Cupped Propellers - Good? Bad? When?

Brett Allred

Brett Allred

New Member
Aug 11, 2023
6
Ruskin, FL
Boat Info
2004 480 motor yacht
Engines
twin Cummins QSM11-C
I'm going through the dry exhaust repair of my twin QSM11 Cummins engines that all QSM11's have or will experience. The solution, by popular vote, is to de-prop the vessel to reduce load on the engines and I completely understand and agree with this argument. My wheels are 28x32 and I plan to back down to 28x30. I've added the Maretron pyro sensors to my project plan so as to monitor EGT via N2K on my MFD. My research is telling me to stay as far below 1000F as possible especially since I'm monitoring at turbo outlet.

The question is should the props have a cup or not. How much cup. What's the purpose of cupping? Who determines the best cup?

Please jump in if you have experience in this area or if you've researched this subject and have something to contribute.
 
When we had our props tuned, I simply told the shop a) what RPM we could make at WOT with "original" tuning, and b) what RPM we wanted to make at WOT+50.

I didn't need to know much of anything about cup... nor about diameter or pitch, for that matter.

-Chris
 
Thanks for the reply Chris. Were you satisfied with the result? Did they scan it before and after?
 
Yes, they did good. And yes, they gave me copies of details for both before and after.

One thing I did NOT do when I was compiling target info, and will do next time, is to turn off the sync function for our engines/gears during my preparatory test runs

The new RPM results at WOT came out to be what I specified, but our load percentage at WOT is slightly different (port engine slightly lower). Next time I want to take load into account too, not just synced WOT.

-Chris
 
Not diesel experience, but it relates. The cup is there to essentially give you a "two-speed" prop; slightly less pitch that the empirical numbers would show so that you have better low-speed acceleration, then, at higher speeds when there's not as much slippage, the cups effectively throw more water as if you were running a higher pitch. Of course, the whole world's a compromise; the cupped props are not as efficient as a straight blade due to the turbulence induced, so you need to evaluate your needs before biting that bullet. If long-range cruise economy is your primary concern, go with straight-pitch props.
 
If you can get them to do it, in my experience most prop shops only want to know your current WOT RPM and work from there.
 
ranger58sb said:
Can you not just substitute load for WOT? "I get X RPM at load Y; I want RPM Z at load Y."
Click to expand...

quality time said:
If you can get them to do it, in my experience most prop shops only want to know your current WOT RPM and work from there.
Click to expand...

I'd guess it might be just some arithmetic? "Give me Z more (or less) RPM than we get now."

??

I dunno if prop RPM is a dynamic curve or static straight line, though....

Seems lite it could be worth a discussion with the prop shop guys, describe the goal, see if they think it's a big deal or not... and if it is, maybe they'd suggest a way forward.

-Chris
 
Having researched this and gone through the same exercise as the OP a few years ago there really isn't a one size fits all formula. Some 480DB's achieve the recommended parameters with 28x28, 28x30, 28x31 etc.. He's just going to have to pick a 1" or 2" reduction and see what happens. Then you can tweak your RPM's in smaller increments using the cup.
 
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