In addition to the above, the engine appears to run very cool - coolant feels little more than tepid after 45 mins running (on the hard using November NE town water, 45 degrees ambient temp). Would this imply that there is no thermostat or is it typical?
This is normal. Hunter went about doing fresh water cooled (anti-freeze circulating in the engine block) QM series engines on many of our boats in an odd way. Rather than installing the fresh water QM models in the first place, they instead installed the seawater cooled versions (seawater circulating through the engine block). Then they converted these to fresh water cooling by adding an extra pump on the alternator belt circuit and putting on an external non-Yanmar heat exchanger. The heat exchanger is made by Sen-dure, model 2550. On some boats the HX is jury rigged on the side of the engine. On my boat it is mounted on a bulkhead under the port settee.
With this modification, the seawater thermostat housing is still utilized. Whereas anti-freeze engine thermostats are set to maintain engine operating temperature at a ~170F range, the QM sea water cooled thermostats open/close to keep the engine not more 125F. (Yanmar specs for the sea water thermostat are begin to open at 42C = 108F and fully open at 52c = 125F). This low operating temp is needed because at not much higher temps, minerals and salt precipitate out of sea water and will deposit within the engine's cooling fluid channels. Once blocked, the engine is ruined.
Note that only a 125F thermostat will fit into the thermostat housing on these sea water to fresh water modified engines. Even though they are now cooled by anti-freeze it is not possible to install a 170F thermostat.
Sometime back, I took the time to make notations about our odd conversion on the same diagram that kloudie1 also posted for you. Attached are pictures of my notations showing both the seawater and anti-freeze circuits.
On the "official" seawater cooled QM models, the pump that is integral to the engine normally pumps seawater through the engine and then out through the mixing elbow. On our converted seawater cooled models, the pump on the engine instead pumps anti-freeze in a continuous closed circuit through the engine, thermostat housing, and the external heat exchanger. A second pump is installed on the alternator belt to pump sea water through the heat exchanger and then out through the mixing elbow.
A positive with the conversion to anti-freeze in the block rather than sea water is that the engine zincs don't need to be checked/replaced. The external heat exchanger however, does have a pencil zinc since it handles sea water. This must be checked and replaced periodically.
A downside to this arrangement is that the Sendure heat exchanger doesn't have enough cooling capacity to keep the engine temp below that alarm trigger value of 60C = 140F (spec from the Yanmar manual) if the ambient seawater temp is high. In San Francisco, water temp never gets above about 60-65F, but if I have been motoring at 80%+ for a while, the temp alarm might sound. Or more frequently, if I throttle back to idle after a period of 80% motoring, the alarm sometimes sounds a minute or two later. I think this is because the slower engine rpm's means less seawater through the exchanger = less cooling capacity. But the engine is still warm from the motoring. After another minute, the alarm stops sounding. I don't get concerned about this because I do have a temp gauge in the cabin. I can see that the engine really isn't overheating to dangerous levels. So what if the engine temp has gone say to 145F? A normal anti-freeze engine operates at 170F+, so lots of head room still remains on my engine before the temp becomes dangerous.
For your reference, I have also attached a drawing I made of my Sen-dure heat exchanger showing the location of each hose fitting. I did this when soon after I bought my boat, I discovered that exchanger was leaking between the sea-water and anti-freeze sides. A local radiator shop with experience in exchanger repair was able to make it good again.
You will need to investigate all this more on your own. The conversion that Hunter made on these QM's isn't "in the manual". So hope the above gives you enough to get started on figuring it out.
