High RPM further info and pitch and props - SailNet Community
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post #1 of 2 Old 02-16-2007 Thread Starter
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High RPM further info and pitch and props

I recently posted concern about the High RPM (4000) on my PSC 34 hull 317, 1998 with a 3jh2e engine. After much discussion with Yanmar, I have the following information. 1) The engine should go to about 4000 rpm with no load in neutral. 2)The problem with my engine is that it only has an RPM of 3200 at full throttle in calm water. That is to slow. Yanmar wants 3600. What is clear, is that my engine is over propped. My maxiprop diameter is 17 inches set at 14 degrees with a pitch of 8 inches. The gear reduction of my transmission is 2.14. This calls for a 15 inch prop with 10 inch of pitch. Pacific Seacraft likes using 17 inch props because they are apparently more effecient considering where the prop is located behind the rudder in front of the skeg. A smaller prop could become less efficient causing a loss of speed even though the engine would have the proper RPM. In summary Yanmar wants its engine to have the correct RPM which is 3600 for my engine but PSC was willing to see my engine over propped to prevent loss of some effeciency (also they disregard the shortened life expectancy of an over propped diesel engine - 5000 instead of 10000 hours of use). On the other hand it is unlikely that I would every put 5000 hour on my engine. This would be more of a problem for a charter boat. Newer PSC have an engine with a maximum of 3400 RPM and a greater gear ratio thus fitting better with a 17 inch prop. Any one have any thoughts about all this? Phil Littman
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post #2 of 2 Old 02-17-2007
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Reduce the pitch on the max prop. I can be set as low as 10 degrees. Per max prop

A two degree change in blade angle will change the engine RPM by 13-15%, at the same boat speed.
so in your case 2 degrees equals about 300 RPM so a 2 degree change should do it.

details here:

Jon D
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