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Old 09-17-2018, 06:05 PM   #1
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Cruising RPM for a Cummins 210

My boat is a 1990 GB 32 (829) with a cummins 210. I have been told the cruising RPM for this engine is 1700 RPM with a max of 2300. In the cummins manual for the engine the cruising rpm is stated at 2300 with a max of 2400. Now I understand this is a 1989 or older engine with about 1200 hours on it but it runs strong with no problems. Additionally, at 1700 rpm I burn about 3 to 4 gal per hour and the manual states at 2300 rpm the burn rate is about 10 gallons per hour. There is a trade off for sure. Any insight into the difference between standard recommendations and the manual are greatly appreciated.
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AJG
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Old 09-17-2018, 06:52 PM   #2
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First off, 2300 WOT is low so you are over-propped or the tach is wrong. Both are common. Purchase a digital photo tach for about $50 and find out.

Second, running 1700 in that condition won't hurt anything unless you are smoking. Running 2300 more than a short test will.
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Old 09-17-2018, 06:52 PM   #3
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What the Cummins manual is really saying is that you can run that engine continuously at 2,300 rpm. That is not the same as saying that the cruising rpm is 2,300.


The cruising rpm is whatever gives you the speed you want, the fuel burn you want and the engine life you want. Fortunately for you, with the Cummins 210 you really can't harm the engine by running too fast, certainly not at 2,300 as long as it is propped correctly to achieve 2,450 at wot.


That boat has a theoretical hull speed of about 7 kts and you will reach that speed at about 1,500 rpm. If that is ok with you then go for it. You can get more speed but it will cost you in more fuel.


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Old 09-17-2018, 06:55 PM   #4
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Over the last 1500 hrs or so, we run between 1600 and 1700 and with the heater and genny use we average right at 3 gal hr burn. So far no issues with her and she has now hauled us for over 10K miles. Take real good care of her and she will take care of you.
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Old 09-17-2018, 06:58 PM   #5
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Run at six knots to achieve optimal speed/fuel consumption trade-off.
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Old 09-17-2018, 07:07 PM   #6
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I have a 5.9 in our motorhome. If I remember correctly that peak torque is around 1700 RPM. We run the motorhome at 1700 and get the best milage there.
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Old 09-17-2018, 07:10 PM   #7
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Something wasn't ringing right so I went to SBMAR.COM.
The 6 BT at 210 hp should be max WOT at 2600 RPM not 2400 RPM. Tony also recommends setting the boat up to achieve 2625 RPM so that it does not run in an over prop condition.
So at 2300 WOT, if true, you have way too much prop, dirty fuel filters, or something else preventing reaching rated RPM.


https://www.sbmar.com/cummins-marine...rmance-curves/
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Old 09-17-2018, 07:23 PM   #8
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Archie is right. My 210's are rated for 2600 max rpm. If you can't get there and your bottom is clean, you may be over-propped or have a fuel delivery problem. I run mine at 1600 rpm which gives me around 8 kts at a total fuel burn of 5 gph.
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Old 09-17-2018, 07:31 PM   #9
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Cummins 210

Thank you to everyone who responded. I do understand that at WOT my engine should reach above 2300 rpm. My question really should have been is it advisable to run at max rpm for other than short test period? Is it safe for the engine to cruise at 2300 rpm not at WOT? There are times running against an aggressive tide and wind we only make between 5.8 and 6 knots running at 1700 RPM. I have not pushed it to 2300 but would be willing to trade off the fuel for some time under specific conditions. Again thanks for everyone input.

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Old 09-17-2018, 08:11 PM   #10
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'Intermittent Rating: This power rating is intended for Intermittent use in variable load application where full power is limited to two (2) hours out of every eight (8) hours of operation. Also, reduced power operation must be at or below 200 RPM of the maximum rated RPM. This rating is an ISO 3046 fuel stop power rating and is for applicationsthat operate less than 1500 hours per year.'

See https://www.sbmar.com/docs/performan...M-90761%5d.pdf

These motors can be pushed pretty hard, but you have to verify that you can get >2600 rpm fully loaded before cruising in the 2300+ range.

An EGT gauge gives a pretty good indicator of loading if you're running close to max.
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Old 09-17-2018, 09:18 PM   #11
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Quote:
Originally Posted by djmarchand View Post
with the Cummins 210 you really can't harm the engine by running too fast, certainly not at 2,300 as long as it is propped correctly to achieve 2,450 at wot.???
Do you understand that WOT for your engine should be 2600rpm? You should determine if your engine can reach that rpm before you do anything else. I had a pair of 210s years ago on a sport fisher and I ran at 2400 all day with no problems! My engines, however, could make 2600rpm at WOT!
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Old 09-17-2018, 10:12 PM   #12
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Yeah. I don't run at higher than 2200 RPM (72% load) rather than 2400 (100% load). Besides, the last 200 RPM increases boat speed by only 0.1 knot.
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Old 09-17-2018, 10:46 PM   #13
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This engine is commonly used commercial fishing and small tugs. Most are run between 1700 to 2000 rpm. An engine kit is about $700.

Even with proper cooling, the internal combustion areas gradually reach excessive temperatures. That's why ratings limit continuous to 6 hours out of 12.
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Old 09-18-2018, 04:27 PM   #14
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My question really should have been is it advisable to run at max rpm for other than short test period? Is it safe for the engine to cruise at 2300 rpm not at WOT?

The answer to your question is NO, unless you can prove your engine can make 2600-2700 RPM at WOT.

More from the SBMAR.com site:

Mechanical engines, due to the nature of the mechanical, non-isochronous governors, must be propped solely on the attainable engine speed at full throttle, steady state conditions. Unlike electronic engines governors which have a precise, predetermined, and consistent governor set point; mechanical engines progressively limit fueling to limit engine speed (known as the high speed governor break point). The high speed governor break point is defined as the speed at which the governor begins to reduce torque below the rated power value, when operating at full throttle. The difference in engine speed between the governor break point and the maximum no-load operating speed (also known as high idle) is called droop. When propping a mechanical engine, the full throttle engine speed must be above the rated speed of the engine and should be below the point where the governor break point begins. Propping beyond the governor break point will result in a linear and progressive reduction in available engine power output as speed is increased to high idle. The governor break point on mechanical CMD engine is typically 100 to 125 rpm above the rated speed. Once an engine reaches the governor break point, it will advance in speed, but power will be reduced. Therefore, if a vessel is sufficiently under-propped so that an engine does not reach 100 percent load before the governor break point, the engine will be limited from producing all its available power. In some applications, it may be necessary to prop the vessel so that the engines are operating beyond the governor break point and not at 100 percent load at full throttle. Typically this applies to production vessels, where the first hull has been loaded to a heavy displacement, tested to comply with reaching rated speed, and then unloaded to conclude what subsequent sister vessels should be propped to when tested light. The resulting prop demand at a light condition may enter the range where the engine speed exceeds the governor break point and load is less than 100 percent.

Note: CMD mechanical engines are designed to produce near constant power, increasing slightly from rated speed to the governor break point.
It is Cummins recommendation that the ideal propping point is to achieve 75 to 100 rpm over rated speed. This maximizes performance, efficiency, and durability of the engine. Figure 16-2 provides a sample engine performance curve for a mechanical engine overlaid with propeller demand curves that graphically depict the above.


Typical Target Propping RPMs for Cummins Mechanical Marine Diesels

Part BHP-MHP Minimum Rated RPM Min Propping Target RPM* Engine

DR6500-RX 210/220 2600 2700 220 Diamond

*Or 90% LOAD (or less) at minimum rated RPM.
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