Fastwin Propeller Slip

R

rjezuit

Chief Petty Officer
Joined
Sep 24, 2007
Messages
418
I have an Evinrude Fastwin that is running well below the recommended WOT RPM's. It has a 11" pitch prop and I am looking at/for a prop with a 9" pitch. Am I correct in thinking that the lower pitch prop will have less slip and therefore be more efficient. I am doing speed calculations and am comparing theoretical speeds with various props and wondered if I needed different slip correction factors. Rick
 
M

MushCreek

Petty Officer 2nd Class
Joined
Jun 20, 2009
Messages
150
Re: Fastwin Propeller Slip

If your RPM is low, the prop isn't slipping, the motor is lugging, which is not good for two cycles. You're correct in assuming that a lower pitch will increase your RPM, all else being equal. Make sure you try your motor out at different tilt pin heights before changing props- it can make a big difference.
 
R

rjezuit

Chief Petty Officer
Joined
Sep 24, 2007
Messages
418
Re: Fastwin Propeller Slip

Thanks, I have already changed all of the variables except for the propeller. All propellers "slip" which relates the actual efficiency to the theoretical efficiency.If you have an 11" pitch propeller and it does not "slip' at all, you have 100% efficiency and you move forward 11" per revolution. In actualiity it is much less. I wanted something to equate the slip or efficency of given propellers at differing pitches. I would think a lower pitch would slip less (be more efficient) but I am not sure. I put together a spreadsheet that shows rpm vs speed, but I need a correction factor for slip. Thanks, Rick
 
W

wilde1j

Vice Admiral
Joined
Apr 15, 2002
Messages
5,964
Re: Fastwin Propeller Slip

Slip = 1 - actual boat speed (mph) X 1056
````````____________________________
```````````Prop RPM X Prop Pitch (in.)
 
R

rjezuit

Chief Petty Officer
Joined
Sep 24, 2007
Messages
418
Re: Fastwin Propeller Slip

Thanks, That is what I was looking for. Off to the spreadsheet. Rick
 
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