View Full Version : Yanmar 4JH3-TE: What are the long-term effects of being overpropped?

06-12-2010, 12:39 PM
Have about 350 hrs. Seems to run good. Per Yanmar, this engine has a max. rpms under load of 3800. I can only reach about 3700-3750 according to the boat tach. under load. This is the best that I have ever done. Comparison with a hand-held photo laser tach at 3800 rpms in neutral, the boat tach is reading about 150 rpms higher than the hand held. Therefore, I need to gain at least 200 rpms in order to reach 3800 max rpms under load. I usually run the engine (in small power boat) in the range of 2000-2500 and try to run at least 5 min. or so at 3000 rpm whenever I am out. I know that I am over-propped and should probably for next season have the pitch reduced an inch or so to gain 200+rpms or swap the prop out with another. Question: What damage or long-term effects does being over propped have on the engine given how I currently operate it? Just how critical is it to achieve max rpms under load? I never get any black smoke just white smoke at times at idle. Please advise. Thanks, Jim

10-26-2010, 07:36 AM
Based on discussions with others (prop shops, Mack Boring, etc.) and my own research on this question I have determined that the long-term effects of being overpropped will be a (unnecessary) strain on the engine throughout the rpm range. This is not good. To remedy this problem, I had the pitch on my prop reduced by 1.25" by a local prop shop. This should (hopefully) now allow me to reach max rpms of 3800 under load which I previously have not been able to do. I will confirm this once my boat is launched next Spring 2011. Hope this helps others who have interest in this issue.