Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 09-12-2013, 18:15   #16
Senior Cruiser
 
colemj's Avatar

Cruisers Forum Supporter

Join Date: Oct 2005
Location: Presently on US East Coast
Boat: Manta 40 "Reach"
Posts: 10,049
Images: 12
Re: Chassis Ground Question

Quote:
Originally Posted by Vasco View Post
The only thing I know is that in a direct lightning strike, nothing much protects you. Been there twice.
Not true. Here is our newly installed lightning prevention system:

Mark
Attached Thumbnails
Click image for larger version

Name:	img_8238_med.jpeg
Views:	81
Size:	102.6 KB
ID:	71820  
__________________

__________________
www.svreach.com

You do not need a parachute to skydive. You only need a parachute to skydive twice.
colemj is offline   Reply With Quote
Old 09-12-2013, 19:33   #17
Registered User

Join Date: Jul 2013
Posts: 126
Re: Chassis Ground Question

Quote:
Originally Posted by Vasco View Post
The only thing I know is that in a direct lightning strike, nothing much protects you. Been there twice.
Proper grounding will protect you from being electrocuted, and it will prevent severe hull damage which could sink your boat. It will limit the damage to other systems too.
__________________

__________________
El Rubio is offline   Reply With Quote
Old 09-12-2013, 21:42   #18
Registered User

Join Date: Jan 2009
Location: Tampa, FL
Boat: Yankee 30'
Posts: 143
Re: Chassis Ground Question

I once had a system aboard Antea protecting my VHF and ham radios. The ham actually had one of those archaic Dr. Frankenstein dual blade switches on a ceramic base. One side would switch the radio's antenna terminal from the tuner to the chassis ground and the other switched the radio's "12V in" from battery to its chassis ground, thereby somewhat isolating the radio from spikes from antenna and the 12V system. The VHF I simple unplugged, both power and antenna, and plugged the antenna coax to an external connector that was shorted and connected to the boat ground. As I used either radio very little at the dock, I would leave the radios in the protected mode.
I live aboard in Florida. The past decade has been relatively quiet but in the eighties, electric storms were very frequent and violent. One time I was having a drink at the club house and actually observed lightning striking the top of Antea's mast. A shower of green sparks cascaded down to the deck. The VHF antenna was vaporized. All instruments and autopilot fried. Semiconductors and chips exploded - little chunks of black epoxy rattling in the boxes. But both the ham radio and the VHF survived. Anecdotal? Just saying. Stan
__________________
sv.antea is offline   Reply With Quote
Old 09-12-2013, 22:23   #19
Commercial Member
 
CharlieJ's Avatar

Cruisers Forum Supporter

Join Date: Dec 2008
Location: St. Petersburg, FL
Boat: Gulfstar Long Range Trawler; 53'; BearBoat
Posts: 835
Re: Chassis Ground Question

A cloud to ground strike typically has a potential of 5,000 to 50,000V and current of 30,000A can travel 50,000 feet from the cloud to the ground, ionizing the air and making it conductive and heating the air to > 20,000C and we think we are seriously going to affect the outcome in the last 50 to 100 feet of its journey?????
__________________
Charlie Johnson
JTB Marine Corporation
"The Devil is in the details and so is salvation."
CharlieJ is offline   Reply With Quote
Old 09-12-2013, 22:59   #20
Registered User

Join Date: Jan 2009
Location: Tampa, FL
Boat: Yankee 30'
Posts: 143
Re: Chassis Ground Question

Charlie J, I think your voltage is actually low (5kV will not produce a very long spark), but why so fatalistic? Lightning strike's energy is contained mostly in portion of radio frequency band. Hence the mysterious paths the current often takes, dodging interiors of Faraday's cages and staying on the surface of conductors (skin effect). A sailboat mast and rigging grounded to an external keel can apparently easily handle the current (few microseconds?). One such Faraday's cage probably was the box of my ham radio. And some say the sailboat rig constitutes a Faraday's cage of sorts as well.
I am sure there are other anecdotes of this kind. A J-24 few boat lengths on my beam in a Thursday night race was struck by lightning. No one was hurt, in fact the crew didn't believe me they were hit until they saw what was left of their Windex.
In any case, what little protection we can provide may not save the chart plotter from direct hit, but may from the spikes (EMP) induced in conductors aboard your boat when a boat down the dock from you gets it. Stan
__________________
sv.antea is offline   Reply With Quote
Old 10-12-2013, 00:32   #21
Certifiable Refitter/Senior Wannbe
 
