Another Picture to Ponder

[JK]

This could be a little tougher.
This is a piece of 5456 H16 aluminum hull plate cropped from the vessel.




Another view:



And another:

[Sébastien]

Stress induced corrosion?

[JK]

Good idea, but not in this case.

[JK]

Different aluminum boat, same thing going on in this photo:

[conrod]

should the skipper be staying away from sand bars !!

[JK]

LOL, though I probably have pictures of that, too!

[JK]

Let's see if I can get this right now...I had our electrical guy explain it to me yesterday to make sure I had it correct, complete with squigly drawings.

What you are looking at is an example of electrolysis on aluminum hull plating. Both examples above were painted originally and have been cleaned off with a wire wheel. Both boats had minor paint damage in the area of the electrolysis. Because the rest of the hull is protected the damage is concentrated here.
What has happened is that there has been a electrical ground. This can be on the boat the damages occurs on, another boat in the marina or the electrical service to the marina. The current passes through the water, through the hull of our boat and into the wiring, then to the shore transformer to utility ground.
This was corrected by installing a transformer on our vessel.

The damage in the first 3 photos has been done in 2 weeks. A fishing boat tied up in the marina had a ground and it caused thousands of dollars of damage to our vessel.