Prop School – Part 4: Blade Cup

Illustration of cup added to the trailing edge of a typical thru-hub exhaust prop.
Blade cup location on our Lab Finished Maximus prop.

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.

An illustration showing cup along the pitch line.
An illustration showing cup along the rake line.

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.

Cleaver Cup.

Cup lication on a cleaver-style propeller.
All Mercury Racing propellers spend time on the grinding wheel.

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.

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11 thoughts on “Prop School – Part 4: Blade Cup”

  1. Thanks for posting. I still have the old “Everything you need to know about propellers” summary had online long ago. That document teached me the basics about the magic props. Still learning every day but reading that piece triggered my attention to prop technology.

  2. The issues I’m having is when I’m at full trim 5250 rpm I will loose about 3 mph so I back off the trim to regain my speed ! I’m running a 24 pitch lazor2 on a 17 ‘ r 78ranger sport. With a 125hp mercury. Will cupping help me or hurt me. ? Or should I try a different prop. ? This is the prop that ranger boats recommend !!! What do u think ?

    1. Eric,
      If your 125 is the 4 cylinder, it has an rpm range of 4750 to 5250 at WOT. And if you’re hitting 50 mph (or close) you may have the best set up already. It is possible to over trim; you’re in the driver’s seat. Adding cup may help the propeller stay hooked with the engine trimmed out further. However, the added cup could have a negative effect by lowering engine speed 150 – 200 rpm at WOT. There is also a good chance the speed will drop 1.5 to 2 mph.

    2. Sounds more like you’re over propped. You could take some cup out to gain higher engine rpm or drop down to a slower prop. Being able to hit top rpm at wide open throttle is a must.

    1. Jean,
      Most prop shops increase or decrease rpm my modifying the cup height, there are a few shops that have a pitch block and will bend the prop to the pitch block.

  3. I’m wondering if cupping will help me I’m running a Javelin Renegade 20 (20-foot foot bass boat) with a Johnson 225, 6 inch Jack Plate, 25 pitch 4 blade Renegade prop,5800WOT. Problem is I’m not getting enough bow lift trimmed all the way up I still feel like I’m plowing a lot of water and I really don’t gain any speed from 5500 to 5800RPM?

  4. I have a 1996 24 feet 5100lbs boat with a 5.7 mercruiser. Someone stole my prop. I’m having trouble finding one that works well. Do you have a recommendation. I’ve been told everything from a 14×17 to a 16×16. Boat is a deep v Hull.

    1. David,
      To close to call, but it is going to be between the 15-inch x 17-inch pitch or the 16-inch x 16-inch. With the information you’ve provided, I’m leaning toward the 17-inch pitch.

  5. Have an inboard with a 5.7l Vortec Indmar with a Holley. Changed my prop from a cast prop with .04 cup to a CNC prop with .08. All other dimensions the same. The changes were fairly dramatic. RPMs dropped by 400 at WOT. Since we us it for skiing, the acceleration out of the hole was smooth and constant with no cavitation. Cavitation in the turns was almost eliminated. Only issue was lost about 4 mph off the top end. To be honest the acceleration out of the hole is so good now noone really cares about the top end. That’s the way skiers think. For reference this is a 1998 Malibu response. New prop was an Acme 449

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