Wotname's Avatar

Cruisers Forum Supporter

Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: Van DeStat Super Dogger 31'
Posts: 7,331
Re: Chassis Ground Question

Please remember that a lightning strike produces several but very different effects.
For instance, there is the current flow produced when the very high voltages ionise the air and thus makes it a very good conductor. This massive current flow creates lots of heat and melts or burns things that are part of it's path.
Then there is the electromagnetic radiation that is produced from the current flow, this has the potential to fry electronics at quite some distance and induce voltages and potential current flows in nearby metal objects.

These are separate issues and require different types of protection or at least mitigation.

There is no silver bullet.
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
Wotname is offline   Reply With Quote
Old 10-12-2013, 01:19   #22
Certifiable Refitter/Senior Wannbe
 
Wotname's Avatar

Cruisers Forum Supporter

Join Date: Jan 2008
Location: South of 43 S, Australia
Boat: Van DeStat Super Dogger 31'
Posts: 7,331
Re: Chassis Ground Question

Quote:
Originally Posted by Canibul View Post
This is confusing me a little. I thought that the actual flow of electrons was from negative to positive, ( hence the very terms themselves) and that would make the positive wiring the "dc return".

If the "dc return" (negative) is not a ground, but is connected to ground at the engine...then it definitely IS a ground....

this is kinda why I asked the question....
Yep, electickey can be confusing and when terms like ground returns, earth loops, single ground points, RF grounds, ac ground, dc ground, shield grounds , isolated circuits, floating voltages, current flow, bonding, equipotential earths and so on get tossed around in the conversation, even well qualified practitioners of this dark art can get confused so really the lay person has no chance of staying abreast of the conversation, let alone fully understanding it.

But help is on it's way Canibul with my simple to follow explanation.

When discussing or thinking about low voltage (12 or 24 volt) DC boat circuits, disregard and don't use any term that contains ground or earth. I would go so far as to remove the word "return" as well. Leave these terms for the others to use. There are only two wires to consider, the positive one and the negative one.

Now remember that all DC electrical devices require current flow in order to operate and this flow must have a path to flow along. Interrupt the path and the item stops working. In this simple explanation, it really doesn't matter which way this current flows or what the current is (more below on that).

The source of the current will be a battery or a solar panel or a alternator and these items have two terminals - one marked positive (+ve) and the other negative (-ve). Conventional flow is considered that the current flows from the +ve to the -ve external to the source. Of course, you can see that internal to the source, this same current must be flowing from the -ve to the +ve but mostly we don't care about where the current flows inside the battery or solar panel or what have you.

So we have a +ve lead taking the current to your desired device or load (light, anchor winch, radio etc) and a -ve lead taking this current back to the source (battery etc). The load has two terminals, a +ve one and a -ve .We connect the source to the load using +ve to +ve and -ve to -ve. That's it in a nutshell.

We usually stick the circuit protection devices (circuit breakers / fuses) in the +ve lead along with controls (switches) and distribution items (switchboard, bus, panel etc) and then lump all the -ve leads together and take them back to the battery.

We could do it differently, we could put say the switch in the -ve side or even everything in the -ve wire but we normally just don't, we stick with the conventional way.

We now have the DC device working and no mention of ground, earth or whatever. These terms do have uses and do mean things, it is just they are not relevant to understanding how low voltage DC circuits work.

Now about the direction of current flow; this keeps changing as the conventions change but really at the end of the day, they are merely conventions. If current is to be considered as the movement of electrons, then they move from the -ve terminal to the +ve terminal (exterior of the source); if current flow is considered to be "holes" or the absence of electrons, then it flows the other way. So you see it really is just convention and the current convention (no pun intended) is that it flows from +ve to -ve which is kinda easy to follow. It goes from where there is plenty (+ve) to where is none (-ve).

Now please, no complicated replies pointing out theoretical inconsistencies of the above, it is just a basic DC primer to point out how to understand a boat DC circuit without resorting to confusing terms like grounds etc. for the uninitiated.

