Thread: Under propped?
View Single Post
Old 03-15-2014, 11:31 AM   #17
twistedtree's Avatar
City: Gloucester, MA
Country: USA
Join Date: Jan 2013
Posts: 4,968
Originally Posted by Baker View Post
Okay, I will play along. What is our max RPM limit??? That is somewhat of a rhetorical question. My only point here is it is gonna take some guessing and assuming to get what you want. And then in the end, you are going to have to believe in your guesses and assumptions.
Actually, I think it's all science if you have the engine's power curve. I've attached an example that shows the full throttle power curve and the approved prop curve for a Cummins QSC 500, which happens to be what's in my boat.

I've marked in a more aggressive prop curve in red (pardon my sloppy art work, but it should convey the idea). With the approved prop load show in the black line, you can see that it intersects the engine's full power curve at max RPM. The entire prop curve is below the engine's max power curve.

In the fictitious red prop load that is more aggressive, it now intersects the engine's max power curve just below 2000 RPM instead of 2600 RPM. Above 2000 RPM, the prop is overloading the engine. This more aggressive curve is OK provided you never run the engine above 2000 RPM, so that's the new red line for the RPMs.

I should have said this in my previous post, but I am NOT an advocate of doing this. I bring it up just because it's what the original poster's friend was doing, and what he was asking about. And people like Dashew talk about this too, so I'm trying to shed some light on exactly what they are doing, and where the hazards lie.

You said that if you came across a boat that only turned 2200 RPM that was speced for 2500 RPM, you would walk away. So would I, and pretty much every surveyor would flag it as a problem that is likely to have caused engine harm over the years. I think it's a bad idea.
Attached Files
File Type: pdf Pages from QSC500Performance.pdf (32.2 KB, 101 views)
twistedtree is offline   Reply With Quote