Propeller Change on Sailfish - slip percentage

[onoahimahi]

Hi - I recently changed the props on my 1994 252 Sailfish from 14 3/4” diameter 19” Evinrude Viper to 15 1/2” diameter 17” pitch Evinrude Rebels. The Vipers are designed for "All-around General Purpose" use while the newer bigger Rebels are designed for "Faster Cruising Speeds and Improved Fuel Economy." I made the change because I wanted the reported better low-speed planing performance which I have realized. Prior to the change the boat would fall off plane too easily at slower speeds. I wanted to understand this better so I made a plot of propeller slip percentage for the two props and see that the old props where slipping a whole lot more then the new props - especially between 3000-3500 RPM where I was having planing issues. The larger Rebels just seem to hold better. Other vitals are that my WOT RPM increased from 5650 to 5850 RPM (light) and max speed dropped from about 50 to 48 MPH. The MPG increase was marginal - maybe about 0.1 at cruise.

Is there a similar propeller style choice that has to be made for Yamaha motors for similar applications?

Cheers,
-Scott

 

[DogBone]

To calculate did you go out and record your speed and RPM and then calculate %slip by computing the theoretical speed using the prop pitch and RPM? That is interesting, I think I might go out and record mine now in case I ever re-prop.

 

[ocnslr]

When we repowered from a single 250HP OX66 to the twin F150s, the F150s had the standard "Black SS" Yamaha prop, in 13.75"x17" size. These were correct in that they allowed the engines to reach full rated RPM at WOT (light), but they didn't have the "bite" needed for a heavy boat when punching through a head sea, or running in a sloppy quartering sea. They would just lose their grip at times.

Tried a number of different props (great service and assistance from Max at Bay Propeller in Chesapeake, VA) and settled on PowerTech 15.25"x16" 3-blade. The F150s can still swing these at 5850-5900RPM at WOT, and we have gained much better bite under all conditions. Lost about 1.5kts on top end, but can hold an 18kt plane with the bow trimmed down in a chop.

Brian

 

[onoahimahi]

To calculate did you go out and record your speed and RPM and then calculate %slip by computing the theoretical speed using the prop pitch and RPM? That is interesting, I think I might go out and record mine now in case I ever re-prop.

Yes - I had recorded speed vs RPM for both props and then went back and computed the percent slip. I use some numerical software called Scilab which is free and similar to Matlab which I use at work but is not free. The equations I used for the 17" prop are:

rebel_prop_speed=17/gear_ratio/(12*5280)*60*rebel_rpm ;// Units are: in*ft/12in*mi/5280*60min/hr=mi/hr

rebel_slip=100*(rebel_prop_speed-rebel_speed_mph)./rebel_prop_speed

There is a website that computes prop slip as well:

http://www.mercuryracing.com/propellers ... ulator.php

-Scott

 

[grady23]

Brian --- I had the same "stock" Yamaha props on my 150's and when I had them reconditioned a few years ago, I had the shop strip all the black of them. I t seemed to make them perform better. I don't have any scientific data to back that up --- It just seems that way.