By all means point out gross inadequacies of my explanation.


Quote:
Originally Posted by btrayfors View Post
Maybe this will help: Grounding

Bill
Nice article Bill, clear and concise, thanks for osting it.
__________________
All men dream: but not equally. Those who dream by night in the dusty recesses of their minds wake in the day to find it was vanity: but the dreamers of the day are dangereous men, for they may act their dreams with open eyes, to make it possible. T.E. Lawrence
Wotname is offline   Reply With Quote
Old 10-12-2013, 10:52   #23
Senior Cruiser
 
Vasco's Avatar

Cruisers Forum Supporter

Join Date: Apr 2004
Location: Toronto
Boat: CS36Merlin, "La Belle Aurore" Ben393 "Breathless"
Posts: 7,138
Re: Chassis Ground Question

Quote:
Originally Posted by colemj View Post
Not true. Here is our newly installed lightning prevention system:

Mark
You going to sail with that?
__________________
Rick I
Toronto in summer, Bahamas in winter.
http://groups.yahoo.com/group/beneteau393/
Vasco is offline   Reply With Quote
Old 10-12-2013, 11:09   #24
Senior Cruiser
 
Vasco's Avatar

Cruisers Forum Supporter

Join Date: Apr 2004
Location: Toronto
Boat: CS36Merlin, "La Belle Aurore" Ben393 "Breathless"
Posts: 7,138
Re: Chassis Ground Question

Quote:
Originally Posted by El Rubio View Post
Proper grounding will protect you from being electrocuted, and it will prevent severe hull damage which could sink your boat. It will limit the damage to other systems too.
My keel stepped mast was grounded to the external lead keel with the appropriate size wire. Everything on the boat was fried except a small Garmin GPS (G67?) and my hand held vhf which was tucked into my bib foulies. There was evidence of "treeing" (burn marks) in the paint at all the thru hulls and at the gudgeon and pintle. The thru hulls were not bonded. All lights blown, alternator shot. Autopilot shot and all instruments shot. Large error in magnetic compass. I was sailing at the time and started the motor when there were two strikes right ahead of me. Direct strike at the masthead.

What could've prevented this? One of those bottle washers? If you get a direct hit what can "limit the damage to other systems"?
__________________
Rick I
Toronto in summer, Bahamas in winter.
http://groups.yahoo.com/group/beneteau393/
Vasco is offline   Reply With Quote
Old 10-12-2013, 11:32   #25
Commercial Member
 
CharlieJ's Avatar

Cruisers Forum Supporter

Join Date: Dec 2008
Location: St. Petersburg, FL
Boat: Gulfstar Long Range Trawler; 53'; BearBoat
Posts: 835
Re: Chassis Ground Question

Vasco- IMHO, nothing could have prevented your damage.

There is some evidence that the system developed by Dr. Thompson at the University of Florida Marine Lightning Protection Inc. can mitigate the damage so the vessel is not severely damaged. Note that, even with Dr. Thompson's system installed, Domino's systems survived one hit/near hit but the systems were damaged some time later by another hit/near hit.

If struck or nearly struck, the vessel's systems must survive a direct surge through the wiring system, an induced surge through the wiring system and electromagnetic pulse (EMP). Surge protectors upstream of mission critical components may help mitigate the surges. EMP hardening of marine electronics will increase the survivability of those electronics. As I understand it the EU is far ahead of the USA regarding EMP hardening.
__________________

__________________
Charlie Johnson
JTB Marine Corporation
"The Devil is in the details and so is salvation."
CharlieJ is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Let's Get to the Bottom of the Lee-Bow Effect Once and For All Dockhead Challenges 774 02-12-2013 12:40
Isolating Transformers, The Earth wire connection conundrum goboatingnow Electrical: Batteries, Generators & Solar 331 04-11-2013 21:29
Distinct Activities: Shackled by a Common Name? Andrew Troup Navigation 728 23-01-2013 21:07
Differences Between Ground, Apparent, and True Wind Direction twistedtree Navigation 92 26-12-2011 14:48
question marks on all channel markers for cmap 93 chart sdowney717 OpenCPN 21 20-12-2011 14:57



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 23:51.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.