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Prop School – Part 4: Blade Cup

Propeller Guide Full.indd

Continuing from Prop School…Part 3. Here I will explain everything you need to know about Blade Cup.

Cup is a curl formed or cast into the trailing edge of a propeller blade.  When done correctly, the face of a cupped prop blade is completely concave.

The first three-blade aluminum props for MerCruiser powered boats featured flat blades, with 15-degree rake. The heavy, deep-vee hull ran best with the drive trimmed up (raising the bow, reducing the wetted surface, and increasing hull efficiency). We got our first experience with cupped, 3-blade aluminum props in the mid ’70s. We immediately realized greater top-end speeds. We also noticed the engine didn’t work as hard. The cupped props were more efficient. Our measurement? The paint was still on the blades at the end the season. Cavitation burns, mostly from abusive teenage kids over trimming dad’s boat, would burn away the paint. The cupped prop definitely made  a difference.

Location. Location. Location.