|
07-09-2020, 03:40 PM
|
#1
|
TF Site Team/Forum Founder
City: League City, Tx
Vessel Name: Floatsome & Jetsome
Vessel Model: Meridian 411
Join Date: Oct 2007
Posts: 7,332
|
Prop sizes
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
__________________
Prairie 29...Perkins 4236...Sold
Mainship Pilot 30...Yanmar 4LHA-STP...Sold
Carver 356...T-Cummins 330B...Sold
Meridian 411...T-Cummins 450C
|
|
|
07-09-2020, 03:43 PM
|
#2
|
Guru
City: Rochester, NY
Vessel Name: Hour Glass
Vessel Model: Chris Craft 381 Catalina
Join Date: Aug 2019
Posts: 7,538
|
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.
|
|
|
07-09-2020, 03:48 PM
|
#3
|
Valued Technical Contributor
City: Litchfield, Ct
Join Date: Aug 2012
Posts: 6,775
|
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
|
|
|
07-09-2020, 04:49 PM
|
#4
|
TF Site Team
City: Brisbane
Vessel Name: Insequent
Vessel Model: Ocean Alexander 50 Mk I
Join Date: Mar 2012
Posts: 4,260
|
Quote:
Originally Posted by 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
|
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!
__________________
Brian
|
|
|
07-09-2020, 05:38 PM
|
#5
|
Guru
City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,738
|
Quote:
Originally Posted by 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
|
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
__________________
Eric
North Western Washington State USA
|
|
|
07-10-2020, 04:52 AM
|
#6
|
TF Site Team/Forum Founder
City: League City, Tx
Vessel Name: Floatsome & Jetsome
Vessel Model: Meridian 411
Join Date: Oct 2007
Posts: 7,332
|
Quote:
Originally Posted by 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.
|
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.
__________________
Prairie 29...Perkins 4236...Sold
Mainship Pilot 30...Yanmar 4LHA-STP...Sold
Carver 356...T-Cummins 330B...Sold
Meridian 411...T-Cummins 450C
|
|
|
|
Thread Tools |
Search this Thread |
|
|
Display Modes |
Linear Mode
|
Posting Rules
|
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts
HTML code is Off
|
|
|
|
» Trawler Discussions |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
All times are GMT -5. The time now is 09:21 PM.