Friend of mine has a 29 ft rib with twin 250 verado's

Spinning (brand new) 23 P Rev 4 props @ 6010 rpm and seeing 70 mph on 2 different GPS

After prop slip calculation theoretical topspeed = 71 mph meaning this rig only has 1 % propslip which is unrealistic

Merc's website stated for Verado 250 only 1 gear ratio 1.85

Where is the error or are these props so perfect ???

GPS's can be off, Tach's are always slightly off.

Example........
True speed...69 mph
True rpm........6250
True slip.........6.22%

Ribs can be real efficiant.

Check that the GPS is not in KPH.

That will only make things worse... 70kph~80mph

No, I was thinking in kilometers but after thinking about it that would yield unrealistic high slippage numbers. My GPS can be set to Knots kts , MPH, or Kilometers per hour kph.

Yes, speed was GPS measured (by two different GPS 1 handheld / 1 fixe in the boat) in kph but converted to mph when putting info on this board.

Changed props last week => almost same story

Rev4 21P turning at rev limiter @ 6400 rpm GPS speed is 106 kph = 66,25 mph
Calculating slip = < 4%

Mirage 3 BL 23P hitting rev limiter 6400 RPM
Top speed was 113 kph = 71 mph = < 5% prop slip

Again very unrealistic low prop slip numbers IMO

Now lets try some Bravo 1 4 blades (or even better Lab finished ones)

Prop is really higher pitch, if it behaves as a 22" then you are up to 9% slip from 4%, still low but better. Gear ratio could be different of course.

With these numbers, Labs will make you fly!

Ah, sorry - my mistake