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Old 21-02-2009, 13:49   #1
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Stray Current Corrosion . . .

I have discovered evidence (vanished anodes, corroded metal in 10 months) on Boracay, a steel Roberts Offshore 44, and I need to find out what has caused it.

It does look like there is some sort of leakage from the propeller shaft to the hull but I need to do more tests.

My only thoughts at the moment is the use a multimeter to go over the boat checking grounding etc.

Does anyone have any suggestions?
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Old 21-02-2009, 16:21   #2
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These may help get you started:

Electrical Study Hall:

The Galvanic Series and Corrosion

http://www.fluke.com/Application_Not...r/B0269b_u.pdf
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Old 21-02-2009, 18:33   #3
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Have you been connected to land based supplies much ?
Is you incoming lead earth wire isolated from the boat bonding system ?
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Old 22-02-2009, 03:05   #4
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"Have you been connected to land based supplies much?"
Yes, I have had a 240 VAC extension lead to my boat for the last two years. However I have only experience this problem over the last 10 months.

"Is your incoming earth wire isolated from the boat bonding system?"
Yes, I have been very carefull to ensure that there is no earth on the boat at all (floating earth system), using double master switches and making sure there is no cantact between the boat electrical system and ground.
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Old 22-02-2009, 03:40   #5
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There's an excellent (imho) downloadable book here Metal boat maintenance-A do it yourself guide by Scott Fratcher (Book) in Engineering
Covers all aspects of real life steel boat maintenance including a section about testing for corrosion.
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Old 23-02-2009, 08:30   #6
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do you run an air conditioner, fridge or freezer from shore supply?
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Old 23-02-2009, 09:16   #7
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Professional Boatbuilder, April/May 2006, Issue 100 has an article by Nigel Calder on checking your electrical system. Unfortunately I haven't been able to access it on line lately so you may have to try to find a copy at your local library or order the back issue. It's possible that the main library in Sidney would have a subscription.
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Old 24-02-2009, 16:19   #8
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First tests...

Just got the boat off the slipway and back to the marina.

I put the multimeter on a few points, engine off, all masters off:-

1) Using the steel stern tube as reference
i) SS Clamps on rubber hose connecting stern tube to stern gland 0.37 - 0.40Volts (surprising as I thought they would be insulated).
ii) Bronze Stern Gland 0.1V
iii) Propeller shaft -.1V (negative)
iv) Gearbox Coupling (propeller side) .08V

2) SS galley sink outlet .06V

3) Head - Bronze through hulls with SS ball valve 0.0V.

4) Cabin lights .03V.

From chala "Do you run an air conditioner, fridge or freezer from shore supply?" No. I sometimes top up the batteries with an automotive battery charger.
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Old 26-02-2009, 02:13   #9
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Ok is your battery charger in good condition? It could be leaking AC to DC or to ground if the casing is made of metal. Do you disconnect the battery at the battery post when charging? Is the battery clean? The way I check a clean LEAD ACID battery for cleanliness but cannot recommend it, Safety Sam would not be happy, is to rub a finger over the clean battery casing and carefully feel it to the tip of my tongue and spit it out. Acid tracking is the most common cause of battery discharge. On my boat I use a 40W solar panel to top up my five 12V battery, it keep them in top condition. I reckon it is the best way of topping up battery and shore supply is no longer required. Bear in mind that if you have a neighbouring boat with a shoddy electrical installation you can still be part of is earth return current or any other earth return current.
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Old 26-02-2009, 05:27   #10
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I've heard that the copper leaches out of bronze props and this may be promoted stray current and/or prevented by zincs.

What's the deal with this?

