Prop School – Part 2: Terminology

Continuing from Prop School….Part 1… Here, I will explain basic propeller terminology and fitment.

Diameter: In “prop speak,” diameter is the distance across a circle made by the blade tips as a propeller rotates.  The proper diameter is determined by the power that is delivered to it and the resulting prop RPM.

A 17 1/2 inch diameter Pro Finish 5-blade CNC prop on an M8 sterndrive.

Type of application is also a factor. How much propeller is in the water (partially surfaced vs fully submerged) plays a role in determining diameter: The more of the prop that is surfacing above the water, the larger the diameter needs to be (so what’s left under water can still push). On rare occasions, diameter may be physically limited by drive type or in close, staggered installations where tips can touch.

Within a specific propeller style, diameter is usually larger on slower boats and smaller on faster boats. Similarly, for engines with a lower maximum engine speed (or with more gear reduction), diameter will tend to be larger. Also, diameter typically decreases as propeller blade surface area increases (for the same engine power and RPM): a four bladed prop replacing a three blade of the same pitch will typically be smaller in diameter.

Physical limits. Mercury Racing engines fitted with the Bravo One XR or Bravo Three XR drives are designed for props up to 16 inches in diameter (Shorty Sport Master: 15 1/4 inch diameter). Sterndrive engines with surface piercing NXT1, NXT6 SSM, or M8 drives run cleaver props up to 18 inch diameter. Our consumer OptiMax and Verado outboards accept up to 16 inch diameter.

A propeller with 21-inch pitch moves forward 21-inches in one revolution.

Pitch: The distance a propeller would move in one revolution with no slip. (Fred Kiekhaefer calls this, “Progress in Jello.”) When we list an outboard  four-blade Pro E.T. prop as a 14 1/2 X  32, we are saying it is 14 1/2 inches in diameter with 32 inches of pitch.

Pitch is measured across the face of a propeller blade. Actual pitch can vary from the pitch number stamped on the prop: 1) Minor distortion may have occurred during the casting process; 2) Adjustments or modifications may have been made by prop shops or 3) Undetected damage from a submerged object may have bent a blade, altering the pitch.

Constant pitch vs Progressive pitch.

There are two common types of pitch: constant and progressive. Constant pitch means the blade pitch is the same – from the leading edge to trailing edge. Progressive pitch (commonly used by Mercury), also referred to as blade camber, starts low at the leading edge and progressively increases toward the trailing edge.  The pitch number, “32” in the Pro E.T. example, is the average pitch over the entire blade.

The Pro Finish Pro E.T. outboard prop features progressive pitch.

Pitch is like another set of gears. Since an engine needs to run within its recommended maximum RPM range, proper pitch selection achieves that RPM. The lower the pitch, the higher the engine RPM. Mercury propellers are designed so that a one-inch change in pitch results in a 150 RPM change in engine speed.

“Propped down,” your prop may provide great acceleration for water sports activities, but your top speed and fuel efficiency can suffer. If you run at full throttle with a prop selected for acceleration and not top-end speed,  your engine RPM may be too high, placing an undesirable stress on the engine. If you select too high of a pitch, your engine may lug at low RPM – which can also cause damage.  Acceleration will be slow. It will be reduced further with a full load of fuel and maximum capacity of people on board.

Proper pitch selection allows the engine to operate near the top of it’s recommended RPM range at light load (1/2 fuel tank and two people). Using this pitch selection method, the engine usually operates near the low end of the recommended RPM range when the boat is fully loaded (full fuel tank, boating gear, live wells full, and maximum capacity). Full load engine speed is usually reduced 200 to 300 RPM.

There are many factors in prop design, especially in more extreme applications. Next post: blades!

Another RPM reduction situation can occur (on naturally aspirated engines) with high heat and humidity: this will reduce engine speed 200 to 300 RPM. Smarter, pressure charged engines like our 1350 will auto-regulate power output for heat and humidity.

In my next post, I will discuss blade rake.

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15 thoughts on “Prop School – Part 2: Terminology”

  1. Great to hear details from an expert. You think you know a lot then you read an article like this and gain a whole new understanding, most informative is the specific RPM changes with the pitch change and load variations. Thanks

  2. Thank you Scott, love your Prop School!

    You wrote that “a one-inch change in pitch results in a 150 RPM change in engine speed”, does that apply to all of the “standard” flo-torq II Mercury props for sale or does the prop design have to be after a certain year?

