Went sailing yesterday, great. When motoring back, checking water flow through exhaust as always and temp guage, temp was about 230 degrees. First thing was to see if water was coming out of exhaust, ok, next went below and felt raw water pump, cold, no water in bilge. Took out of gear and reduced engine speed and no change in temp. Then I smelled coolant, shut it down and opened port cockpit locker and bellowed steam. Coolant was coming out of the overflow. Finally got towed back to the slip where I checked over all hoses, by this time there was a little coolant in the bilge, less than a quart. No hose leak. The only sign I found was where it came out the top of the overflow, cap is rusted through. My thoughts at this time are 1) thermostat frozen, 2) air in the system, 3) low on coolant ot 4) heat exchanger. By the way the temp did, I think it might be the thermostat. Any other thoughts. I did not recheck the oil to see if water was present, but engine was running fine. It is a Universal 5424 (M30).