I don't have the Yanmar but rather a Perkins 4-108 which is rated at 4000rpm intermitent and 3600 continuous. I also have a max prop which allows me to adjust the pitch to achieve various WOT readings. I finally settled on adjusting the pitch to 14 inches( 17 inch dia) and achieved 3000 WOT on my tach. However this year I discovered that the tach was out of calibration and when normally cruising at 2500 rpm, I was actually doing 1900. So with much thought I considered going back and adjusting that pitch smaller to achieve closer to the continuous rating, but now I'm not sure if that is necessary. One of the main factors on engine life is how much
fuel flows through it and as long as operating temperatures are achieved then operating at lower rpms should be OK. I've read about over proping an engine resulting in lugging and while I can tell that I'm "lugging" my engine in my jeep when I don't down shift going at a lower speed, I have yet to tell that i'm doing the same with the boat. The only disadvantage that I see operaing at a lower rpm is that the horsepower available is less. My WOT reading is only 2300 rpm which is quite a reduction from the 3600 rating.