Baffeled by RPM !

[Sam]

After upgrading the top half of my 350 this past winter I am perplexed by an rpm issue. I have been testing two props on my sixteen, a 23" and a 21" Mirage Plus. The the 23" get's me to 4800 rpm the 21" only takes me to 4900 rpm. I thought I was hitting the rev limiter so I switched ignition modules to eliminate that. I go out today and I get the same results. In theory I should be around the 5200 mark with my 21" based on 1" of pitch = 200 rpm up or down dedpending on which way you go.
So what are your thoughts ? Any sugestions you can offer would be greatly apperciated.

THX
Sam

[CDMA]

Has either prob been labbed, cupped, modified? Are they both in perfect shape? Are you sure the new module has no rev limiter?

Chris

[Formula Jr]

The 21 is probably cupped and or of slightly larger diameter than the 23. Therefore it performs like a 22 inch prop. Which would es'plane the 100 rpm diff.

[Ralph Savarese]

I am running a 14 1/2x21 tried it today with my new heads and manifolds 5200rpm full tank 2 people . would you say re pitch to a 23 should get me to about 4800 and go faster correct?
Ralph

[Sam]

Both props are brand new out of the box. I switched from a Thunderbolt V to a IV to eliminate the rev limiter. I need to explore the difference in diameters (Madpoodle's observation). It seems to me that there was a post recently explaining what the difference in dia would do to rpm. If anyone remembers let me know.

THX
Sam

[Sam]

Madpoodle here are the numbers I posted last week, what do you think.

21" Mirage
37.5mph @3000
45@mph @3400
50mph@ @4000
54.5@ @4400
61.5 @4900

23" Mirage
38mph @3000
48mph @3400
53mph @3800
56mph @4000
62mph @4800

Sam

[Sam]

Scott, I wouldn't know what to check for, the box was sealed when I got it. It looks the same as the 21", both are finished the same with no differences in the edges. I am going to call York Propeller today in Califorina and pick his brain. I was thinking about having one of the props labed but I think it's time I talked to an expert. Let me know if you come up with anthing else and thanks for your input.

Sam

[Rootsy]

Wow that's weird... 54.5 @ 4400 with the 21 +... i have a straight Mirage 21P (don;t know the diameter) and i gps'd @ 60 flat @ 4550 rpm last summer and that was kinda on the ragged edge... generally i GPS'd anywhere from 58.5 - 59.5 @ 44 - 4500 rpm with the prop... it is a repaired prop though so as far as i know it may have been repitched, has a helluva cup in it too.. no dings or dents or missing parts though... but the surface finish isn't the greatest...

Due to a late friday night at work i kinda missed my sunday firing schedule (and believe me i royally thrashed sat and sun).. but i'm at 95% completion... just a few odds and ends... we may have to keep an eye on the upcoming weekend weather and if i get her done this week do you want to possibly run somewhere and tweak Sam? I know of a few inland lakes that'd be good testing grounds...

[Donzified]

What If?
your 23 pitch has more stern lift.Stern lift causes your bow down,causing more wetted surface, slowing you down,kind of like a see saw.You compensate by trimming up the drive more. More drive trim = more speed only to a point.Lift is as important as pitch,diameter and r.p.m's I have a 24 pitch prop with a monster cup and it wants to drive my nose into the dirt.
:D Wednesday is "Boatin Day" keep in touch.

[Sam]

Keith, it will chine walk with he best of them at WOT trimed out, no hull sucking here.

Sam

[Rootsy]

What is the difference between the Old Mirage props and the newer Mirage Plus props? aren't mirage props intended to generate more hull lift for use with larger offshore boats?

[Sam]

The guy at York Prop said that the new Mirages have more cup than the older ones.

Sam

[smokediver]

have the 23 mirage plus labbed and enjoy !!!

[Sam]

Looks like that is the plan.

Sam

[Bryan Tuvell 33ZX]

Sam, do you have a local prop shop that will let you test their props? I didn't know you could even do that! At ours you leave a credit card and walk out with the prop you want to try!
Good luck, and keep us posted...
Bryan

[Sam]

Not that I am aware of Bryan, although with more registered boats in Mich than any other State you would think so.
I'm going out one more time this weekend and then I will more than likley send the 23" out to be labed. I have spoken to York and Chad Hill any other sugestions other than Merc.

Sam

[Rootsy]

You may want to try Erie Prop. I don't know how extensive they are but i know they repair props and do a good job and might be able to lab too... they are located in Point Place on the michigan/ohio line. they are close and worth a try i reckon... i don't know if michigan prop would do a merc prop or not???

Does everyone think the 23 mirage + is the best choice for this application being a light boat and all? what other good choices are there which are readily available?

[Bryan Tuvell 33ZX]

Bob (Donziman2) has his done at Merc, just got two back from them in fact, a bit pricey but perfect, on the other hand, Chad H. has an outstanding reputation on OSO and did a bunch for $100 each over the winter. I paid $125 for mine, it is not as thin as a merc job, but when it is put on the measuring machine each blade is identical and I did gain a couple hundred RPM.
Bryan