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.
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.
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.
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.
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.
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.