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. Cupping is most beneficial on blades which are surfacing, either from transom height (X-dimension) or trim angle. The cup retains water on the blade for a longer period of time, enhancing thrust and efficiency. Racers and performance boaters were the first to realize the benefits of cupping. Now, most recreational props come standard with some cup.
The first three blade aluminum props for our MerCruiser powered boat 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 in our application.
Location. Location. Location.
Originally, cupping was done to gain similar benefits as you get from progressive pitch or higher blade rake. In fact, cupping reduces full-throttle engine speed 150-300 RPM below the same pitch prop with no cup. The location of cup on the blade determines the affect it has on performance. When the cupped area intersects pitch lines, pitch increases. Cupping in this area will reduce engine RPM. And, as I experienced with our family runabout, cupping can also prevent prop cavitation or blow out. Blade rake can be increased when the cup intersects the rake lines. Slip is a measurement of propeller efficiency as it turns through the water, the normal range is 10-15%. Most racing and performace boats slip can be as low as 5-7% where as sterndrive powered step bottom boats with high X (drives mounted high) can see slip as high as 22-24% at WOT.
Adjusting cup on cleaver-style propellers is more difficult. First, the material is stainless steel. Second, the trailing edge is very thick and runs straight out on the rake line. Pitch can be altered some by grinding away some of the cup. Rake may also be altered slightly. The rake can be reduced by decreasing the cup near the tip of the blade. Rake can be increased by reducing the cup near the prop hub. Remember that any change in cup affects engine RPM.
The Bravo I propeller family is a good example of how cup changes RPM and the attitude of the boat (see my “Bravo I for Outboards”).
I will discuss blade configurations and factors that effect propeller efficiency in Prop School – Part 5.