Propeller selection

[PAMeyer]

I have a 1976 Johnson 75 HP on a 17 ft bow-rider. Using an OMC 14x13 prop, that does not appear cupped, I run 6000 RPM and 29 MPH. I believe the tach is correct, I'm not sure about the speedometer. Switching to a new HUSTLER cupped 13.75x15 prop, I run 5000 RPM and 32 MPH I had expected 5500-5600. I switched props a couple of times and get the same results. I am surprised at the RPM drop. Is this due to the cupping? The engine seems to run well. If I use a cupped 14x13 do you think I'll run in the 5600 RPM range? <br />Thanks again for your continued help.

 

[moderator1]

Re: Propeller selection

Moving to Prop questions and topics...

 

[walleyehed]

Re: Propeller selection

the 15" cupped would be the same as a 17" un-cupped, so really, you jumped 4" in pitch creating the 1000RPM drop.<br />A 13" cupped would most likely be around 5600-5700.

 

[PAMeyer]

Re: Propeller selection

That explains my results. So cupping effectively adds 2 inches to the pitch...is that correct?

 

[walleyehed]

Re: Propeller selection

It doesn't really add pitch, it reduces slip by having more bite. (More efficient over-all)

 

[Realgun]

Re: Propeller selection

Pitch increase 1 inch for cupped prop. So a 15P is really 16 picth if cupped.<br /><br />Those are nice props. I killed 2 already and working on #3. Gotta rember there has to be water under the prop not rocks or it does not work very well.

 

[walleyehed]

Re: Propeller selection

All manufacturers will tell you if using "X" un-cupped, go to "X,-2" cupped. all manufactures are different too, so 1 in a 19, may run less RPM than anothers 21. The props have to be the same blade design for any of the know variables to be close to true.