Galvanic isolator

You can get galvanic corrosion when your boat is 100 miles from anything. It simply means that any 2 dissimilar metals in fresh water or salt water will make a very weak battery. That simple. And then a very small current flows between the ( all ) metal parts in the water. Metal begins to transfer from 1 to the others. That is why newer boats ground ALL the metal parts that make contact with the water. As long as the boat is in water it will be attacked.

 

Roger
It will only help IF you have a shore power connection AND you have some leakage current making its way to your propshaft.

On a mooring it will do nothing. If your anode continues to rapidly deplete when the boat is moored then You have 3 options (or combinations).

Paint the prop and shaft with an epoxy and antifoul.

Use a bigger anode on the shaft to get a longer time. (can be done by internal bonding to a close hull anode)

Use a slower acting anode. A mild steel nut can protect most prop and shaft installations very well in sea water and corrodes at a much slower rate than the zinc. Simply run a bright steel nut (unpainted uncoated) on after your prop lock nut. I have surveyed several boats that use this and they are always in good order.

However I recommend painting the whole assembly and stick with the zinc anode. it always seems foolish to have a large area of bare bronze on a stainless shaft immersed in a very effective electrolyte.

 

The problem with painting is that if the coating chips anywhere, you get violent corrosion at that spot

 

Thanks for the response

I guess I'll suppliment the current anode with a hanging anode - this will be easier to change and install than a hull anode.

The boat is only connected to the mains in the winter and on visits to marinas with shore power so I may add a galvanic isolator at a later date.

Thanks for the help.

 

You can read about electrolysis at A NON TECHNICAL EDUCATION ON BOAT ELECTROLYSIS and about Galvanic Isolators at THE GALVANIC ISOLATOR EXPLAINED.

 

Information

Try this for some information



http://www.geocities.com/bill_dietrich/BoatElectrical.html#ElectricalCorrosion

 

Dont even bother reading that unless you want a laugh.

You had better have a look at the post http://boatdesign.net/forums/showthread.php?p=60345#post60345

where there is a review of the site ! It's hopeless (and thats being nice).

Cheers
JIm

 

Jim is full of critical rhetoric but no facts.
Criticism is easy but vacuous if unsupported.
The article doesn't pretend to be authoritative the title says "Non Technical".
In the forum referenced:-

"They confuse Galvanic cells with Electrolytic cells " Galvanic cells are not mentioned anywhere in the article.

"they state that the anode is more positive" As any high school student will know, ANODE is positive by definition.

"state that electrons travel from Positive to Negative" The word "electron" or "electrons" appears nowhere in the article. Current is traditionally considered to flow from positive to negative.

"I haven't looked through the rest of the site I thought the preamble said it all." That appears to be true - he obviously didn't read the article.

"Also, that the ground and neutral being bonded at the dock wiring makes a difference. Ultimately they are the same: Earth's potential. " No they are NOT the same. Mis-wired boats can return the AC current through the ground wire instead of the neutral. The ground wire in docks is often a light gauge since it is not designed to carry heavy and continuous currents - just sufficient to blow a circuit breaker under fault conditions. So if you are in a dock in the vicinity of the faulty wired boat, your ground lead will have AC voltage on it which represents the voltage drop between the boat injecting current into the ground wire and the eventual dock bonding location where ground and neutral are connected. I've measured as much as 5 volts AC between a dock outlet ground and "Earth's potential".

"They're trying to sell their $195 isolator, that's all." I guess that is true however they are selling an isolator that exceeds ABYC recommendations, carries an UNCONDITIONAL warranty and sells for under $100. That makes them unpopular with vested interests who have been ripping boaters off for years.

The site is often criticized for their commercial connections and the West Marine brand name but they've been in business for 13 years and the only marine electronics manufacturer that offers unlimited warranty on all their products.

 

RCD vs GFCI:
An RCD compares the current in the 'live' conductor to that in the 'neutral' conductor. If any difference (due to leakage to earth, perhaps) is noted it breaks the circuit.
A GFCI monitors current in the ground wire and breaks the circuit if any is detected.
GFCIs are usually installed as the last step before an appliance, especially one that is near water. RCDs are typically installed on the master power feed. A good (ie. safe) shorepower system usually includes both.

