Prop sizes

[Baker]

Ok...my main set of props are 22x24. They are perfect....2700rpm on a 2600rpm rated engine(6CTA-450). My spare set is 24x24. We can deduce that I am overpropped with those...and I most certainly am...2500rpm on same engines. I was thinking of taking two inches of pitch out of the 24 inchers to make a 24x22. Another thought would be to cut off an inch and take an inch out to make 23x23.

Now let's assume all configurations deliver the proper RPM. Would any one configuration be more desirable and why?

TIA

 

[rslifkin]

To figure out what works best on your boat may take a few tries. If you're a bit short on blade area, you might see a slip reduction with the bigger diameter and might not lose speed, even with less pitch. If you don't need the extra blade area, the smaller diameter / higher pitch will likely perform better.

De-pitching the 24x24s to either 23 or 22 pitch will get you in the ballpark for engine load with those. I'm not sure if an inch of pitch will be enough or if you'd need to go to 22.

I did find some data for boat speed and trans ratio for your boat. Based on that, the Michigan Wheel calculator says that 24x22 should get you 2700 rpm if the boat does 27 kts. If you want 2650 rpm or the boat does 28 kts (and 2700), then the 24x23 should get you there. Forcing the diameter to 22 agrees with your 22x24s.

 

[DavidM]

Definitely reduce pitch, not diameter. A 1-2" pitch change doesn't have much affect on overall efficiency, but a 1-2" diameter change will. Also a larger diameter prop is almost always more efficient than a smaller diameter one.

David

 

[Insequent]

Definitely reduce pitch, not diameter. A 1-2" pitch change doesn't have much affect on overall efficiency, but a 1-2" diameter change will. Also a larger diameter prop is almost always more efficient than a smaller diameter one.

David

I agree. Provided that you will still have adequate clearance to the hull when using 24" diameter, then pitch reduction is the way to go. It should be reversible if needed also. Reducing diameter is a one-way only step!

 

[Nomad Willy]

Definitely reduce pitch, not diameter. A 1-2" pitch change doesn't have much affect on overall efficiency, but a 1-2" diameter change will. Also a larger diameter prop is almost always more efficient than a smaller diameter one.

David

Unless it results in friction or parasitic drag to become too great. Or put another way resulting in too much blade area.
Power loading and pitch/diameter ratio also very important.
Eric

 

[Baker]

To figure out what works best on your boat may take a few tries. If you're a bit short on blade area, you might see a slip reduction with the bigger diameter and might not lose speed, even with less pitch. If you don't need the extra blade area, the smaller diameter / higher pitch will likely perform better.

De-pitching the 24x24s to either 23 or 22 pitch will get you in the ballpark for engine load with those. I'm not sure if an inch of pitch will be enough or if you'd need to go to 22.

I did find some data for boat speed and trans ratio for your boat. Based on that, the Michigan Wheel calculator says that 24x22 should get you 2700 rpm if the boat does 27 kts. If you want 2650 rpm or the boat does 28 kts (and 2700), then the 24x23 should get you there. Forcing the diameter to 22 agrees with your 22x24s.

Very interesting. Those speeds are dead on the nuts And based on the other replies it sounds like the 24x22 would be the better choice. And trust me. I know. The prop shop has thousands of my recent dollars getting things dialed in. Looks like they’ll have one more go round.

Thanks again.