RPM limit on props

[Steve K]

Is there a limit on RPMs for props, is there a point where they start to "slip"?
I have a 40 thrust lb electric motor on my kayak, measuring my speed by GPS, at full throttle lets say I get 4.5 mph, if I lower the input voltage ( turn down the throttle) I pick up about 1 mph to 5.5

Thanks
Steve

[CDK]

Not so much an rpm limit but a maximum blade tip speed, so small props can spin faster than big ones before reaching the critical point where a vacuum starts to form and the effective prop diameter decreases. The leading edge shape also plays an important role.

All props slip, but not to the point where increasing rpm result in a lower speed.
In your case you probably need a prop with a better shape and/or smaller diameter.

[jehardiman]

Actually, unless you can see the cavitation, I think what you are seeing is a torque effect.

Props absorb power as torque at a given rpm and prop torque increases the more "slip" you have (actually a lower advance ratio: J = Va/nD) regardless of actual speed. Many motors on the other hand slow down and draw excessive power as you load them up with torque. To prevent overloading the motor, many stock trolling motor props are matched to the motors with flexable blades and excessive sweep. What this does is to twist off the pitch to limit torque when the wheel is heavily loaded. The side effect is that the reduced pitch generates less thrust for a given rpm. Optimiumly, you want to run at a motor rpm and prop slip just below the point that the blades begin to twist off pitch.

If it is cavitation on a stock prop, you might want to inspect the leading edges of the blades. Nicks or ragged edges can cause local cavitation below the back pressure limit which then can then spread to the rest of the blade.