Prop School – Part 3: Blade Rake

Rake is the relationship of the blade face and the propeller hub.

Rake is the angle of a propeller blade face relative to its hub. If the blade face is perpendicular to the hub, the prop has zero-degree rake. As a blade face slants back toward the rear of the prop, blade rake increases. Rake is either flat (straight) or curved (progressive). Most lower horsepower (“lower” by Mercury Racing’s reckoning) outboard propellers, like Black Max aluminum and Vengeance, have 15-degree rake and are designed to operate fully submerged to push a boat across the water. Typically, higher horsepower outboard and sterndrive propellers have a higher flat or progressive rake.

A good view of our CNC cleaver prop on a NXT1 drive.

A greater rake angle generally improves the ability of the propeller to operate in a ventilating situation. Ventilation occurs when blades break and re-enter the water’s surface — such as occurs with 1) a Bravo sterndrive installed with a high “X” dimension, 2) a surfacing drive (NXT1NXT6 SSM or M8) or 3) an outboard installed or jacked high on a transom. In surfacing operation, higher rake can hold the water better as it’s being thrown into the air — deflecting it aft and creating more thrust.

This lightweight Allison outboard drag boat has natural bow lift.

On lighter, faster boats with a high prop shaft, increased rake often will improve performance by holding the bow higher. This results in higher speeds due to less hydrodynamic hull drag. However, on some very light boats, more rake can cause too much bow lift. That will often make a boat less stable. Then, a lower rake propeller (or a cleaver style for outboard) is a better choice.

Looking at examples:

  • A runabout with Alpha sterndrive usually performs best with a lower rake Black Max or Vengeance pushing the boat. The aim is broad capability and utility for many recreational activities.
  • A lighter weight runabout with Alpha drive may increase performance with higher rake Laser II or Enertia propellers lifting the bow offering less wet running surface (lower drag).
  • Bass boats can vary widely because of the design differences among hulls in the market. Mercury offers several propellers with high rake for these applications Laser II, Trophy Plus, Tempest Plus and Fury. Mercury Racing offers specialty props for the bass market with the Lightning E.T. , Pro E.T. and Bravo I XS.
  • The Bravo X or XR drive, used with higher horsepower multi-length and weight applications, typically use props with high rake and large blade area — such as the Mirage Plus, Revolution 4, Bravo I and Maximus.
Surfacing #6 drives on a Cigarette 46 Rider.
Our Pro Finish 5-blade CNC Cleaver prop is available with 15, 18, or 21-degree rake.

Performance applications using Mercury Racing’s CNC Pro Finished Cleavers with NXT1, Six SSM or M8 drives have three rake choices: 15, 18 or 21 degree. Most “V” and step “V” bottom boats utilize a 15 degree rake — unless the center of gravity is forward of the helm; then, 18 degree rake works best. The higher rake helps lift the bow — positioning the boat to ride appropriately on the steps. Air entrapment hulls (catamarans and tunnel hulls) pack air and lift during forward motion; they typically use props with 15 to 18 degree rake — since air pressure does most of the lifting.

Your head probably hurts by now, so I will discuss blade cup in Prop School – Part 4.

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10 thoughts on “Prop School – Part 3: Blade Rake”

  1. WE love these informational propeller posts. Its good for us and our customers to refer to when trying to fit a prop to a Motor and Hull for optimal efficiency and power. Being that were in the San Francisco Bay Area, there are many fisherman that we must teach these very same principles to. Even the High performance guys need help… Thank you for the post

    1. Thanks, Jason. Pleased to help. We held the same belief: more knowledge is good.
      Even high-perf guys,” you say? Especially high-perf guys! 😉

  2. Great Stuff!!
    Ok a couple of questions, I read in an old prop book that props are generally more efficient when they turn slower. Can you shed some wisdom on this? Do you focus on this a lot when finding your self choosing gear ratios for High Performance products? Is there an ideal shaft speed you shoot for?
    Are you going to cover multiple blades in a future post? The CNC race props seem to defy the laws of drag because you continue to add blades and the boats just go faster!!!
    Thanks and keep it up!

