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Discussion Starter · #1 · (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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Those sound like really high RPMs for a Yanmar diesel. I had that same engine in my C&C 36 and I don't think I ever cruised at over 3000 rpms, although I couldn't swear to it. I am not, however, an expert in pitch or props.
 

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Those RPMs are high. You should be running at hull speed at around 2800-3000. Pushing it any faster is a waste of fuel and a great waste of wear and tear on the engine. The things you need are the hull speed of your boat and the torque curves for that particular engine. Torque curves can be found in the repair manual. Hull speed, torque, and fuel consumption need to be balanced. My 3GM30 runs at 26-2800 at the boat's hull speed of 6.5 (or so) knots.
 

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Discussion Starter · #4 ·
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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A lot depends on your prop, my 8000 lbs 30 footer has a 2GM with a Gori prop. I can cruise at 6 knots at 2200 RPM. A number of friends with the same boat and engine have Martec props and they have to be close to 3000 RPMs to get 6 knots.
I contacted Gori directly to get the recommended prop size for my boat.

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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'.
 

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That seems very high, above my 3 cylinder engine's maximum RPMs. I cruise at 2,000 - 2,200 RPM on my Vetus M3.10 for maximum fuel efficiency, making 5+ kts. in calm water with a clean bottom and 4.5 in normal wind and waves.

The operating manual for my Vetus M3.10 lists 3,600 as the max RPM. 75% would be 2,700 which would consume much more fuel for not much more speed, based on my experience.

Here is video at 2,000 RPM making 5.4 kts:

 

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Discussion Starter · #8 · (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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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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Discussion Starter · #10 ·
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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