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Old 22-12-2007, 01:21   #1
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Question Voltage in water ??

Hi all you knowledgeable folk. A question for you electrical whizzes. We are currently in Sebana Cove Malyasia, very quiet very nice but a bit run down A friend has just found his flanges on his water cocks have dissolved away, new 1.5 years ago. So doing a little exploring we have found .5 volts between finger and boat. On our steel boat, we have found 1 volt between boat and water ?????

Whats the likley cause and what's the likely impact on the boat and how long for this to happen ???, or is it normal, I've never measured it before. The marina is up a river so the water is not very saline.

Thanks, Enzwell.
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Old 22-12-2007, 01:39   #2
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OK firstly some questions.
First major question is, are the Through hull fittings bonded to one another via a good earth cable or strap?
Is that cable then connected to the earthing point on the motor.?? This is essential or you will run the risk of having what has happend happening. Eating the through hulls that is.
Are you connected to the dock via shore power? Have you checked the state of your earth on the boat, the cable and on the shore box?
Is it possible even, that you have jury rigged your cable to adapt to the malaysian power system?
Now for some slightly different questions.
How long have you been where you are? It is possible the real damage was done elsewhere.
When you measured 0.5V between finger and boat, what was it that you actually measured on the finger. Is it the power box? If yes, you should have no voltage and to "see" such means you have a poor earth somewhere someway.
And finaly, how long has it been since you checked the Anodes??
If you go to the "study Hall" at the top of this electrical Forum, you will find a table on nobility and what voltage you shoudl read for what metal you need to protect. Umm, at least I think the table was placed there. I better look.
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Old 22-12-2007, 02:00   #3
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Thanks Alan.
Firstly, wed are steel so everything is bonded I believe.
The boat is not connected to shore power, we do have a lead coming aboard but to a multi point running the toaster etc.
We put new anodes on 6 months ago.
We have no damage, it is a friends glass boat that has the damage.
The voltage was measured by dipping one end of the multi meter into the water and the other on the boat, and then one end in the water and the other on the metal edge of the finger.
Hope that explains it more clearly.
Enzwell
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Old 22-12-2007, 02:11   #4
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enzwell - Don't know if we have met up there or not or how long you have been there. I have not heard of a stray current problem at sebana cove but it is surely possible. As you probably know George (Amable) has been there for a long time. He would know of any history and more importantly how to get action to get it fixed if it confirmed to be a shore power problem.
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Old 22-12-2007, 03:05   #5
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Quote:
Originally Posted by enzwell View Post
The voltage was measured by dipping one end of the multi meter into the water and the other on the boat, and then one end in the water and the other on the metal edge of the finger.
The voltage between the boat and the sea is zero because the steel yacht is short circuited to it. The voltage you are measuring is essentially meaningless as it is just the voltage of the cell made up of the immersed metals your boat on one side and the metal that the probe of your multimeter is made from on the other, with the electrolyte being the salty water (actually, even fresh tap water will give a result).

For example, if you attach a piece of different metal to the multimeter probe than the probe is made of, say a piece of aluminium, and just immerse part of that metal instead of the probe itself then you will get another voltage.
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Old 22-12-2007, 05:05   #6
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The voltage referred to could be as simple as described above. On the other hand, improperly gounded boats at most marinas can cause huge problems. If you have a steel boat and have plans to use shore power, you absolutely need an isloation transformer; in fact, most fiberglass boats should have them since their props, prop shafts and through-hulls are all metal.

Boats connected to shore power all have a common ground. The end result is that the best grounded boat ( steel, if there is one) will become the ground for all other improperly grounded boats connected to the same shore power system. When that happens the anodes become overwhelmed and all other metal items below the waterline will suffer the same fate as on your friends boat.

The other advantage to an isolation transformer (apart from isolating you completely from the shore ground) is that it can also be constructed as a 'step-down' transformer. This enables you to convert the 240v used in may European and some Caribbean countries to 120 volt, so that you can continue to use your appliances and tools.

Brad
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Old 22-12-2007, 12:37   #7
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Once I was docked next to a 70 foot powerboat that was having problems with the grounding system on the shore power. They unplugged from the dock because they said that they were putting seventeen amps of current into the water. They had an aluminum hull. The marina did something to the shoreside ground, and the problem went away.

