what creates prop slip?

[93maxum4.3lx]

I've got another thread going here regarding prop selection, but after having calculated slip several times, i'm wondering what causes prop slip and why mine is relatively high.

mercruiser alpha drive, 18 ft 1993 maxum open bow runabout, 1.81 ratio.

several prop slip calculations:

19p vengeance / 2 people / stingray foil: 18%
19p vengeance / 4 big people / no foil: 14%
20p bayproII / 4 big people / no foil: 16%

all calculations were with prop stated pitch, GPS speed, and dash tach.

the hull is clean; no speedo pickups or transponders hanging off the back of the transom, etc... attached a few pics of boat for reference.

any thoughts beyond just replacing the tach and hoping it's off?

 

[junior1113]

Re: what creates prop slip?

has 4 blade prop crossed your mind they are great andd always lower slip.

 

[Frank Acampora]

Re: what creates prop slip?

Nothing creates prop slip (except for cavitation/ventilation, but let us assume that is not happening). It is a function of the design and the use to which you are putting it.

Remember: If your boat is tied up at the dock and you give it full throttle, by the definition of actual boat speed versus theoretical speed, you have 100% prop slip. (not really, you would need to calculate the speed of the water stream driven backward by the prop--but the idea is there.)

 

[hwsiii]

Re: what creates prop slip?

Maxum, one way to look at prop slip for a particular prop on a particular boat is to consider it as a kind of efficiency rating of that prop with your boat and the normal uses you bought that prop for. If you have a 10% prop Slip then the prop is NOT losing 90% of your forward speed from the prop losing traction in the water and if it is less than 10% then you are at maximum traction.


H

 

[wca_tim]

Re: what creates prop slip?

Try here... and scroll down to the section on slip.

http://sites.mercurymarine.com/port...26_49291:126_49299&_dad=portal&_schema=PORTAL

It's the mercury propellers page on terminology,etc... slip sounds bad but it isn't. It is required to turn rotational torque into forward thrust. if you're slip is below about 10% on most hulls, then you're probably wasting power turning a prop with too much diameter and/or blade area.