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.
In South Africa doing lots of boat stuff. Departure north from Cape Town around December 15th. Arrival in the Caribbean around the beginning of February, after 5300 miles or so.