    1. Peter,

      Thanks for the comment. Yes, in most cases, 1”change in pitch equates to an approximate 150 RPM change in engine speed. There are a few exceptions but, for the most part this general rule applies. For example, I had a customer who was discouraged with his boat performance. He had a 225 OptiMax on a center console. He was using a 21″ pitch Mirage Plus propeller. The recommended engine operating RPM is 5000-5750. His engine would barely turn 5000 RPM, especially when loaded with fuel and passengers. Upon my recommendation, he installed a 19″ pitch Mirage Plus. He gained 300 RPM. The boat has much better hole and carries the load much better.

  3. Thank you Scott for taking the time to respond to the “common” man about prop selection.
    With so many choices, it is hard to pick one.

    I used the Mercury Prop Selector to see what would be reccomended for my current project.
    We are repowering a 25′ rescue boat for a local fire dept with twin 200 hp L4 Verados that will replace their 140 hp carburated Evinrudes with 19p props.

    This is a big heavy boat ( 5200 lb hull). I am leaning towards an Enertia due to past experiance with the same motor on a Wellcraft center console. The Enertia was better than a Tempest we tried.

    1. Hi Dan,

      I’m glad you’ve experienced the Enertia and the Tempest. The other three-blade prop to think about is the Mirage Plus. The Mirage, with its large diameter, will probably offer the best control in rough seas but it won’t be as fast as the Enertia or Tempest. If you need stern lift, I would lean toward the Revolution 4 (4 blade). An estimated prop slip, given the gear ratio and the weight of your rig, tells me you need to go with 19″ pitch for the Mirage and Rev 4. You may be able to move up to a 21” pitch with the Enertia, given its smaller diameter.

      Let us know where you end up.

      Scott

  4. I have a 25′ Robalo 2520 center console boat with twin carbed 200hp motors (and 1.87 ratio c-rotating 21p Mirage (not plus) props. With an average load and conditions, I see 52-54mph TOPS, but at only 5,000-5,100rpm wide open…should be a bit higher Cruise at 4,000rpm in calm conditions is around 38-42mph depending on trim. Hole shot is fair; don’t have much to compare it to. I just ordered 19p Revolution 4’s; what should I expect?

    1. Your current performance speeds are impressive with the Mirage props. What you can expect with the Revolution 4 19” pitch props is a shorter time to plane. Mid-range and top-end speeds might drop 2-3 mph because of the lower pitch.

      1. I have had the 19p Rev4’s on for a couple seasons, and they are very impressive. I’m in the 5,000-5,400rpm range with both engines and a lighter load; and with a lot of engine trim, I’m still seeing early-mid 50mph on GPS. What I really noticed was that the boat sticks like glue in tight turns and rougher water, the entire hull lifts while planing…which is of course stronger, and I can run slightly slower planing speed. Slow speed handling around the docks is improved over the Mirages, too. Fuel economy…what’s that?

  5. I have a 21 foot Key West 210 bay boat, Suzuki DF200, 6-inch Atlas jackplate. I have tried three props so far; OEM Suzuki 3x16x25 – WOT 54 MPH at 5800 RPM with blow outs while making turns. OEM Suzuki 4x15x24 WOT 53.5 MPH at 5750 RPM. Better, but not great. Rev 4 23p, WPT 53.6 MPH at 6050 RPM. Much better across the board. All props run with the same fishing load. Could you give me your opinion of the Rev4 with 1-inch increase of pitch or is 6050 RPM ok? Or maybe Bravo I FS?

    1. Gerald,

      Looks like the Rev 4 works well on your Suzuki. It’s hard to know what size prop is best for your rig without knowing your engine RPM operating range. The next pitch up for the Rev 4 is the 25″ pitch. If you were to try it, plan on dropping 300 RPM from the 6050 you achieved with the 23″ pitch Rev 4. I’m not sure if the Bravo I FS will work for you. All my experience is with Mercury Outboards.

      Scott

    1. Gerald,
      Thanks for the great comment. The Bravo I FS is an amazing prop. Your right! It’s like running a whole new boat.
      Thanks again,
      Scott

  6. I have a 21 cc trophy with a 225 optimax 2003 I have a black max 19 inch on it wot is 5700 rpm at 38 knots loaded for fishing 3 adults too ..cruise at 3800 rpm at 25 knots I happy with that ..i had a solas 18 inch wot was 5800 and 4400 rmp was crusiing at 25 knots ..take off better but fuel economy was not good with that ..so I put black max back on ..any suggestions on the mercury range was thinking eniatia 19 or 20 in prop or should I go for a bigger diameter prop like mirage plus or eniatia eco ???

    1. Shane,
      The ECO and Mirage are great props. Your Trophy would benefit from the Enertia; offering the best overall performance. Consider looking at the 18” or 19”; the 18” pitch will offer quicker planing but the 19” could offer 1 mph greater top speed.
      Scott

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