 

No, a GFCI or GFI works the same way as you described the RCD. It has a triple coiled transformer with the hot in one primary coil and the neutral in the second opposing each other so if the current is equal there is no output in the secondary coil. Any imbalance caused by current going to ground on the downside of the GFI causes an imbalance voltage to be generated in the third coil which trips the breaker.

You can verify this because GFIs will work in two wire circuits and don't need a ground connection - check your hair dryer.

Independent of their "title" fault interrupters are a significant safety factor however they can be a nuisance on older boats. The accumulation of fine salt mist over years can build up on exposed equipment and slowly increase the leakage to ground until you start getting nuisance trips on your interrupter. Unfortunately the trip level is not adjustable on conventional interrupters so the interrupter ends up being bypassed. If you are considering installing a master GFI or RFD as suggested by Matt, it would be well worth your while investigating a commercial, adjustable type assuming they are available.

 

It would appear, then, that we are each using the term Ground-Fault Interrupter to describe a different device.

 

Actually I think we are using different descriptions to describe the same device. The term "Ground fault isolator" is somewhat misleading since the ground wire is not involved in the device itself. It detects a "fault to ground" by detecting the missing current on the hot side that escaped to ground.

In fact the leakage doesn't have to be to ground. There could be leakage to another phase that has nothing to do with ground and it would still trip the GFI.

 

Galvanic Isolator Size?

Not aware of but two (2) models (but I am sure someone somewhere will say there are more): one rated for 30 amps shore power services and another for 50 amp.

Anyway, :"the latest ABYC standards specify that the isolator must be capable of carring 135% of the rated SHORE POWER current, in the event of malfunction in the AC breaker/circuit protection." (www.boatelectric.com/Galvanic%201solator.htm).

For what it worth, I guess it is safe to insure that whatever brand is considered, it should state that it meets the latest ABYC standards.

Also, noticed that WM Model 3705233 installation instructions state (regarding their 50 amps AC (30 amps DC) "Use on any shore power connection up to 50 amps. AND ONLY ONE REQUIRED FOR DUAL 30 AMP CONNECTIONS". Have not seen the use of "one" on dual 30 amps anywhere else in my research of this subject matter. I guess you can double check with tech@yandian.com which is referenced at the bottom of the installations instructions.

 

The reason you can use one 50 amp isolator for a dual 30 amp supply is based on the probability that you would (hardly) never get two simultaneous faults to ground both of which were carrying 135% of the maximum 30 amp current at the same time.

Most faults blow the circuit breaker but wiring errors can cause the ground to carry neutral current at 30 amps continuous without tripping the circuit breaker and that is why Galvanic Isolators are required to carry 135% of rated current continuously.

The West Marine 50 amp isolators are individually tested at over 70 amps, at maximum rated temperature, so in the event that both 30 amp circuits were wired to ground instead of neutral and in the unlikely event that both had a simultaneous load at the 30 amps maximum, the isolator would still be able to carry the load.

If you download the owner's manual, http://www.yandina.com/acrobats/GalvOwnManual.pdf, the second page shows a schematic for the dual 30 amp arrangement.

You are absolutely correct - they must meet ABYC standards however ABYC added a new requirement recently that I feel is unnecessary and vague. They are requiring a remote monitor, visible to the boat operator, indicating the status of the isolator. Since you never use shore power underway I fail to understand the need for remote visibility to the operator. In addition they fail to say what parameters should be monitored. Some manufacturers claim to have met this requirement but are in fact only checking that the hot line and neutral line are not reversed which has nothing to do with the integrity of the galvanic isolator. A monitor that does what I think they intended would double the cost. If you are going to spend that much money on a monitor there are much more imperative items that could benefit from monitoring - bilge pump failure - engine exhaust temperature - etc..

I have seen one reference to a status monitor that looks like it might meet the intended specifications at http://www.pirella.com/products/galvanic-remote.htm. It is made in Australia and they don't have a price showing.

 

May i surgest u fit a isolation tranformer instead