Kind of dumb on my part. I noticed a trickle of antifreeze in the cockpit locker. Then I noticed the expansion tank was spitting out it's plastic lid. I didn't pay too much attention as the temp guage was it it's normal area, and I'd just motored about 15 hours to Monhegan from Isle of Shoals. Maybe the system was over full, I figured.

But then when I throttle back after reaching the island, suddenly the Temp guage began to rise, so I shut it down. It was well above where the alarm should have gone off. I guess the sender, despite working with the test button, is not working.

As I throttled back, the cooling system lost enough to clear the thermostat housing, where the temp sender is, hence the quick rise in reading. No problem luckily, but over a long period of time, the expansion tank had allowed about a half a gallon out of the coolant system, which is close to 3 gallons.

I think I'll replace both the oil and temp alarms on the engine.

The dumb part is, I've before filled my raw water strainer with eel grass from the Cape Cod area, I should have opened and cleaned it before leaving.