I noticed two things on the boat today when we powered to the pumpout:
1) The tach wasn't working. Eventually I found a split alternator belt in the bilge. Bummer, but not critical.
2) The water/coolant temp hit 230 F. I noticed this AFTER docking. I shut the engine down and noticed that the expansion tank had shed some coolant into the bilge. I took apart the primary water filter (a Perko basket in a glass type), and blew out the engine intake hose. I also took off the cap to the standpipe to make sure a fish hadn't met an unfortunate end in there...it hadn't. Aside from a bit of stray vegetation and "bits", I found nothing amiss.
After the engine had cooled, I started it again and noted that the output appeared normal, i.e. plenty of gush out the exhaust. The engine took five minutes to reach its customary 170 F or so of coolant temperature. All seems well.
My questions are these: I assume that it is possible I sucked in partially a plastic bag or some more rigid obstruction in the standpipe...it currently lacks a screen and is just a three inch hole in the bottom of the boat. Said mystery blockage caused the overheat and when the motor was stopped, it dropped to the bottom or floated away unseen. Thoughts?
Second possibility is a cacked vane on the impeller, which is new this season. I can't discount this without dissembling the raw water pump, which is a pain in the ass, but will have to be done if this issue persists. As I said, output at dock seems normal (spurt, gurgle, spurt), the lines are visibly clear, the filter is clean and the standpipe holds no dead fish (you can see the water all the way down with the cap off!)
Advice, theories, conspiracies are welcome. Right now, I assume I just have to source a 44 inch alternator belt tomorrow morning to continue my sail.