80HP evinrude 10x9-1/4P (go bigger)?

[a70eliminator]

My 67 speedifour engine is factory equiped with a puny little 10x9-1/4 prop, I'm reading posts where some 70-80HP engines are turning props in the 15 and up pitch range, are the actual ratings just that different for older motors, HP at the prop vs hp at the flywheel? My 16' MFG does about 30MPH @ 4700 RPM, it's rating is 4500-5000 range so I'm good but just curious why such a small prop.

 

[hwsiii]

Re: 80HP evinrude 10x9-1/4P (go bigger)?

The reason your older motor turns such a small prop is because the Gear Ratio on your motor is 1.15:1 and a new 70 HP Yamaha has a gear Ratio of 2.33:1 and your motor was rated for 5,000 RPM and the Yamaha is rated for 6,000 RPM. Here is a chart showing the same speeds with with the different gear ratios and approximate same prop slip. Look at the SHAFT RPM that is the difference, almost twice as many RPM.

Prop Slip

H

 

[a70eliminator]

Re: 80HP evinrude 10x9-1/4P (go bigger)?

Excellent thank you for clearing that mystery up.

 

[hwsiii]

Re: 80HP evinrude 10x9-1/4P (go bigger)?

I am glad I could help.


H