I installed a 2YM15 recently, with the 3.2 to 1 reduction gear. In calm water, in gear, my full-throttle maximum rpm is 2900, which is 76% of the no-load maximum (intermittent, not continuous) rpm for this motor (3830 on my tach).
"John" the diesel mechanic tells me that I should get a maximum rpm of about 3450 under load, under way, which is about 90% of the no-load max of 3830. He says I need to get my prop re-pitched so that my engine is turning about 550 rpm faster at full-throttle (3450 - 2900 = 550).
When the prop is adjusted, John says I should cruise at about 2800 rpm which is about 80% of 3450. He says until my prop is fixed, I should cruise at 2300 rpm which is about 80% of my current maximum loaded rpm of 2900.
1. Does John know what he is talking about? If not, what are reasonable numbers?
2. At full throttle now, I can run for hours without overheating. And, at full throttle now (2900 rpm) I attain more than theoretical hull speed. Is the engine really working too hard if it is not overheating?
3. John mentioned that until I get my prop modified, I should cruise at about 80% of my current max. rpm of 2900, which is 2300. Given my existing prop, if the engine is happy at 2300 then why modify the prop so it turns faster to attain the same speed?
Notes: My sailboat is 28' and 9500 lbs, with a full-length keel. Hull is clean. Prop is a 14x11 3-blade "Campbell Sailor". I sized the prop with the prop calculator at boatdiesel.com and with the manufacturer's input.