Diesel cruising RPM

JPGator

JPGator

Active Member
Apr 25, 2019
495
Treasure Coast
Boat Info
2008 Sundancer 40
Engines
Cummins QSB 5.9 425HO
This is my first diesel boat and I’m having trouble finding some information. For reference, 08 40 w/Cummins QSB 5.9 420 HO

I’d like specific information on cruising RPM. The broker told me to cruise at 2400 but where is that written? What temp is too hot? What load percentage is too high?

I have all the manuals that came with the boat but I can’t find anything. Suggestions?
 
Should be able to find info on that engine. I googled it - saw on the Sabre Yachts site it listed max continuous cruise is 2800 RPM. I wouldn't bet on that number - just what popped up.

Bottom line, there should be a rated cruise RPM from the manufacturer - then you will get into all kinds of thoughts and recommendations.
 
What is your rated wide open throttle? 85% of that is where you should be
 
We had this boat with this engine configuration for 2 seasons.

WOT RPM should hit 3060 rpm

Personally the boat always felt nose high, even with tabs down, at anything under 2700-2800 rpm. General suggested cruise is at 80% load.

To us the boat felt like it wanted to run at 2800 RPM.

Your vessel view should have a screen to show load %.
 
I truly appreciate the input but are any of you able to
techmitch said:
Not sure the info. you seek in on this page https://www.sbmar.com/cummins-marine-diesel-performance-curves/ but if not, I'm sure it is on this site somewhere. You should sign up if you haven't already.
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Hey Mitch, thanks for that. I actually have a version of that document in my manual set without the graphs.

What I am unable to find is what is considered optimal. In this thread it has been mentioned that 80% load or 85% is optimal but again where is that written. I promise I am not trying to be an a$$ I'm just curious.
 
80% load is a general rule of thumb that applies across the spectrum of diesel engines. Follow it and you can't go wrong. You may be able to find that guidance in your Cummins Owners Manual, but you might not. Also, depending on how the boat is loaded (fuel, water, waste, crew and provisions) and bottom and prop condition, RPM's at 80% load will vary.
 
Most recreational boats with Cummins are fitted with what Cummins calls "HO" or "High Output". Basically this means the engine is rated for only brief periods (no more than 1 in every 8 hours) above 80% load, and extended running at 80% or lower.

below is an excerpt from Cummins website at https://www.cummins.com/engines/marine-ratings

High Output (HO)
This power rating is intended for infrequent use in variable load applications with a load factor of 10-30 percent. Full power is limited to one out of every eight hours of operation. Reduced power operation must be at or below 80 percent load.

Engines with this rating are intended for powering recreational use vessels only. Use of High Output ratings in commercial applications will at Cummins' discretion void the warranty. Commercial use is defined as any work or employment-related used of the product, or any use of the product which generates income, for any part of the warranty period, even if the product is only occasionally used for such purposes.

Typical High Output applications are sportfishers, motoryachts, and cruisers.
 
I posted this question on the Cummins forum and got valuable data back. I’m probably over proped because my RPM’s don’t match my fuel burn. Going to run some tests this weekend and report back to Cummins.
 
Understanding that it's a different engine, I run a 2018 QSB 6.7 480. It is a HO-rated engine. Documentation from Cummins states that the max cruising load factor for my engine is 90%. My max RPM is 3350; therefore my target RPM is 3015 or less. I carefully monitor the the load % via a data field on my Garmin and target 85% load or less. My trip this weekend when running with the tide I was hitting 27-28 mph at about 77-79% load, and burning 19 gph.

I know a lot of folks will say that conventional wisdom is the target cruising load is 80%; I'm going by multiple Cummins-issued documents for my engine, and confirmed by a Cummins tech during my captain's briefing.

For temps my engine typically runs between 167-171 when warm and at cruising speed. Over temp alarm occurs at 205.
 
There is no documentation that stipulates percentage of load for my engines other than conventional wisdom. Cummins said I should be hitting a specific fuel target based on RPM and that’s what I am going to see this weekend. Otherwise its a prop issues. Currently with my engines I’m getting 21-22 fully loaded or 22-23 not.
 
Previously I had Cummins QSC 600's. If memory serves WOT RPM was slightly over 3300. I typically ran the boat at 26-2700. Felt that was my best fuel burn to distance traveled.
 
On the QSB and QSC engines designed for recreational use, Cummins recommends that the max continuous RPM is rated RPM minus 300. Rated RPM for the QSC 600 is 3000 so max continuous is 2700. I usually cruise between 2200 and 2500 RPM with my QSC 8.3 600.
 
JPGator said:
The next question is are these number with or without trim tabs full down?
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I always wondered something similar. We prop the boats to WOT, fully loaded, but I always see it noted to be trimmed out (IO trimmed up, inboards with tabs retracted)

but, it seems to me a boat should be propped to its normal use. In other words, if you run with tabs at 50% all the time then you should be propped to the rated WOT with tabs at 50% since that changes engine load. Maybe it’s insignificant, but with so much more wetted surface it stands to reason it impacts loading to some degree

I remember a recent post where EGT increased/decreased with tab usage so that indicates a change in engine loading
 
I know this is a Cummings thread, but Cat recreation is the same at 80%. That said, when I talk to Cat tech that supports everyone, he thinks we should be more like 65-75% load if you want engines to reach manufacturers overhaul which is 10,000 hours. So I run at 18kts doing 65% of load.
 
JPGator said:
There is no documentation that stipulates percentage of load for my engines other than conventional wisdom. Cummins said I should be hitting a specific fuel target based on RPM and that’s what I am going to see this weekend. Otherwise its a prop issues. Currently with my engines I’m getting 21-22 fully loaded or 22-23 not.
Click to expand...

What numbers are you referencing?

upload_2020-9-10_22-5-33.png


I have QSC 540's and you want to be propped below the curve... for example at 1000 RPM i burn 2 gph at 1200 I burn 3.5 gph at 2250 I burn 17.5 at WOT 2680 rpm I burn 27.4 gpm.

We cocktail cruise from 1000-1200 and fast cruise at 2250 rpm and this is about 70% load.

If you follow the advise on sbmar.com which many of us have found very valuable for cummins engine support if not the best they suggest all cummins engines be propped to stay below the curve above
 
Exactly those charts Zach. For my QSB’s 2400, 2500, 2600 are the numbers. My fuel data is off by about 3 GPH per side but we typically run with the tabs full down which increases burn. The props were recently done and the prop shop has the Hull ID so they know the manufacturers specs. Fueling up the boat today and going to do the tests Cummins requested.
 
Should be able to build a fuel burn curve and bring to a good prop guy and he can make adjustments to the prop. We have had ours adjusted from factory spec.

There have been many discussion on prop adjustments from factory spec to get the loading correct.
 
I do not understand running with full tabs at cruise. I had a 2005 390 Dancer with 380 Cummins that was rated at 90% for cruise rpm. I ran that boat at 2600-2700 and the numbers were fine and I seldom used more than a small amount of tab. You can watch your GPS to find the correct amount of tab needed for the conditions you are in.
If you do the fuel chart as recommended above you may find at higher RPMs your numbers will be more in line with the cummins fuel curve because the boat is over the hump.
All of the advice on this topic should start with serviced after coolers, heat exchangers and fuel filters.
 
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