Prop Terminology

Prop Terminology

  • It’s the theoretical distance of forward movement of a propeller in one revolution. For example, a 21-pitch propeller would move forward 21 inches in one revolution.
  • Note: On ACME props, it’s the second number listed after diameter on the hub.
  • Note: This spec affects performance.

Repitch This is where we increase or decrease the pitch by 1 inch. A repitch will change the RPMs by approximately 200.
  • Measurement that is twice the distance from the center of the hub to the tip of the blade.
  • Note: On ACME props, it’s the first number listed after the part number on the hub.
  • Note: This spec determines fit, and changes in diameter affect performance.

  • A small curvature or lip on the trailing edge of the blade. The trailing edge of an ACME prop blade is also the straightest edge.
  • Note: This spec affects performance and RPM


As a shape passes through water at an increasing speed, the pressure that holds the water to the sides and back of the shape is lowered. Depending upon the water temperature, when the pressure reaches a sufficiently low level, boiling (i.e., the formation of water vapor) will begin. The collapsing action, or implosion, of the bubbles releases energy that chips away at the blades, causing a “cavitation burn” or erosion of the metal.

The initial cause of the low pressure may be nicks in the leading edge, too much cup, sharp leading edge corners, improper polishing, or, sometimes, poor blade design. Massive cavitation by itself is rare, and it usually is caused by a propeller that is severely bent or has had its blade tips broken off resulting in a propeller that is far too small in diameter for the engine.

Ventilation occurs when air from the water’s surface or exhaust gases from the exhaust outlet are drawn into the propeller blades. The normal water load is reduced and the propeller over-revs, losing much of its thrust. This action most often occurs in turns, particularly when trying to plane in a sharp turn or with an excessively trimmed-out engine or drive unit. Ventilation can also be caused by aerated water from step bottom hulls.
Lab finishing, refers to the process used to insure the accuracy of the propeller rake angle, pitch, progression, pitch variation and cup. This hand process involves thinning the blades and sharpening the leading edges.

Blueprinting refers to the process used to ensure the accuracy of the propeller rake angle, pitch, progression, pitch variation, and cup. This hand process involves thinning the blades and sharpening the leading edges.

The advantage of blueprinting is that it reduces the horsepower required to turn the propeller and generates more usable horsepower for thrust. On an accurate, well-built propeller the speed gain may be 2-3 miles per hour. On a lesser quality propeller, the increase may be 3-4 miles per hour.

  • Forward: The suction side — facing the bow/front of the boat.
  • Aft: The pressure side — facing the aft/back of the boat.
  • Root: Where the blade attaches to the hub of the propeller.

  • Looking at the back of the boat, viewing the pressure-side of the propeller (or the side with ACME prop specs), the rotation is the direction in which the propeller spins. Right-rotation props rotate clockwise to propel the boat in forward. Left-rotation props rotate counter-clockwise to propel the boat forward.
  • Note: This spec determines fit.
  • Note: ACME right-rotation props have an even part number. ACME left-rotation props have an odd part number.

Bore is the hole through the hub. There are two common styles of bores:

  • Splined — a “teeth” style bore typically used for Mastercraft boats (2003 – present)
  • Tapered — a smooth style bore with keyway
  • Note: This spec determines fit.
  • Note: Tapered Bore size is equal to the overall diameter of the shaft.

  • Hub is the center part of the propeller where the blades attach. Propellers can have varying hub lengths so its important to be aware of the hub length when replacing your current propeller.
  • Note: This spec determines fit.