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Problem-solving with Nigel Calder via e-mail
Author: Dean Plager
Lat/Lon: N/A
Location: Davis Strait
Course: Heading toward Breevort Islands
Speed: N/A
Weather: N/A
Sightings: N/A
Log date: August 08, 1998
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As you know, we had some concerns regarding the interaction of the bronze grounding plates I installed for our radio and the wrought iron rivets which hold the boat together. Here is an interchange of e-mail between me and Nigel Calder.
Nigel's Calder's message:
Dean,
Just got back from doing a couple of seminars to find your forwarded E-mail re: the Snorri. Sounds like you've got a galvanic couple at work between your grounding plates (presumably, sintered bronze) and the rivets,although its a little difficult to visualize the mechanism. We might also have some stray current corrosion, but once again it's hard to visualize the mechanism. I would like some details on construction, radio installation (and any other DC equipment, including an electrical schematic; is there an SSB with a transmit capability, and if so how is it grounded and how is the antenna rigged?), where the corrosion is occurring, the waterline, and the level of any accumulation of bilge water within the hull (or any other moisture). Also all metals used in construction and their location (maybe there's some other galvanic couple at work). And some sense of the rate of corrosion (I don't need this quantified; just a subjective impression).
I guess you should tell the crew to pitch their radio equipment overboard and get a proper sense of what it's like to be a Viking!
Nigel
My response:
July 19, 1998
Nigel,
I appreciate your rapid reply. I'll try to answer as many of your questions as I can now. First off, I've asked our technical support person at Lands' End, Randy Lagman, to fax you the schematic of our electrical system.
As far as boat construction: SNORRI is a replica Viking ship, a knarr, of lap strake construction which are held together by wrought iron rivets. Some 2000 in total. There is really no other metal in the boat's structure. That's why we added two of the largest Dyna plates we could get, 6 by 18 more or less, so we could have an SSB. It's these sintered bronze grounding plates near all the rivets that have us concerned.
We haven't checked for actual corrosion yet since the boat's only been in the water since June 15 and the water is pretty chilly here. We're just above the arctic circle in the town of Sisimiut, Greenland. There's more information on the boat's construction at our web site <beyond.landsend.com> if that's helpful.
We have a Sea 235 SSB with 3" copper foil connecting the tuner and the grounding plates. A run of about 10'. The antenna, about 50' long, is just standard black antenna wire hoisted from the stern to the mast head where the backstay would be if we had one. It's not structural at all. Merely hoisted with a flag halyard. I have put a multimeter between the grounding strap and one of the nearby rivets. I get readings of 0.3 volts and a current of about 0.7 milliamps.
We have quite a list of electronic gear (see below) but I think this is completely isolated when the master switch is turned off which is when I've taken the readings. I'm sort of assuming our problem - if we have one - is simply between the two dissimilar metals on the hull: bronze and iron. My thought was to attach a zinc to the bronze with the notion that this would then have the zinc give up to the iron instead of the iron to the bronze. Would that work?
We certainly appreciate your assistance.
Thanks,
Dean Plager
SNORRI
List of Electrical Equipment:
2 gel cell batteries 200 AH each
Sea 235 SSB with tuner
mini-M sat phone
computer
mast head tri-color light
VHF radio
Ampair 100
Link 20 battery monitor
smart charger
Nigel's reply:
Dean,
I haven't received anything since I wrote re: your millivolt readings, etc. Just in case you didn't get this message, here it is again:
A 300 millivolt reading between the bronze and the iron is what you would expect to get (bronze reads around -300 mv against a silver/silver chloride reference cathode; iron around -600 millivolts). The current you are reading (0.7 milliamps) is being created by the electrical connection you are making with your multimeter.
In normal circumstances, it is difficult to see how there could be an external (independent of the seawater) electrical connection between the bronze and the iron (unless the two are in physical contact where the ground plates are mounted, in which case the ground plates will eat the heck out of these rivets). In the absence of this direct electrical connection, corrosion will not occur. When you put the meter in the amps mode and touch the probes to the ground plates and rivets, you make the circuit and the corrosion begins, hence you get a reading. Adding zincs is not likely to help. Unless the rivets have a direct electrical connection (e.g. a bonding wire, which is clearly completely impractical) to the zincs, the zincs will not protect them. All the zincs will do is generate galvanic current between themselves and the bronze plates, which in itself may have some undesirable consequences.
If you are not having problems, I would leave things alone and watch the situation closely (if it ain't broke, don't fix it). If you find you are having problems, we'll need to take a closer look at things.
Nigel
The result of this dialogue being...
So we are no longer concerned. Besides we aren't using it for anything now anyway.
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