Which prop pitch

[Nickmcdaniel4]

Hello everyone. I have a question about prop pitch. I am currently running a 13 x 21 semi cleaver on a yamaha c115 1998 model. I turn 6300 rpms at 54 mph gps. I have run a yamaha pro series 13.5 x 22 and it had terrible hole shot and would run @47 mph and 5250 rpms.
I'm guessing that I probably need to stay with the semi cleaver style bc it helps the motor spool up faster but not sure. I have a triton tr17 and I'm on the third hole down on the motor. It puts my cav plate about .5 inch below bottom when trimmed all the way down. I'm running about 13% slip with both props. I really just want to run in the 5500 to 5600 rpms at wot. Any help or suggestions? I was looking at a yamaha painted ss prop in the 12.5 x 25, mathematically, this would be the right prop.

 

[fhhuber]

Best would be to find a local boat shop that will let you borrow test props. The local dealers each have a table full of props labeled "Demonstration, not for resale" that they loan out for people trying to determine what would be best on their boat.

 

[Sea Rider]

Was 6300 wot rpm achieved with lightly loaded boat and you alone ? Bear in mind that as soon you add more weight to boat rpm will drop accordingly. If hole shot is spot on personally would stay there. If boating solo throttle less and watch tach...

With combo well weight balanced trim to a hole that achieves boat and AV plate running parallel to water level when on plane. If no back or over transom water splashes you're fine.

Happy Boating

 

[Texasmark]

I'll agree that thrust being parallel to the direction of travel (AV plate parallel to water level) maximizes the trust from a propulsion unit, however there are other things at stake where you are running.....aka wetted area of the boat....aka drag. To minimize that you need to get the bow up and that means that the AV plate will usually be tilted down in the front, and possibly above the water line. tip of the prop blades at or near the surface, running what and how you are running......in my estimation.

I ran your numbers through Go-Fast.com's prop slip calculator and agree with your slip numbers.....14 for the cleaver and 15 for the 22P. That says both props are grabbing the water. So you have the same boat, same engine, assumed same environment, different "gears" in your gear train and you loose a whole lot of speed and rpms. So what gives? Well I'm going to say that the cleaver has more "Rake" which gets the hull out of the water, reducing drag, thus letting the engine come on up the power curve meaning the torque curve on that engine is relatively flat on out the RPM range.

22/21 with the same slip is 4.8% thus expecting the same reduction in rpms and same or higher speed.....same speed: 54/47 = 14.9% drop and rpms: 6300/5250 = 20% drop. Something dreadfully wrong here.

Since your numbers indicate that the props are biting adequately and that the engine has a reasonably flat torque curve, and HP = (Torque x RPMs)/5252 and HP is what turns the prop shaft, rotating the prop creating thrust, then the only thing left is hull drag. Since you are running the same boat, assumed same everything else, then it's gotta be prop shape and I'll go for the 22 having less RAKE providing less bow lift, more hull in the water, and more load on the engine.

 

[Nickmcdaniel4]

I was wrong about the 22 pitch prop. I run @ 52 mph with it. The semi cleaver 21 will lift the back of the boat higher than the 22. It also doesn't seem to matter about the weight load with the 21. It will drop maybe 200 rpms with another person in the boat but will still over rev on a longer run say half mile or so. The semicleaver is great for straight runs but doesn't hold well in turns rpms jump 2 to 300. Haven't experimented with the 22 a whole lot, it will chine walk to the point that you can't drive through or out of it. I think this is just going to be a trial and error type of thing. Most forums about this boat have a mercury , Johnson or evinrude 115 and the pitch props vary between 19 and 22. Everything I've read says the 115 yammi is almost the same as the 130. (Different heads and exhaust porting). The 130 is rated for 6k rpms where the 115 is 5500.
I thought about sending one or both of the props to a professional but don't know who to send them to.
I appreciate the help and welcome all suggestions.

 

[Nickmcdaniel4]

I've tried 3 props.

Unknown brand 13 x 21 semicleaver 6300 rpms 54 mph.
Yamaha performance 13.5 x 22 5250 rpms @ 52 mph
Mercury trophy plus 4 blade 13.75 x 23 5000 rpms @ 47 mph.
Prop shaft 5" below keel. Third hole down on transom, no jack plate.
The boat does have a stepped keel. Measurement is from true bottom, not step.

 

[Texasmark]

James' Propeller Service, Carrollton,TX. did mine. Located just NW of Dallas

 

[Nickmcdaniel4]

I have a newer prop. The only numbers on it that I see are 78 753. The hub has 13 x 22 on it. I've tried to look up the numbers but haven't had any luck. Anyway.... this prop has some rake and some cupping. Hole shot is really good and boat will run 5800 rpms at 52 mph gps. I'm still getting the 13. Something slip though. I was wondering if I go down one hole with the motor would help that. Prop shaft is 4.5 inches below hull bottom. Av plate looks to be 1 to 1.5 inches out of the water when on plane.
Any suggestions?

 

[Sea Rider]

Stay at a trim hole that achieves a parallel combo running on water surface, provided that's well deck balanced. With that AV plate height will experience aireation on choppy/windy water cond and at close tight turns, check. Will need to lower OB a bit, but that's probably impossible. Post some pics off the combo shot rear side...

Happy Boating

 

[Nickmcdaniel4]

I can go down probably 2 to 2.5 inches. The motor is mounted in the third hole down. It does blow out in corners probably 3 to 400 rpms worth. I will take some pictures this weekend. Thanks for your help.