Over the years I've seen rudders, shafts and props damaged by leakage current. I once touched a saildrive in a boat yard and recieved a tingling shock. Out of curiosity, I measured the saildrive's voltage to ground, and it was eighty volts of AC. They were hauled out for new rudders because the old ones had severe electrolysis.

For all of these reasons, I try to anchor out whenever possible.
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Old 22-12-2007, 15:32   #8
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Measure the DC voltage between the boats ground and the shore power ground. Stray DC currents are what will eat up your anodes and then the metals that you want to keep around.

At the very least, if there is any stray DC voltage then you need to add a breaker in your shore power ground that opens when the voltage is low and closes when the voltage exceeds a certain threshold voltage to do what a ground is supposed to do...protect you from electrocution. This breaker switch is automatic. The name of the switch you put in your shore power ground escapes me right now.

You said you did not have shore power but you were getting electricity from somewhere...the bottom line is you have shore power and your ground could be acting as the conduit for electrolytically protecting all the other boats at the expense of the metals on your boat. You don't want to be hard wired directly to all the other boats, only to have stray DC currents destroy your metals.

I have an aluminum boat that was subject to the same stray DC currents running through my shore power ground. Adding that little switch box in my ground worked.

Also do not ground any DC to a metal hull. You want what is called floating system and it is how the DC system on metal boats should be wired. Don't make the path of least resistance, at any point in a circuit, your hull.

There could very well be other things going on as well. I would ask for a different slip and see if electrically, things calm down.
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Old 22-12-2007, 22:49   #9
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David M:

Do you mean a isolation transformer?
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Old 23-12-2007, 01:55   #10
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Charlie, an isolation transformer is not simply a switch. There are two coils which, in a boat, should be epoxy coated, separated by a copper plate. It completely separates or 'isolates' your AC ground from the ground at the dock. In essence, it is a separate source of electricity for your vessel that is merely powered by the shore power. They are quite heavy and they are not cheap. They also generate quite a bit of heat.

These are to be distinguished from 'galvanic' isolators which, as I understand it, will alert you to a problem with reverse polarity or a ground fault, but will not fully correct it. If you are planning on using 30 amp shore power you will need at minimum a 3.6 kV isolation transformer (120 volts x 30 amps); if you want 50 amp shore power, a 6.0 kV. Charles manufactures some nice units for marine purposes; however, you can also have one manufactured to marine specs for less money by a number of manufacturers. If you are doing this, ask that it also be set up for use as a 'step-down' transformer (from 240 V to 120 V). Usually this will only require changing the leads and they will/should have instructions on the case. Also make sure you specify epoxy encapsulated coils and a powder coated case.

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Old 23-12-2007, 08:30   #11
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Hi David.

E.L.C.B. or RCCB.

Earth leekage circuit breaker or reisue current circuit breaker. That the type the hotels have on any power outlet, say a razor or hair drier, in a bathroom
In therory they knock of the curret befor it kills you when you put it in water. Normally in mili seconds.

I actually tested one. ( With rubber gloves on ) it worked.

Steve
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Old 23-12-2007, 14:15   #12
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Yes, ground-fault outlets will save your life. Unfortunately, they are in place after your boats main breaker and grounding system, so the same problem exists. If your boat is well grounded and is on the same dock supply system as an improperly grounded yacht, your vessel (and its anodes and underwater metal) will be sacrificied for the other yacht: there is a common ground. As far as I know, only isolation transformers will eliminate this risk if you are going to use shore power.
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Old 23-12-2007, 23:08   #13
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Quote:
Originally Posted by Charlie View Post
David M:

Do you mean a isolation transformer?
Charlie,
No, this is different. Its not a transformer. All it does is keep stray DC currents from running through your shore power ground. It has nothing to do with your hot or neutral as does an isolation transformer.

David
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Old 24-12-2007, 01:41   #14
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ABYC 'A-28' - GALVANIC ISOLATORS
Follows the Project Technical Committee report (Aug. 2007), expected to be “passed” at the January 29008 meeting. The revised requirements would become effective July 2009.
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Old 24-12-2007, 15:03   #15
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Thanks Gord!...darned if I could not remember.
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