Editing Propeller (section)

===Propeller geometry===
The geometry of a marine screw propeller is based on a [[helicoid]]al surface. This may form the face of the blade, or the faces of the blades may be described by offsets from this surface. The back of the blade is described by offsets from the helicoid surface in the same way that an [[aerofoil]] may be described by offsets from the chord line. The pitch surface may be a 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.<ref name="PNA ChVII" />

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. This is called the ''boss'' in the UK.
''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:<ref name="PNA ChVII" />
* Pitch ratio ''PR'' = propeller pitch/propeller diameter, or P/D
* Disk area A<sub>0</sub> = πD<sup>2</sup>/4
* Expanded area ratio = A<sub>E</sub>/A<sub>0</sub>,  where expanded area A<sub>E</sub> = Expanded area of all blades outside of the hub.
* Developed area ratio = A<sub>D</sub>/A<sub>0</sub>,  where developed area A<sub>D</sub> = Developed area of all blades outside of the hub
* Projected area ratio = A<sub>P</sub>/A<sub>0</sub>,  where projected area A<sub>P</sub> = 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