Prop tweaking advice needed

[Pale_Horse_Idaho]

I have a 14ft mirrocraft, 25hp F25 Yamaha 4 stroke 5-6krpm range (new) and here is my issue.

9 7/8 x 11 1/4 works fine with me or me and the dog. I get 5800-5900rpm. (29-30 mph) Add wife and it drops to 5450rpm.
9 7/8 x 10.5 with me + dog + wife it goes right into the rev limiter (6150rpm) 2 seconds after getting on plane. (26mph)

So there is no 9 7/8 x 10.75 but there is a stainless 9 7/8 x 11 and an aluminum 9 7/8 x 11. Would dropping 1/4 pitch bring my rpms up from 5450 into the 5800 range?
Could I go from 9 7/8 diameter to 9 1/2 for less drag? Or maybe the 11 pitch stainless would be more efficient and turn more rpms?
What I need is an 10 7/8 pitch prop.


Who would have though dropping 3/4 pitch would make such a crazy jump. Im running 90-100 % throttle with 11 1/4 but only half throttle with 10.5 to stay off rev limiter.

 

[airshot]

Near top rpm with a light load and still within operating range with two people is about as good as it gets. Your trying to fine tune your prop to get it perfect in all cases, really not worth the trouble for 1 or two mph. Also consider the fact that when you add the extra person your changing the weight balance so pitch change would mean even less. Do you really want to carry around three or four props to keep changing to get the perfect rpm under each circumstance ??

 

[Pale_Horse_Idaho]

Near top rpm with a light load and still within operating range with two people is about as good as it gets. Your trying to fine tune your prop to get it perfect in all cases, really not worth the trouble for 1 or two mph. Also consider the fact that when you add the extra person your changing the weight balance so pitch change would mean even less. Do you really want to carry around three or four props to keep changing to get the perfect rpm under each circumstance ??

So is the 11 1/4 at 5400rpm ok? It sounds like its working hard vs the 5800 range.

 

[MDCrappie]

My guess is that one of those 2 props is mis-marked and not the exact pitch. You're talking less then 1" of pitch difference (6.6% different) that is leading to a 700rpm (12.8%) difference. What is the speed when the 3 of you are in boat and turning the 5450rpm? If those props are correct. knowing this speed will get you close to knowing what the rpm increase should be with 11".

 

[FuzzyGrub]

Are the two props from the same mfg, style, and type? While blades, diameter, and pitch are the primary drivers of performance, rake, material, and cup, etc, will alter results.

I agree with Airshot it is going to be hard to find one prop that will be perfect for all loads.

 

[Pale_Horse_Idaho]

Both props came from a Yamaha dealer and are OEM Yamaha. One came with the motor.
Speed of 11 1/4 is 28-29 mph and by myself on glass water I can hit 30mph 5850-5980rpms.
Speed of 10.5 is 24-26 with 26 hitting rev limiter. 24 mph is aprox 58-5900 rpms.

 

[MDCrappie]

Both props came from a Yamaha dealer and are OEM Yamaha. One came with the motor.
Speed of 11 1/4 is 28-29 mph and by myself on glass water I can hit 30mph 5850-5980rpms.
Speed of 10.5 is 24-26 with 26 hitting rev limiter. 24 mph is aprox 58-5900 rpms.

You've given us those speeds already. I was asking what the speed went down to when you say "Add wife and it drops to 5450rpm."

What doesn't make sense to me is that (not sure of exact ratio but used a L/U ratio of 2/1 in calculations) you are going from a slip percentage of 5% to approx 15% when you add the wife AND drop the pitch.......who from the pics appears to be maybe 105lbs.
Knowing the speed with the larger prop dropping to 5450 you can figure out the slip and see where the problem might be.

 

[Zum]

Your still in range at 5400rpm, so not an issue.
If she is with you most of the time you may want to try the 11 aluminum.
Depending on what brand prop they can all be abit different, even same brand props can be different alittle.
I use 3 different props(really 2 mostly) depending on load. 30hp on a 1652

 

[thill]

Props can vary widely by performance. I would be inclined to try the 11P stainless one. My last boat, it was incredible how much better the stainless performed both in low AND high speed. It was crazy. I gained MUCH better holeshot AND higher speed, and ran right at WOT. And it didn't matter if I was alone or if I had 3 others with me. It really was the "magic" prop for that boat.

I hope you find the perfect prop. Keep trying until you like the one you have.