Bigislandluv
Junior Member
Running universal M-25 what is a good rpm to run all day. Is it good to go to higher RPS.
Universal M-25 crusing
- Thread starter Bigislandluv
- Start date
IF... this family of Kubota engine blocks is very similar to my former OEM Universal M25XP, our manual called for a cruising RPM of 2400 to 2600, or close to. I founnd a service manual on-line that specifies a max. RPM of 3200.
Once over about 2200, it is kind of a balance/choice/compromise between noise, vibration, and speed thru the water.
With a really clean bottom, we power along at 7.0, and at 6.5 with the usual thin slime layer, for instance.
We displace 10600#.
Your bio does not show which hull you have, but the M25 model (21 hp) was used in several Ericson's, from 32 to 34 feet, in the early-mid 80's.
Once over about 2200, it is kind of a balance/choice/compromise between noise, vibration, and speed thru the water.
With a really clean bottom, we power along at 7.0, and at 6.5 with the usual thin slime layer, for instance.
We displace 10600#.
Your bio does not show which hull you have, but the M25 model (21 hp) was used in several Ericson's, from 32 to 34 feet, in the early-mid 80's.
Bigislandluv
Junior Member
I updated profile. Yes it is the older Kubota 21 hp universal. Is there any benefit at every once in a while maxing out the RPM's? Thanks for the information.IF... this family of Kubota engine blocks is very similar to my former OEM Universal M25XP, our manual called for a cruising RPM of 2400 to 2600, or close to. I founnd a service manual on-line that specifies a max. RPM of 3200.
Once over about 2200, it is kind of a balance/choice/compromise between noise, vibration, and speed thru the water.
With a really clean bottom, we power along at 7.0, and at 6.5 with the usual thin slime layer, for instance.
We displace 10600#.
Your bio does not show which hull you have, but the M25 model (21 hp) was used in several Ericson's, from 32 to 34 feet, in the early-mid 80's.
Alan Gomes
Sustaining Partner
I would say yes. This can help blow out the carbon and make the rings happy. I have done this with my little Yanmar 1GM and it helps keep down oil consumption. Most of the time, though, you will want to run it at about 80% of max RPM for general cruising.
This all assume are properly sized/pitched prop.
Below is an AI summary, but it fits with most of what I was going to say (esp. the paragraph on Periodic Loading). For low speed cruising, I use about 1800 rpm on my M-25 (9800# E-32). For high speed cruising, I use 24-2500 rpm. Occasionally, I'll run it up to 2700 for 20-30 min to burn off the carbon. Can't think of a good reason to go higher than 90%/27-2800 rpm.
The best continuous cruising speed for most marine diesel engines is 60% to 80% of their maximum rated RPM. For optimal longevity and fuel efficiency, many operators prefer to cruise around 65% to 75% of the maximum RPM. Running at this range provides a good balance between fuel economy, speed, and preventing engine issues like bore glazing.
Key Considerations for Marine Diesel RPM:
The best continuous cruising speed for most marine diesel engines is 60% to 80% of their maximum rated RPM. For optimal longevity and fuel efficiency, many operators prefer to cruise around 65% to 75% of the maximum RPM. Running at this range provides a good balance between fuel economy, speed, and preventing engine issues like bore glazing.
Key Considerations for Marine Diesel RPM:
- Optimal Cruise: Generally, 60–80% of max RPM is recommended for continuous duty, keeping the engine in a comfortable power band.
- Fuel Efficiency: While lower RPMs generally save fuel, running too low (e.g., under 50% for extended periods) can lead to inefficient combustion and carbon buildup.
- Maximum RPM Definition: It is crucial to know your engine's maximum RPM when it is under full load (properly propped), as this is the baseline for these percentages.
- Periodic Loading: It is recommended to run to the engine at higher RPMs (e.g., >80%) for short periods, especially after long periods of low-speed, low-load operation to clear out carbon deposits.
- Engine Type: Smaller, multi-cylinder engines (15-40 HP) often cruise around 2100-2500 RPM, while some larger or older engines might have different "sweet spots".
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Prairie Schooner
Jeff & Donna, E35-3 purchased 7/21
Great info above. - At our initial survey we were surprised to be told that our tach was reading many hundreds rpm low. We later confirmed that with a moderately priced optical reader. I highly recommend getting one to be sure you are dealing with accurate info from your tach.
HerbertFriedman
Sustaining Member
I just completed a 6 hour run on my 25X engine (1987 E34) at 2200 rpm. The max rpm is only about 2500 rpm, I think the previous owner over propped the system with a 3 blade, 15 inch diameter x 8 inch pitch, thus limiting the max rpm. But a calculation based on the boat specs indicate that a three blade prop should be 14.7 x 8.8, so I am not sure over propoing is the reason for the limited max rpm. But 2200 rpm is what the previous owner recommended and it seems to work well good fuel economy, ~1/3 gal/hour, about 5.5 kts in flat water and no wind.
I run 5 or at most 6 knots with the E38. It is purely for reduced engine noise. That's in the 2300 rpm range with my 5432 and a three-blade fixed prop--far from 2800 max.
I do like to run periodically at very high rpms, mostly to reveal the state of the heat exchanger (HE). If you can run balls out for 20 minutes and maintain the usual operating temperature, the cooling system is in fine shape. The state of the HE can be monitored by such forays. Salts buildup over time requires a lower and lower RPM to stay at my running temp of 180. When high RPMS drive the op temp to 190, then 200 and above, I know it is getting to be time to boil out the HE.
I do like to run periodically at very high rpms, mostly to reveal the state of the heat exchanger (HE). If you can run balls out for 20 minutes and maintain the usual operating temperature, the cooling system is in fine shape. The state of the HE can be monitored by such forays. Salts buildup over time requires a lower and lower RPM to stay at my running temp of 180. When high RPMS drive the op temp to 190, then 200 and above, I know it is getting to be time to boil out the HE.
I agree with Christian’s idea of occasionally running at max rpm for a while to see if engine temp remains in range.
Otherwise, marine diesels actually prefer to run at about 80% of the engine’s ‘wide open throttle’ (WOT) spec, which should be in the manual. AI says the M25’s WOT is 3,000 rpm (when propped appropriately), which indicates that 2500 rpm is the correct cruising target. It might also be worthwhile asking a major prop vendor for the ‘ideal’ prop configuration for the particular boat (LWL and displacement), engine, transmission and shaft diameter.
I recently had to get a new prop, and did this check with Michigan Wheel Propeller before ordering (I didn’t tell them what I was replacing). They were happy to give me the ideal diameter and pitch, which turned out to be 1” more pitch than the previous prop had, so I’m glad I didn’t order a direct replacement.
Otherwise, marine diesels actually prefer to run at about 80% of the engine’s ‘wide open throttle’ (WOT) spec, which should be in the manual. AI says the M25’s WOT is 3,000 rpm (when propped appropriately), which indicates that 2500 rpm is the correct cruising target. It might also be worthwhile asking a major prop vendor for the ‘ideal’ prop configuration for the particular boat (LWL and displacement), engine, transmission and shaft diameter.
I recently had to get a new prop, and did this check with Michigan Wheel Propeller before ordering (I didn’t tell them what I was replacing). They were happy to give me the ideal diameter and pitch, which turned out to be 1” more pitch than the previous prop had, so I’m glad I didn’t order a direct replacement.
Bigislandluv
Junior Member
Thanks for all the info. Found out our boat is not a M-25 it is a M-30.
Looks like it was rated for about 2800 RPM max. I would guess that cruising revs might be about 2300 or 2400. Maybe?