I took delivery of my new Hunter 33 (2004)last June and motor/sailed to my home port some 150 miles away from Toronto. During the trip, I was very tender with the new engine (Yanmar 3YM30) nursing it at between 2000 and 2600 rpms for most of the time but for short periods between 2600 and 2800. After about 20 hours engine-time I had an over-temp light/alarm, sound while in pretty rough weather. I was at 2600 rpm. I immediately reduced to about 2000 rpm and the alarm ceased. This experience occured on and off through the rest of the summer/fall. On every occasion I checked the raw-water intake strainer and only on one occasion was there some junk in the strainer, which I cleaned out. At all times I had good water flow at the exhaust end. Alas, for the last few boating hours last year the problem appeared to go away. Not so fast! My boat was launched last week and about five minutes after turning on the engine, the alarm and temp light came on again. I had to keep the engine running (at low rpm)to get to my mooring - in very breezy conditions. On arrival at the mooring I turned off the engine, which by then was very hot. That was after less than 10 minutes operation at low rpm. I let the engine cool down for about 25 minutes - did some checks and then restarted. Almost immediately the alarm and temp light came back on so, I turned off the engine immediately. To cut a long story short I contacted the Yanmar rep who put me in touch with a deisel mechanic in my area. This mechanic suggested (by phone)I run the engine with the fresh water pressure cap off and very very slowly top up the coolant (anti-freeze mix) level until all signs of air bubbles disappeared. Carry on the exercise until the coolant level is stabilized and tank remains full. This took a full 40 minutes as I worked up the rpm from idle to 3000 rpm which was held for 10 minutes. At the end of the exercise there were no bubbles and no overheating. I then ran down the rpm slowly over fifteen minutes and idled for five minutes. The mechanic had previously suggested making sure the tube connecting the plastic recovery tank to the spigot at the coolant pressure cap be firmly secured. It is also important to ensure the tube within of the plastic recovery bottle is firmly in place and immersed in the coolant. In all, I added 1.6 litres of coolant directly into the freshwater system. Th mechanic later told me sometimes the coolant is poured too quickly into the system, which creates bubbles and then air locks. He said this could have been done when the boat was commissioned thus the reason for the overheating appearing so early in the life of the engine. Of course, my occasional topping up of the system was not done slowly, so it may have aggravated the air lock. Through all of this the dealer was not much help. The distance away from Toronto dddoes aggravate that situation. So if you have an engine overheating problem - airlocks in the freshwater tank may be the culprit. If so, the solution appears to be relatively simple. As you see, I am not a deisel expert so the causes of engine overheating could be more than just airlocks.