Hi Gord, it's the bottom curved surface of the stern pieces. There are damaged trimming boundaries in these spots:


And there's a kind of mutated micro shelf in this area:



How did you create this piece, was it a result of a Shell operation? It kind of looks like the sort of stuff that Shell messes up on.

It's also possible that this very thin area here could be considered an area of self intersection:



It's not generally good to have opposite sides of a solid squish down to such a thin area.

If you have any earlier stages of these pieces if you can post them I might be able to tell you where things started to go wrong with them.

- Michael