Cummins QSB 380 Operating Loads

[JMY]

I have a 2009 40 Dancer with Cummins QSB 380 and have been cruising in the 2500RPM range (24 - 26MPH) which the Smart Craft states is 75% engine load. My question is what is the maximum engine load that I should operate at for extended periods?

 

[Mr Salt]

All the diesel mechanics that I have asked that question to have told me that diesels like an 80% load.

 

[fwebster]

Varies by manufacturer..........

Most quote loading in terms rated hp @ a certain rpm, which should be your WOT loaded (not %; full fuel, water, and normal gear and humans) and is 100% load. They will also quote a "max sustained cruise" setting in rpm's which is the answer to your question.

Cummins may publish % load factors..I don't know. If they do, check your engine operation manual.

 

[Rocket]

My Cummins Tech advised 80 to 85% of rated max RPM but he did recommend WOT for 1 of 8 hours. He claims the motors will have no problem at WOT for the hour. I dont run WOT for more than a few minutes.

The 450C motors are rated for 2650, but at WOT i am at 2760, i still use the 2650 for the my max rpm. I guess searay overpropped this boat.

 

[Four Suns]

Wow.. I've never had a Cummins person tell me to run WOT for an hour... My manual doesn't say that either... No way I would do that.

The best thing to do, as Frank said, is look at the manual. On my Cummins engine, cruise is "80% load" which is 2100 RPM out of a rating of 2300 RPM for WOT. The reason that is important is not to go over the 80% load when running the boat. For example, if I am in heavy seas, I'll see "80% load" at 1900 RPM... So that's where I don't exceed.

Generally speaking, I try and keep the boat around 75% load and the RPM will depend on the fuel on board, amount of shoes, waste, water, etc. As the boat lightens up during a cruise, my RPM's increase but I keep them below 80% load per the manual.

My 2 cents.

 

[Rocket]

Gary - i questioned him further and he claims that you should record your operating temps, oil pressure at WOT. Apparently if your record normal temps and pressure at WOT, and it varies later on, this will give an indication that something is wrong. At 80% problems might not show up until its too late. Sort of a diagnostic test.

This also was the owner and guru of the local cummins rep in the area, we have an independantly owned service co. I agree with you and take caution at 100%. Also i dont have a load gauge, only RPM.

 

[Four Suns]

Yeah.. an hour seems a bit much.. I test WOT probably weekly when I'm cruising but don't really see a need to do that more than a minute or two... but I'm not a Cummins guru...

 

[wetpaint]

You should always vary the rpm's, but diesels love to run and can run all day at 85% load........but I cruise at 2300 rpm's with a 70% load.....It purrs at that....

 

[SR400DB]

My cummins guy told me I "could" run my engines at WOT every 1 out of 8 without voiding warranty/risking damage. I have never done this. I check my WOT at least monthly for a minute or two and typically vary my cruise from 2050 to 2150 rpm

 

[osd9]

....The 450C motors are rated for 2650, but at WOT i am at 2760, i still use the 2650 for the my max rpm. I guess searay overpropped this boat.

Just to clarify, and not to nit pick, but that would be underpropped.

 

[RollerCoastr]

Keep in mind that the more sophisticated your engine electronics are, the more they will betray bad habits at the time of resale. No-one wants a potential buyer or a surveyor to study an engine computer report that claims that the engines where run at WOT and/or overloaded 20, or even 10% of their lifetime. Electronic diesels have many benefits, but they can also have a "big brother" component.

The 450C motors are rated for 2650, but at WOT i am at 2760, i still use the 2650 for the my max rpm. I guess searay overpropped this boat.

Rocket, did you mean to say underpropped? At any rate, assuming she's not suffering from performance issues, that's about where I would want 'em.

 

[Rocket]

Just to clarify, and not to nit pick, but that would be underpropped.

Having trouble with that, under propped would mean that the props are too small? And that would cause the motor to go over RPM?

 

[tobnpr]

Having trouble with that, under propped would mean that the props are too small? And that would cause the motor to go over RPM?

Yes.

"Overpropped" means too much-too large- a prop. Any combination of pitch, dia., cup, etc. whereby the engine cannot reach max rated rpms when loaded.

"Underpropped" is the opposite- your case. If both your engines are able to turn 110 rpm over spec when the boat is fully loaded, you should consider adding a bit of cup at your next haul-out. I believe you could lose 50 rpms or so and increase your cruise speed and efficiency a bit without compromising engine longevity.

 

[JMY]

Thanks for all the feedback, everyone enjoy the coming weekend on the water.

 

[Rocket]

Yes.

"Overpropped" means too much-too large- a prop. Any combination of pitch, dia., cup, etc. whereby the engine cannot reach max rated rpms when loaded.

"Underpropped" is the opposite- your case. If both your engines are able to turn 110 rpm over spec when the boat is fully loaded, you should consider adding a bit of cup at your next haul-out. I believe you could lose 50 rpms or so and increase your cruise speed and efficiency a bit without compromising engine longevity.

Ok thanks - I got it now :smt038

 

[Robski97]

Just To add to Rockets Post. I was told the same thing at my Captains meeting.

I was also told which I think i posted before You could run the boat all day at 80% load like Gary said . ( 200 Rpm less then WOT )


Rob

 

[osd9]

...I was also told which I think i posted before You could run the boat all day at 80% load like Gary said . ( 200 Rpm less then WOT )


Rob

Rob....that sounds high! I think your WOT is around 2650.....running at 2450, all day long, on a pair of 'QSCs' seems high.....that's more than 90%....approahcing 95%.... of full throttle....

 

[Mike 51 Sundancer]

I alsol own a 40' 2008 Sundancer with the QSB380's and was told by my cummins mech. to run wot for 1 hour out of every 8 and check temp's, oil pressure, rpm's and speed and record this information. I also run at 2500 rpms for my normal speed.

 

[Robski97]

Dom ,
Yes my WOT is 2650. I was told % of load was my guide not % of throttle.

Rob

 

[Mike 51 Sundancer]

By the way wot on my 40' is 3000 rpm's and the cummins mechanic said I should hit that plus another 1000 rpm's or something is wrong. I also had my props computer ballanced and checked and it made a big difference in how smooth the boat runs. Also at 2500 rpm's my speed is 22.5 to 23.5 mph depending on total fuel, water and people on board.