Cruising RPM

[Stewartn]

2014 21 EC with Yanmar 3m y30 w/factory 4 blade prop.
What is best cruising RPM? Running now at 2400 and difficult to get much over 5kn (GPS). CAN I safely go to 2700- 2800?
Thx
stewartn@optonline.net

 

[bob daily]

Stewartn, the Yanmar 3YM30 data sheet showed the max rated RPM as 3600, 75% =2700RPM
And 80% would be 2800 rpm, so yes you can curse all day (and night) at that rpm and not hurt the engine. As far as speed, I usually just sit back with a cold one and enjoy the ride, but then I'm the guy you see going to town doing 35 mph with my left blinker on. Bob

 

[BB marine]

What is your WOT Max rpm the way the boat is loaded operating with the factory installed 4 blade propeller ? If the max rpm is at 3600 rpm. Running all day long at 2800 rpm would not be an issue. Based on the Yanmar fuel consumption chart at max WOT the engine is burning approx 1.9 gph. I would estimate this to be 100% load. Using fuel burn as an estimate for load and using the Yanmar fuel consumption chart. At 2800 rpm you would be at approx. .8 gph which would be approx 45% load on the engine. Easy equation ( Max fuel 100% load) - ( 20%) = fuel burn @ 80% load. Max fuel burn =1.9 gph -20% = 1.5 gph. Using the fuel consumption chart 1.5 gph 80% load would be approx 3400 rpm. ( if propped to maintain recommended WOT 3600rpm)

Based on the numbers posted by Yanmar and the ability to turn Max rpm 3600 rpm. Cruising at 2800 rpm would be an optimal rpm for cruising and maintaining longevity of the engine. Cruising,and burning 1 gph would be ok to operated your engine at if this speed provides a comfortable ride.

I would note that many of the fuel consumption charts are not that accurate in the real world. Most engine manufactures use 2.5 prop load which only seems to be accurate at WOT when comparing the charts to actual. These load illustrations are averaging the loads of a planing hull and a displacement hull. I believe the displacement or semi displacement prop loads are closer to 3.0. I'm a bit weak at mathematical equations so I familiarize myself with the actual fuel burn rates in my boat at different rpms to figure my approximate engine loads. When doing this I confirm manufactures maximum RPM rating achieved and gph matches the manufactures posted max fuel consumption at WOT.

Sorry for the long explanation but the answer may not be as simple as yes or no. Max rpm 3600 (yes) Max rpm 3400 ( maybe) anything lower than that I would be looking to re-prop so the answer can be yes.




http://magelia.demo.epitomy.com/frenchm ... chData.pdf