, you have remembered that while the cursed things were still working. You DO make mental note of your Lat/Long every half hour or so and remember it don't you :-)?

So all you fancy gear goes out, and even your steering compass seems to have developed St.Vitus' Dance. And you, thanks to said skepticism, have paper charts aboard. So no sweat, eh:-)?

You "forgot" the charts, you say? So how do you get home from 57º45'N/10º40'E? Well. I'd be VERY surprised if you can't see the southern shore of Samsø from there. After all it's only 4NM distant. On a nice close reach. They even have a harbour at Kolby Kås just around the corner. And you can take a taxi home from there :-)! The northern tip of Fyns Odde is 6NM. So with the wind a tad aft of the stbd beam, as you probably would have, you could skive in through Gabet and be in perfectly protected waters in Odense Fjord in about 2 hours, but easier than that, since you'd be able to see Odden. Just put in to Korshavn. Under sail, you'd be in behind the breakwater in about an hour. They have taxis even there, though in my youth they were horse-drawn :-).

I really don't think you have much to worry about!

God vind, du gamle ;-0)

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Old 19-07-2021, 15:05   #15
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Re: Electric propulsion - lightning protection ?

I'd ground the shrouds not the mast. JMHO
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