Replace engine 1984 31

[Mark Nooney]

I have a yanmar 2gmf (rated at 13hp) and I have put a Campbell sailor prop on her this year. We can't get over 3.8 to 4 knots at 2600 rpm. Has anyone re-powered a 31 and what did you use? I would like to make 5 to 6 knots depending on conditions. Thanks for your input.
Mark
Dream Too

 

[IH82BL8]

Amateur opinion: It doesn't sound like the engine is the problem. If you're putting 2,500rpm to the prop shaft, the engine is doing its job. I don't know what to suggest the problem might be after that. If I'm correct and you don't need to re-power, you should be relieved.

 

[mr c]

I have the same boat. I have the original 2 blade prop and I can reach almost 6 knots with a clean bottom. The rpms are at 3100 when this is accomplished.

 

[mr c]

Maybe your new prop has the wrong pitch. Or your not revving it enough. That engine is supposed to run all day at 3200 rpms.

 

[kloudie1]

First, an accurate tachometer reading is needed. An inexpensive digital handheld will give you a good reading. The Yanmar tacho is notoriously inaccurate.
http://www.harborfreight.com/digital-photo-sensor-tachometer-66632.html
Once you know the real RPM, and the propeller pitch and diameter, then you can start troubleshooting. You should be able to read the diameter and pitch of the propeller under water. It will be small numbers probably stamped into the prop hub somewhere..

EDIT: Speed measurement accuracy is important as well.. if using GPS, make sure there is no current.. if using a knot meter, check its calibration against a GPS in no current conditions..

 

[Nelsonhow]

Why did you replace the prop in the first place... You may have to much pitch or to big a prop..

Getting a laser tach is a good idea. You may not be reaching max rpm.


If you are getting black smoke out the exhaust with a drop less than wide open throttle ? This is a sign that you may be over propped.

 

[gr8_2_av8]

Same boat, 2 blade, 5 knots at 2500 RPM all day long.

 

[Steve Dion]

Mark:

Is this a new prop? What was the prop that you previously were using?

If nothing has changed except the prop you should return it and have it repitched. Otherwise you need to go thru all the previous posts.

Your engine should rev to 3500-3600 with everything being correct.

 

[Mark Nooney]

Thanks all,
This all started two seasons ago. Had a mechanic point out that our original prop 15 X 11 had one blade bent forward. I ordered perfect pitch prop from CDI and relied on the company to provide the correct prop. Bad move! They advised smaller and larger pitch. We didn't get the performance from the prop. For this season, we decided to get the Campbell sailor prop. They advised smaller still and bigger pitch 13" diameter x 8" pitch. So here is the rub.
Yanmar suggests
Model *****2GM
Reduction***2.6:1
2 Blade*****16 X 9, 15 X 11
3 Blade*****15 X 9, 14 X 11

The basics - all tach readings were done with a laser off the main fly wheel.

I have emailed Campbell and I am hoping for a good outcome. I'll post again when I hear from them.

Thanks,
Mark

 

[kloudie1]

You can play with this:
http://www.rbbi.com/folders/prop/propcalc.htm
using a slip of 50% which is around where you'll be, it says you should be closer to 10 or 11 inches instead of 8" of pitch. I am not a propeller expert, but usually larger diameter at the same pitch will decrease slip slightly.

 

[Mark Nooney]

Update:
I heard for Campbell - they want me to send them RPM at WOT under load and the same for No load.
MN