What happens to bronze when the copper leaches out? Can enough be removed to compromised the integrity of the propeller?
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Old 28-02-2009, 11:14   #11
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Hello Boracay.
Ok let’s try to protect your ship. That your zinc block vanish means that they are protecting something. Did you ever see French sailors with their alu boat, in marinas they dangle electrodes all around their boat to protect them. I used to own some marinas in Darwin that I rented to passing seafarers. The one with metal boat used to dive to check there zinc and always reported they vanished faster in the marina. What I suggest that what you can do is get small zinc blocks (Tailors Marines used to have a good choice), and do like the French. Unless you find better advice, I would suspend them with multi strand gal wire then you can check with a multimeter if they do any work, adjust your meter to the lowest scale in amps, volt is not good, you like to see a current flow, if you see nothing it can be that you have a bad contact, your meter is not sensible enough or the zinc is not needed, move around. Protect the boat with lasting insulating material where the wire touch the boat you don’t won’t to arm the paint. How many zinc blocks do you have on your boat and how big are they?
Good luck.
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Old 03-03-2009, 00:42   #12
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From chala

"Ok is your battery charger in good condition?" It is in as new condition.
"Do you disconnect the battery at the battery post when charging?" Yes, all is disconnected, save the bilge pump.
"Is the battery clean?" Yes
"Acid tracking is the most common cause of battery discharge." No sign of any acid spill.

From defjef

"I've heard that the copper leaches out of bronze props and this may be promoted stray current and/or prevented by zincs." There was some minor evidence of dezincification when the propeller was reconditioned (it is the zinc in the manganese bronze that is affected, copper being more noble).

From chala

"Ok letís try to protect your ship. That your zinc block vanish means that they are protecting something." I have put some zinc blocks (old prop anodes) on some tinned wire down the side. They are showing up to 0.05 volts, which is consistent with the galvanic difference between zinc and steel.

"How many zinc blocks do you have on your boat and how big are they?"
I had 6 blocks from 0.4kg to 2.4kg. While on the slip I put two additional anodes on the skeg and increased the hull anodes to 4kg each. This is possibly not enough. I will probably increase the size (and possibly the number) of the anodes when I next slip, depending on what I see.
I also put two coats of International "Primocon" steel primer on all bare metal. When I slip next I will evaluate whether the steel needs to be abrasive blasted and if a welder is needed.
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Old 27-09-2010, 20:47   #13
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How did you go with your stray current? I'm in a similar situation, in a similar boat you could say. While connecting up the aerial for the stero I was playing with the multi-meter just checking the whole length of wire was continuous, for some unknown reason I put one end of the multi-meter on one of the bolts holding a portlight in place. To my surprise it indicated a circuit. I went to the bible (Nigel Calders book) and went about testing for stray current. It would appear there is a problem but electricity is like trying to understand Chinese for me, I'm hopeless at anything other than the basics. Yes thats a problem if your going cruising but hey at least I recognise its a problem! Did you manage to sort out your stray current? I'm in middle harbour, can you recommend anyone that I could get to have a look at this for me to either put my mind at ease or fix the problem. I could spend months looking at this and get nowhere, sometimes you need to get a little helping hand!
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Old 27-09-2010, 21:03   #14
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You can buy or borrow a "Galvanic Corrosion" meter and then test each section in the boat. It is basically a milli-volt meter with two very long leads. One has a silver bulb on one lead that is placed over the side into the water. The other lead has an alligator clip on it and you attach it to the various metal parts of the boat e.g., prop shaft; engine; through-hulls; etc. The scale on the meter is calibrated for the different metals and there are green "zones" for each metal to let you know if that particular item is within normal potential. If one part of the boat is "out of the green" then there is a problem that needs to be found.
- - It is not uncommon to leave appliances plugged into an outlet and then the appliance develops an electrical "leak" to ground. The meter can help find that item.
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Old 27-09-2010, 21:52   #15
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Quote:
Originally Posted by ianhef View Post
While connecting up the aerial for the stero I was playing with the multi-meter just checking the whole length of wire was continuous, for some unknown reason I put one end of the multi-meter on one of the bolts holding a portlight in place. To my surprise it indicated a circuit.
By "a circuit" do you mean that the multimeter was reading 12v with the probes on the hull and the + lead from the sterio?
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