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Best RPM for Yanmar 3HMF?

9711 Views 10 Replies 7 Participants Last post by JimsCAL, Oct 16, 2014

Loki9

Discussion Starter · Aug 15, 2014 (Edited)

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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Loki9

Discussion Starter · #4 · Oct 14, 2014

Just a quick post to close the story.

I asked a few knowledgeable people and they seemed to think my motor sounded about right when at a good cruising speed, regardless of the tachometer reading (which is high, 3600+ RPM). For my own peace of mind, I went out and bought a diagnostic tachometer and learned that Yanmar 3500 RPM = real world 2900 RPM. The stock in-dash tach just doesn't read accurately. I'll sleep better now knowing that I'm not running the motor at crazy high speeds.

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Loki9

Discussion Starter · #8 · Oct 15, 2014 (Edited)

You guys did not read post #4.

It turns out that my Yanmar is not turning 4000 RPM, the tach is reading wrong.

After a bit more investigation I discovered that the tach in my boat is the wrong one. I have a p=97 tach but I should have a p=114 tach. Anyone have a p=114 Yanmar tach that they don't need?

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Loki9

Discussion Starter · #10 · Oct 16, 2014

RichH said:
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'.

Well of course, that is what I'm doing now. I'm not planning to buy a new replacement (~$200). However, if I found a used tach for sale or trade, I'd consider it.

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