Prop School – Part 5: Blade Efficiency

For this, the fifth installment of my Prop School series I will review the various propeller blade designs and how they – along with rotation – affect propeller efficiency and overall boat performance.

Rotation. Propellers come in both right and left-hand rotation. Standard rotation for both outboards and sterndrives is right-hand: the prop spins clockwise when in forward gear.  Left-hand props spin counter clockwise.  Left-hand props are typically used with multi-engine applications. The counter-rotation prop works to balance (or reduce) the torque effects from the right-hand prop. Most twin engine applications are setup with the props “turning in”; the port engine spinning right-hand and the starboard engine spinning counter clockwise.

Hull types and designs respond differently to prop rotations. Some need additional stern lift to reach maximum efficiency and performance. To obtain this, the rotation of both propellers is set up, so they rotate away from each other. We call this turning the props out.  The left-hand rotation prop is on the port side and the right-hand rotation is on the starboard side.

For example, a high-speed catamaran loaded with gear and passengers often runs best with 5-blade cleaver props with 15-degree rake. Turning the props in pulls the stern down, enabling the boat to float over chop. With lighter loads and ideal conditions, the same cat can gain 6 to 8 mph when using 18-degree rake, 5 blade cleavers “turned-out.”

Performance combo pack. Racing’s 4-blade Bravo I FS and Pro Max propellers.

Number of Blades

In theory, two blade props are most efficient since they have the least amount of surface dragging through the water.  Two blade props are commonly used on lower horsepower outboards and trolling motors. Three -blade and four-blade props are the most common designs used today. The added blades reduce vibration while maintaining most of the efficiency of a two-blade design at a convenient size and reasonable cost.

The Pro Finish 5-Blade CNC Sterndrive Cleaver propeller.

Racers and performance boaters raise sterndrive mounting heights (x-dimensions) on ventilated, stepped hulls. The steps create air bubbles, raising the hull off the water on a drag-reducing cushion. This, combined with reduced drag from the higher drive heights, improves hull efficiency. This trend has spawned an evolution of prop designs featuring four, five and even six blades.  The additional blade surface helps offset slip induced by air bubbles flowing from the ventilation steps toward the props.

Cross section showing the thickness of a common prop blade.

Blade Thickness

For efficiency, blades should be as thin as possible to reliably handle a particular power range. A cross section of a typical constant pitch prop blade reveals a flat section on the positive (pressure) side and an arc surface on the negative (suction side) of the blade. Edges are usually 0.06″ to 0.08″ (1.5 mm to 2.0 mm) thick for aluminum props, thinner for stainless steel.

The cross section of a cleaver style prop blade is shaped like a wedge.

The blade cross section on surfacing props such as our T.E. Cleaver  and Pro Finish CNC Cleavers is wedge shaped. The thick trailing edge adds strength. Surface air ventilates a low-pressure cavitation pockets behind the trailing edge, enhancing efficiency.  The contour or shape of most propeller blade tips (other than cleaver) are round.

I will discuss propeller slip more thoroughly in Prop School – Part 6.

 

 

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18 thoughts on “Prop School – Part 5: Blade Efficiency”

  1. Scott,
    I’m curious, when it comes to upper level competition teams, when do they replace props? What is the process they use to check for weakness or fatigue? I know we lost a blade on a “Brand X” OTH cleaver and we were told that it was because it had welded blades and they fail sooner than cast blades. Do you agree with this? Are the Mercury Race Props investment cast like the production props?

    1. Brent,
      Most professional racing teams replace props fairly often. Some run new for every race. Others may run a season. It usually depends on race conditions and blade thickness. However, every time out, racing teams check their propellers for nicks on leading and trailing edges. A nick is the leading cause of a blade tearing off.

      All stainless steel Mercury and Mercury Racing propellers are investment cast, welded blades are just not part of our vocabulary.

  2. Scott,
    Had a question regarding prop direction. I know the Race Edition Vessel View will give real measured slip, I just don’t have a boat that has it so here goes. How do you see the slip % change when you go from turning in to turning out. Is that why the lift changes with a directional change? Does the logic hold true for V bottoms and Cats?

    1. Slip will increase or decrease depending on prop rotation and boat design. Traditional and stepped vee bottoms typically run best with lower rake props turned in (port prop spinning clockwise; starboard prop spinning counterclockwise). Turning the props in sucks the stern down into the water — enhancing bow lift on traditional vees and improving the ride and efficiency of stepped bottom hulls. There are some stepped bottom vees that need more bow lift than the 15 degree rake props can offer. Moving to 18 or 21 degree rake will help generate the necessary lift. Turning the props out (or away from each other) increases stern lift, which can increase prop slip. Many catamaran applications run best with standard 15-degree rake cleavers turned in. This setup enables the operator to “float” the boat over wakes using positive trim. Similar to the vee bottom example, turning 15-degree props out on a catamaran typically raises the stern. Again, prop slip increases resulting in a loss of speed and efficiency. Switching to 18-degree rake props, turned in, raises the bow. The bow lift enables the operator to run the boat with neutral or slight negative trim. Neutral trim results with less drag and reduced prop slip for increased top speed and enhanced overall boat performance. Slip can drop 4 to 6% depending on the application. Keep in mind, not all boats are the same. What may work for one application might not work for another. You never know unless you try.

  3. im looking for a top end prop for a 1979 checkmate exciter any recomendations?looking for the high end of 80 if not 90 thanks

    1. What a fun boat! At 18’ long, the Exciter was rated for a 150 horsepower outboard.
      You’ll need heck of a lot of horsepower to hit 80 or 90 mph. You’ll also need to consider: can the boat handle 80 or 90 mph?
      If you’re running a 150 HP, and if you can swing a 3 blade 25” or 26” pitch Tempest Plus, you should be able to hit in the low 60’s.
      I had the 21′ with 235 HP and was happy to see 72 mph.
      Have fun!

  4. My husband recently had to replace the propeller on his boat, and we also found that hull types and designs respond differently to prop rotations. We found that ours needed an additional lift to be able to reach its maximum capacity. We are so glad that we replaced our propellers and now our boat is running great!

          1. When the speed is 60 mph, the turns are 4800 rpm I want to send Christmas Greetings also to Fred Kiekhafer I met in Sarasota at breakfast 1989 as a guest of Wellcraft Powerboat Exhibition and test drive

          2. Hi Karl. Fred left Mercury Racing at the end of 2012. We will extend your greeting the next time we see him which will most likely be the Miami Boat Show, February 15-19.

  5. I have a 1987 berretta super sport what prop will make this boat the fastest it can go as I use this for tubing and fishing

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