I think it just rusted through but did not look any different than when I bought the boat four years ago. The stub could be a real issue since it broke off almost flush with the surface of the exhaust coupling. I have a replacement nipple and we are not going anywhere until the end of cyclone season so Aprilisih. I think I might have a go at it in the next couple of weeks and then I can take it back to Canada when I go for Christmas. At least the machine shops there will be familiar with the thread - they use metric or some British standard thread here.
Finished the circumnavigation in early February in Grenada. Have to work on a book project for the next several months so the boat will be waiting for next year.