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Best RPM for Yanmar 3HMF?

9714 Views 10 Replies 7 Participants Last post by  JimsCAL
The 27hp Yanmar 3HMF in my 38' 15000lb boat will peg the tachometer at 4000 RPM and just barely reach hull speed (or maybe not quite). Right now I cruise at about 6.5kts @3500 RPM. I'd like to get the speed up to 7-7.5kts and the RPM down to 3000 or ?? not sure. Is that reasonable or possible? The prop is a two blade folder. I couldn't find any markings on it, so I'm not sure of the pitch. I thinking of re-pitching or replacing it, but I'm not sure at what RPM range I should be aiming. I race the boat, so fixed props are out.
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Standard practice for long term life in service for just about any diesel is to be at ~75% of max. rpm when under 75% of full load.
4000 X .75 = 3000 rpm
If you continually run at 4000 rpm, plan on a premature 'rebuild'.

Hull speed is the speed of the waves generated by the boat, and supporting the boat at both ends; above that (wave) speed the stern wave moves aft and no longer 'under' and supporting the stern ... and the boat will then 'squat'.
Why go to the expense of changing tachs?

You now KNOW that its not turning 4000 rpm but actually running 2900 (about 75% of max. rpm which is 'good'). Just mentally correlate, and know that 4000 on the gage is really ~3000 ... or the gage is reading 1/3 higher than actual, if you can do the mental math.
Just guestimate-multiply all gage readings by .66 or 2/3rds 'in your head'.
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