Pitch (propeller)

Propeller pitch is the axial distance a blade section would advance in one revolution if it moved through the water like a screw through a solid nut, with no slip. For a section at radius r set at pitch angle phi to the disk plane, the geometric pitch is P = 2 pi r tan(phi), so a constant-pitch propeller has tan(phi) varying inversely with r. Pitch fixes how much the blade bites per turn: it sets the design advance ratio and, with diameter and rate of revolution, the achievable thrust. Real advance per revolution falls short of P by the slip, V_A = n(P - slip), because the blade works in a moving, viscous flow rather than a solid medium.