Rpm and top speed loss after changing reeds?

[1224bret]

Before changing the reeds and injectors I was running around 54mph at 5800 now I see around 48-50 at 5200 those speeds are all lightly loaded. Been thinking about going to a 19 just wasn't sure what would have caused the drop...

[1224bret]

Oops just realized I said 23 pitch I'm running a 21 pitch mirage right now not a 23 pitch

[THEFERMANATOR]

Your prop slip is REALLY high for both sets of numbers. 54 at 5800 is 13% prop slip, and 50 at 5200 is 10% prop slip. I would be rechecking your speed and tach as a mirage prop should have slip numbers in 4-8% range MAX, but many of us have seen slip numbers in the 0 to negative prop slip. I know mine ran -4% prop slip when I ran a mirage type prop.

[jvitiel]

How can you have negative prop slip? Sounds impossible unless you are going down hill no?

[THEFERMANATOR]

It sometimes happens. Remember, the pitch is the measurement of the blade and how far it should theoretically move with each rotation. Almost all stainless props are also cupped to help the prop bite, and reach that theoretical pitch number. Sometimes a blade is so efficient it runs near 0 slip on it's own, and when you add in a nice deep cup, it adds an extra inch or 2 of pitch to it basically giving you negative slip numbers. Also keep in mind most props are mass produced, and you can see quite a bit of variation from 1 prop to the next. It's not 7ncommon to run 2 or 3 identical props all from the same manufacturer, and all of them turn different rpm, speed, and sometimes different manners.

All's I know was I ran a michigan wheel apollo large diameter 15x17 3 blade prop, and routinely could hit 51 mph gps, at 5700 rpm's verified on my mac digital meter, which came out to -4% prop slip.

[jvitiel]

Interesting. I never considered the cup. Thanks for the insight!

[Destroyer]

Originally Posted by jvitiel

Interesting. I never considered the cup. Thanks for the insight!

There's a lot of things that determine prop slip. Pitch, Cupping, Rake, Material, Diameter, Number of blades, Leading edge, Trailing edge, Blade tip, Blade root just to name a few.

A good quick way to determine what is your prop slip is to use this calculator from Mercury Racing. While it may not be accurate for a racing enthusiast, it's more than sufficient for the average Joe.

http://www.mercuryracing.com/prop-slip-calculator/