Originally Posted by Valiant Knight
Thanks for the info.
Just one comment.
The recommended RPM range for the DF 90 is 4.5 k - 5.5. k.
Using a 17", 3 blade propeller the RPM at WOT was around 6.2 k, above the recommended range.
What is the expected result in moving to a 17", 4 blade?
I understood from Google the max rpm was 6300 on that motor and expect an electronic rev limiter will prevent over revving.
If it can pull max rpm with a 19" then stick with that, a four blade will give you more stern lift and better acceleration because of less prop slip which varies throughout the rev range. At high speed the water behaves more as a solid and slip is reduced, so it's at low speed and mid range where most benefits are felt. Your prop slip could be as high as 25% at lower rpm's so reducing slip could improve fuel economy and give you the better holeshot that a 17" prop would give you.
You would do well to note the precise speed and rpm every 500 or 1000rpm throughout the rev range and this will help enormously to understand how your current prop is behaving. A prop has a power curve like the engine has a power curve, ideally both are matched.
After talking to Steele Developments and looking carefully at Solas four blade props it seems they adjust the diameter and/or prop design to compensate for the extra blade area over a three blade prop. Therefore the usual advice to go down a prop size when fitting a four blade is incorrect regarding Solas. Stick with the 19" pitch. Welcome comments from others to confirm this with real life experiences.