What is a good maximum rpm for the 18 hp 2GM Yanmar diesel engine on the Hunter 320. I have been operating it at about 3000 rpm and was wondering if this is excessive.
Wesley: This is right in line with the recommended RPM. It is suppose to be 70-80% of the max rated RPM which should be 3600 rpm. The actual recommended range is from 2500-3000 rpm for cruising. If you run a little lower you may get a little bit better fuel consumption. I'd base it on your running speed. If your knot meter get you more speed out of more RPM's then it would be okay to run it higher. If you get the same speed from 2500 as you do 3000 then there is no need to rev it up.
My experience w/ the same engine on my 1999 H310 is a max. of 3400 RPM under load w/ a clean prop and a clean mixing elbow. At 3000 RPM I get 6.2 to 6.4 knots. Top speed seems to be about 6.8 knots at 3400 RPM so you don't get a whole lot more opening the throttle completely up and I imagine fuel comsumption goes up quite a bit. If you idle your engine alot or use it only to get in and out of the harbor and don't get the RPMs up under load the mixing elbow will coke up causing back pressure which causes power loss. It can also block water flow causing over heating. This will happen gradually so you might notice a little steam from the exhaust this is not normal unless it is very cold outside. I began to experience symptoms of clogged mixing elbow at about 100 hrs. Cleaning the mixing elbow made a dramatic difference. Probably more information than you wanted but better to learn here than the hard way.
Wesley: "Do not operate for extended periods below 80% rpm. The cylinder bores will glaze causing an increase in oil consumption and blue smoke." There is a lot of information on this site www.yanmarhelp.com Take a look and see if it doesn't help.
Wesley, and I always cruised at 2500 RPM. This would move the boat along at about 6.8 knots. Increasing the RPM made only a slight difference in boat speed. At times I would run it up to max RPM for ten to fifteen minutes just to burn off any deposits. Terry
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