I removed the thermostat entirely and am heading out to test whether the engine still overheats without it. The goal is to isolate whether the thermostat is the cause, or whether there is weak freshwater coolant flow or another freshwater cooling issue. If the engine does not get up to temperature with the thermostat removed, that would point strongly toward the thermostat. If it still overheats, the coolant may not be circulating properly, or there may be another cooling-system restriction. Yesterday, after coming in, I measured the raw-water exhaust discharge with a thermometer and got about 65 F while the engine gauge read about 220 F. That suggests the raw-water side is moving cool water and is probably not the limiting issue; if raw-water flow were the problem, I would expect the exhaust water to be very hot. This points suspicion toward the freshwater side and specifically toward poor circulation through the heat exchanger. The next 30 minutes of testing should clarify whether the freshwater coolant system is the problem.

With the thermostat fully removed, the engine is still overheating at the same rate. I am now sailing back under genoa only with a following breeze. This result makes the thermostat unlikely to be the cause. My best guess now is that something is wrong with the freshwater pump or freshwater circulation, and that the freshwater pump needs to be serviced.