    1. Good questions. The old wisdom was based on one element of efficiency – submerged blade hydrodynamic drag, where slower = less. (That still works for ships.) On higher speed craft, there are many more variables. We don’t focus on prop shaft speed; that’s an output of the design, not an input.

      We start with the application: Based on experience, how fast will a boat (of a particular design and range of operating weights) go with the applied power? What style of prop design is proven to work in that application? What pitch works well at that speed and still provides enough thrust at low speed to get on plane? What is the peak engine RPM range? So, expected prop pitch (on the output end) and engine RPM (on the input end) determines the desired overall reduction. The range of applications and boat weights (speeds) then points to the combination of prop pitches and gear ratios that are needed. Obviously, there are limits at both ends: Zero pitch and there is no thrust; infinite pitch and the blade slaps the water like a paddle wheel (only sideways). There’s science behind the designs that gets us very close; however, final selections remain empirical (There are also drive and gear design considerations related to size, drag, torque capacity and life, but that goes well beyond your question.)

      As for the higher blade count on our CNC Pro Finish propellers, think about the surface piercing world in which they operate. More blades make them smoother and reduces the impact on blade entry (because parts of other blades are still under water — pushing and stabilizing). Drag is a balance: Higher prop shaft elevation reduces the drag as less blade area is in the water; That compensates for more blades which would increase drag. Generally, more is better, but there is a practical manufacturing limit: Even with a 5-axis milling machine, there is a point where the tools cannot reach in to machine all the surfaces and neither technicians or media finishing can obtain the final surface quality.

  3. Wow definitely a ton of things you guys get to think about while staring at your clean sheet of paper, or is it empty CAD file?
    OK so I am just making sure I am thinking of this right. Would you say rake has more to do with lift and pitch has more to do with thrust. I know they both have to influence each other but can you make that broad of a statement?
    Looking forward to the blade design post. I am very interested in the Cleaver vs Chopper, Over the hub vs Thru hub, etc etc.

    1. You’re spot on with the rake and pitch. For me it’s staring at running boats along with getting out to events, listening and talking to customers. This weekend it’s the FLW Cup in Hot Springs, AR.

      1. One thing I forgot to ask is what influences a prop to create bow vs. stern lift. We talk a lot about Tempest vs Trophy vs Hi Five and Fury and everyone has an opinion. What is your opinion based on a typical bass boat? How do the different flavors of Merc props have different types of lift?

        1. Opinions, everybody has them and for good reason because there are so many different boats, motors, set backs, engine heights and weight of equipment combinations.
          It’s awesome that Mercury offers so many different propellers and as a general rule of thumb, most 3 blades (not Cleavers) will offer more bow lift than a 4 or 5 blade. In simple terms the 4 blade props typically offers more stern lift, some say picking the whole boat up.
          Fury doesn’t require allot of trim (compared to Tempest Plus) which can decrease gearcase drag improving top speed.
          The Tempest Plus is a fast prop and offers more trimming compared to the Fury. If there’s allot of fishing gear in the front compartments the boat may plow through the water. The Tempest will offer a tremendous amount of bow lift offering good top numbers.
          The Trophy is used in many high speed applications where boat control is key (80+mph). The Trophy offers less steering torque compared to 3 blade because there’s always some metal in the water.
          The HighFive offers awesome hole shot but expect 3-5 mph loss on the top compared to 3 blade.

  4. I am over in the UK and have a Phantom 25 with twin 200xs ss motors counter rotating( outwards) with 1.87 sportmasters on 5″ setback jackplates.
    At the moment i have Et lightnings 14.25 x30 pitch. The boat really takes loads of throttle to get to plane with engines fully down and trimmed in.Also with about 50kg plus of water up front in the bow tank(literally impossible without weight up front) .With engines fully down the bullett is level with the bottom of the hull. Someone has suggested Bravo ones would be better but i did not think they were surface piercing props.
    Any suggestions would be great.

    1. Nick,
      The Bravo I prop lines can be used for surfacing. Consider the Bravo I OC. The barrel design channels the exhaust through the blades for enhanced venting and improved hole shot. The props are Lab Finished to 14.75” diameter for a quick spool-up and offer added cup at the maximum diameter. A total of eight adjustable PVS vent holes regulate the exhaust venting during planing and the shortened barrel minimizes drag at wide open throttle.
      Scott

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