prop corrosion on aluminium boat

[malou]

Hello,

The prop of my OVNI 28 (aluminium sailing cruiser) was changed two years ago (said to be copper-aluminium). It already appears to suffer of heavy corrosion. A zinc anode which was fitted a month ago close to the prop is now well damaged, and corrosion on the prop itself continues to grow visibly in a month time.

Suspecting an electric problem, I took few measures (engine stopped) :
Beetween hull and shaft: 600 mV.
Beetween hull and engine (Yanmar 20GM2 changed 1993): 510 mV
Beetween engine and shaft: 90 mV.

Can this cause such corrosion ? What can explain such voltage, and what is to be done to cure?

I’m sure some engineers here can give appreciated help.

Regards.
Olivier.


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[MainlySteam]

The first advice I would give is to not act on anyone's advice unless you are sure that it is correct. The subject is full of misinformation.

Without a lot more detail it is very hard to state what is happening, but as it is a new problem in an existing vessel the prime things to look for are things which have changed:

1. The metallurgy of the prop, it may not be as good a material as may be claimed. If the shaft is insulated from the hull, then the prop metallurgy is probably the most suspicious thing (good bronzes and ss shafts will live quite comfortably together with no protection whatsoever).

2. Was the shaft drawn when the prop was replaced - if so was it insulated from or was it electrically connected to the engine and hull prior to that, and was that situation preserved when the shaft was reinstalled.

3. Are you now using a shore power cable, and if so do you have a galvanic isolator in the earth conductor (or an isolating transformer). However, if this was the problem one would expect you to have more active problems with the hull than the prop. If the neutral is incorrectly connected on the boat to the boats ground when the boat's shore power switch is made, then this will bypass the galvanic isolator to ground on shore.

4. Any electrical replacements, additions or modifications at all since before the prop was replaced. If so, suspect their installation.

5. The possibility of electrical leakage to the hull from corrosion or damp especially in the engine area, around the batteries, around the charging circuitry and particularly any wiring in or close to the bilge.

When troubleshooting, the key things to keep in mind are:

1. Metal boats are essentially shorted out batteries - the zinc being the anode and the hull and other metals attached to it the cathode, and the terminals of the anode and the cathode are shorted by the hull. If the zinc starts dissolving quicker and the boat is not connected to shore power, then another metal has been introduced somewhere, or more of the existing metals have been exposed to the seawater (eg through loss of paint).

2. If the shaft and prop are insulated from the boat, they exist entirely as their own system. Nothing you can do to the rest of the boat will change that, so if they have a problem it is entirely to do with the prop and shaft themselves.

3. Unless you are electrically connected to them by a shore cable through the marina distribution, the boats around you have no effect on corrosion on your own boat - a very common misunderstanding exists that you should not moor aluminium boats near steel ones (I am ducking the flac already, but it is true).

4. The possiblity of electrolysis from electrical leakages, especially if wiring is also in poor condition. However, if such was the case, one would expect even more obvious signs in you hull.

I think that means, in the event you are sure that nothing else has changed, then it is the metallurgy of the prop which is the problem.

John




<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by MainlySteam on 29/08/2003 02:59 (server time).</FONT></P>

 

[qsiv]

I also have an aluminium boat (Trintella) and the current prop (a standard maxprop) is the original prop fitted 5 years ago, and has no visible corrosion.

I would doubt the prop, and suspect a leak within the engine - the voltages all sound a bit high, and I have engineers reports tracking down differentials as low as 50 mV (which turned out to be some oil spilled over a connector). I'd get the services of someone with provable experience in this area, and get them to investigate and fix.

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[Phoenix of Hamble]

Yikes!

Those are very high readings... two over half a volt.... that is capable of doing serious damage in pretty short periods of time....

Get them checked out (and follow the good advice... use someone who understands this problem) SOON.... new props aren't cheap, but hull repairs are even more expensive....

as a really short term activity, get all the electrical connections in and around the engine, shaft etc really really clean, and check whatever cable that is visible for damage and then get the engine/shaft/area spotless.... even if it doesn't solve the problem, its good practice anyway...


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