3GM20F cruising RPM

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IgorC

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Apr 12, 2008
16
Hunter 29.5 -
I was using a motor on my boat a lot recently. We had delivery trip from Ohio to Toronto with light wind so we had to motor sail a lot or we would not make our schedule. I checked the forum for my model and some suggest using 2300 RPM for best speed/fuel consumption combination. I was motoring at 2800 RPM for a while and was able to get almost to 6kt. I thing Max RPM is 3600. I don't know if that is good idea to run motor at max RPM.

What are the best solution/practices? Is it possible to motor at higher RPM without damaging the motor? Any advice suggestions would be appreciated?

Thanks,
Igor
 
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Don12480

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Aug 9, 2007
31
Hunter 35.5 White Rock
75-80% of max

I have heard that the engine can run forever at 75-80% of max RPM. If the prop is pitched right you should be able to get it up to max RPM. Assuming the bottom is clean you should be able to go 6 Knts all day long.
The engine should also get up to proper temp to prevent carbon build up.
 
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Granfalloon

Square Root of W/L

I have a Yanmar 2GM20F in a Hunter 310.

It has a Waterline Length of 28 feet. Square Root of 28ft is 5.29 (call it 5.3kts). I get this at 2500 rpm. I believe this is a max range figure. I burn about .4 gph. Anything above this fuel consumption goes up significantly.

Hull Speed for the Hunter 310 is 7.1kts (SR of 28 x 1.35). At this speed it burns 1.2 gph.

I've heard 75% of max rpm also - I go about 6kts. But for best fuel specs I cruise at 5.3kts and go to max speed for 2-3 minutes each hour to burn off any carbon build it. It also tells me the condition of my bottom and any growth on the prop shaft and prop.

I put my boat in the water(Cheasapeake Bay) mid May - I dove on the prop mid July - bottom looked great but had the some build up on the prop and prop shaft- I scrapped it clean with a small paint scrapper. Next time out I picked up 200 rpm and 4ths of a knot at 2500 rpm.

Granfalloon
 
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Steve Dion

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Dec 2, 1999
15,184
Hunter Vision-36 Rio Vista, CA.
Actually 2800-2900 is correct.

Igor:

Actually 2800-2900 IS the correct RPM for these engines. If you are really worried about fuel consumption I suppose you can run it a little lower.

The website that is attached will give you a lot of Yanmar specific information and the possible results of not heeding their advise.

I believe that the Yanmar manual also has a recommendation too.
 
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