Yes, mine also pretty suddenly clogged up. I also noticed black soot in the exhaust - particularly when I tried to add more power.
The diesel mechanic suggested that a major cause is lack of use.
By just idling out of the marina, and then the advantage of in-mast furling, we have sails up quickly and away. The reverse at the end of the day.
He reckons this very light use at relatively low revs encourages a build up in the diesel motor. He recommended a long relatively high speed long trip every month or so. But I have not done this - if you are on the boat - then sail it.
Also the 'bent elbow' system used on the Catalina looks like it encourages build up. I bought the wrong Yanmar outlet first up - it is a straight, slightly bent piece of stainless with a water inlet. It seems a much more efficient way to exhaust and would not clog up, also nice smooth stainless. However, the correct part for below waterline motors is a clunky thing of cast metal which you fit with a range of threaded fittings. All in all not a smooth removal of exhaust gas.
