I guess you're not kidding here. Running your engine until it cuts out from overheat and using that as your impeller wear indicator is just a bad idea. Probably not good for the engine, but also, this event might just happen at the worst possible time--when you really need your engine. There are lots of sailboat parts that give nice obvious indicators when they've failed, but best not to go there.I should have read this forum before sailing but luckily none of that happened to me. I hear everyone and I mean everyone who sails saying change your impeller. I changed mine and the new one sucked.....so I took it apart again and but the old one back in. Works like a charm. Plus, they work fine even with tabs broken so I don't see the huge emphasis here. When the engine overheats....change it. I don't have a temp sensor on my YSM 12 but it shuts off automatically when it gets too hot. This is usually, when I leave the seacock closed or the mixing elbow on the exhaust gets clogged/blocked which seems to happen about every 24 hrs of run time. I hear everyone praising the Yanmar engines but I've never had more trouble with an engine. The one thing I would praise is I've been able to fix everything....pretty easy to work on. Well almost....currently I'm mounting a little 8 hp evinrude to the transom...not sure whats wrong with the Yanmar now. Won't spin. Probably water in the chamber which .... rebuild or replace with something better. It's on a Hunter 30 and only ran 4.5 knots. I'd love to get something that'll get me 6.0 knots.