Prop shaft bonding brushes?

[CharlieJ]

@boatpoker #56
Thank you for the reference. I already have it in my library. I am confused as to why you referenced a tech bulletin for a saildrive. The OP's boat is a conventional shaft and prop.

[CharlieJ]

@mdenize #52

I am not sure why Beneteau would put such a huge hunk of iron in salt water without protection part from cheapness over lead or design obsolesce however as others have said I would try to find out why and how it is supposed to be protected.

For galvanic corrosion to occur, there must be two dissimilar metals, electrically connected immersed in the same electrolyte. What your friend experienced could be a keel made of scrap "iron" of different compositions thus creating local self corrosion cells at the microscope level. Said another way, a quality component, isolated from other metal components generally has a self corrosion lifetime of many, many years.

So the boat builder was thinking correctly that the keel was isolated from dissimilar metals but may have failed to understand the ramifications of the manufacturing process and materials of the keel. A sacrificial anode on the keel would be in order.

[YANDINA]

Originally Posted by CharlieJ

@Yandina #57:


Please explain the underline part above. A malfunctioning galvanic isolator could have everything to do with accelerated anode wastage.

A galvanic isolator is installed, and if functioning correctly, will block cathodic protection current from leaving the vessel via the safety ground wire. Without a galvanic isolator, cathodic protection current can leave the vessel and:

1. protect underwater metal components connected to the safety ground system; e.g., metal sheet piling, exposed concrete rebar, etc.
2. protect adjacent vessels also plugged into shore power that have inadequate sacrificial anodes for their underwater metal components.

A galvanic isolator blocks DC current going in or out of the boat while still permitting AC to flow. We need the AC flowing to provide safety protection. We don't want the DC causing electrolysis problems.

A. If the isolator was not there and the ground is connected directly then both DC and AC can flow. Safe but corrosion.

B. If the isolator was not there and the ground is disconnected then neither DC or AC can flow. Not safe and no corrosion.

If the diodes in the isolator "fail safe" as required by ABYC specifications, they act as A above so safe but corrosion.

If the diodes "fail NOT safe" (open circuit instead of short circuit) as you measured in your isoolator, they are acting as B above and you still don't have corrosion but risk killing swimmers in the vicinity.

[boatpoker]

Originally Posted by CharlieJ

@boatpoker #56
Thank you for the reference. I already have it in my library. I am confused as to why you referenced a tech bulletin for a saildrive. The OP's boat is a conventional shaft and prop.

Sorry, thought he had a saildrive with an aluminum prop. maybe I was confused with another thread.

[EngNate]

The reading of 2V in one direction on the GI indicates that there is an open diode in that direction. The unit will not conduct AC fault current as it should, and should not be considered safe. However, it will still block galvanic current. A shorted diode will still conduct fault current but will not block galvanic current. These are the two failure modes.

The diagram attached shows the internal components, 4 diodes and a capacitor. The capacitor isn't important in the current discussion. Diodes have a characteristic voltage drop - applied voltage must exceed this level before the diode will conduct. Further, the diode conducts in one direction only, all current is blocked in the reverse direction. The nominal drop for a standard diode is aprox 0.6V, two diodes in series drop 1.2V, a little more than the range of galvanic voltages in sea water, that's how the isolation is obtained. The galvanic current is DC and flows in one direction, so just one of the two strings of diodes provides the desired galvanic isolation, while both strings are involved with conducting AC fault current, one for each half-cycle of the AC. The capacitor conducts AC only and fills in the 1.2V gap so that all AC current will be conducted through the isolator.

The OPs observation with the GI test doesn't indicate that it isn't doing its job as far as isolating, but it does indicate that it may not conduct AC fault current effectively. However, I would repeat the test directly at the GI connections without involving the shore cord. The reason is that a false reading may have been obtained if the capacitor was not fully discharged between tests in each direction.

It would be most productive to further troubleshoot with galvanic potential readings taken with a multimeter and a reference probe made from a zinc pencil anode.

[suiramor]

I have done a bit more fiddling and discovered that there is a fixed 0.7V potential between my prop shaft and the nearest through-hull (nothing is bonded). I do not understand where its coming from. I do have a working (tested) galvanic isolator. Should I be concerned? Should i disconnect everything and slowly re-connect to see where the voltage is coming from?

[CharlieJ]

You are simply measuring the corrosion potential between two dissimilar metals immersed in the same electrolyte, a direct analogy to testing corrosion potential with a reference cell.

No need for concern.