true helicoid or one having a warp to provide a better match of angle of attack to the wake velocity over the blades. A warped helicoid is described by specifying the shape of the radial reference line and the pitch angle in terms of radial distance. The traditional propeller drawing includes four parts: a side elevation, which defines the rake, the variation of blade thickness from root to tip, a longitudinal section through the hub, and a projected outline of a blade onto a longitudinal centreline plane. The expanded blade view shows the section shapes at their various radii, with their pitch faces drawn parallel to the base line, and thickness parallel to the axis. The outline indicated by a line connecting the leading and trailing tips of the sections depicts the expanded blade outline. The pitch diagram shows variation of pitch with radius from root to tip. The transverse view shows the transverse projection of a blade and the developed outline of the blade. The blades are the foil section plates that develop thrust when the propeller is rotated The hub is the central part of the propeller, which connects the blades together and fixes the propeller to the shaft. Rake is the angle of the blade to a radius perpendicular to the shaft. Skew is the tangential offset of the line of maximum thickness to a radius The propeller characteristics are commonly expressed as dimensionless ratios: Pitch ratio PR = propeller pitch/propeller diameter, or P/D Disk area A0 = πD2/4 Expanded area ratio = AE/A0, where expanded area AE = Expanded area of all blades outside of the hub. Developed area ratio = AD/A0, where developed area AD = Developed area of all blades outside of the hub Projected area ratio = AP/A0, where projected area AP = Projected area of all blades outside of the hub Mean width ratio = (Area of one blade outside the hub/length of the blade outside the hub)/Diameter Blade width ratio = Maximum width of a blade/Diameter Blade thickness fraction = Thickness of a blade produced to shaft axis/Diameter Cavitation Cavitation is the formation of vapor bubbles in water near a moving propeller blade in regions of very low pressure. It can occur if an attempt is made to transmit too much power through the screw, or if the propeller is operating at a very high speed. Cavitation can waste power, create