Uneven anode wear

[lumphammer]

After two years in the water we were lifted out this weekend. The prop nut anode on our flex-o-fold prop has eroded about 50% but the shaft anode and the pear anode on the hull haven't lost any zinc at all. Just some corrosion on the surface.

All the anodes are within a metre of each other and all are electrically connected to each other and to the shaft and prop. The pear anode is connected directly to the engine block.

Is it normal to expect all the wear to happen on just one anode? If the anodes are not eroding does this mean they are not protecting anything?

 

[Norman_E]

I have seen similar, even to the point that where I have two keel anodes which are on the same bolt, one each side of the thin aft section, I have even seen one side half eaten away, and the side other barely touched. There just does not seem to be much logic to it, except that where the prop and shaft are concerned, the propeller is a big lump of metal with plenty of surface area, and a good deal more "noble" than the anode, so I expect (and usually see) more loss of metal from the prop anode than from the shaft anode.

 

[lpdsn]

Maybe the other anodes are not so well bonded. I've had that one year where I accidentally used too much sikaflex to bed the anode on one side of the keel and it got into the thread when I bolted the anode on. Not a big issue as the anodes generally last a couple of years but it did give me uneven wear.

On the other hand, I recently had an issue where the LEDs on my Galvanic Isolator indicated a problem which I traced to a nearby boat. I hummed & haa'ed about leaving my shorepower connected whilst the marina got in touch with the owner of the other boat and eventually decided to do it as that would mean both my prop and keel were cathodes, whereas otherwise my keel and prop would form a path for the stray current with the prop being the anode.

Try checking the voltages at keel and prop against a shorepower earth.

 

[VicS]

After two years in the water we were lifted out this weekend. The prop nut anode on our flex-o-fold prop has eroded about 50% but the shaft anode and the pear anode on the hull haven't lost any zinc at all. Just some corrosion on the surface.

All the anodes are within a metre of each other and all are electrically connected to each other and to the shaft and prop. The pear anode is connected directly to the engine block.

Is it normal to expect all the wear to happen on just one anode? If the anodes are not eroding does this mean they are not protecting anything?

As already said youd expect the prop nut anode to be consumed fairly rapidly

Check the continuity between the other two and the prop

 

[Tranona]

The anode on the prop is all you need. It wears first because it in contact with the prop. The others are not doing anything, and won't until the prop anode is gone.

 

[Mandarin331]

When we lifted in June we knew the prop anode was already gone from the purachase survey last December. However we were surprised to find the main anode in perfect condition, until we looked closer, then we found one of the bolts almost completely wasted away, with just half the thickness of the threaded square plate remaining to hold the bolt in the hull and the anode hanging on the aft bolt This presumably due to a poor connection between the bolt and the anode with the bolt acting as an anode. this erosion was not mentioned in the survey so happened in the last six months.

Had we left this another few months its quite possible we would have had the bolt pop out of its hole and a flooded boat. Another reason to check anodes frequently. We now have a nice new prop anode and new bolt with zero resistance.