How much slip is normal?

[krisnowicki]

How much prop slip is normal? I ask because I have garmin 320c with a spining wheel style speedo on it and with 21p at 4000 on a 1.86 gear drive it says i am doing 25 when i should be doing 38-39. IF I were doing 25 IT WOULD MEAN I HAVE 41.5 % SLIP. so I am wondering whats what ? This is with about 600 lbs of people in my 21 foot four winns and a yamaha 200hp the prop is also new as is the hub.

 

[Silvertip]

Re: How much slip is normal?

You determine slip at wide open throttle -- not at some arbitrary rpm less than that. Slip is not a fixed characteristic, just like speed is not linear. In other words speed and rpm do not track 100% across the rpm range so that means slip does not stay constant. You can watch the tach and speedometer or GPS to understand that. Slip is the difference between theoretical and actual speed and rpm. Since a prop operates in a liquid, it can never have 0% slip.

 

[steelespike]

Re: How much slip is normal?

Only way to get correct speed is with a GPS.Otherwise you could be off as much as 5 mph either way.

 

[stevens]

Re: How much slip is normal?

Slip will vary a great deal across the RPM-range, as evidenced by the graph below for a boat like my own. When comparing between props, one usually refers to the slip at the maximum RPM.