Editing Propeller (section)

==Theory==
[[File:RMS Olympic's propellers.jpg|thumbnail|left|Propellers of {{RMS|Olympic}}. The outer two are counter-rotating.]]
{{Main|Propeller theory}}
In the nineteenth century, several theories concerning propellers were proposed. The [[momentum theory]] or disk actuator theory – a theory describing a [[mathematical model]] of an ideal propeller – was developed by [[William John Macquorn Rankine|W.J.M. Rankine]] (1865), [[Alfred George Greenhill|A.G. Greenhill]] (1888) and [[R.E. Froude]] (1889). The propeller is modelled as an infinitely thin disc, inducing a constant velocity along the axis of rotation and creating a flow around the propeller.

A screw turning through a solid will have zero "slip"; but as a propeller screw operates in a fluid (either air or water), there will be some losses. The most efficient propellers are large-diameter, slow-turning screws, such as on large ships; the least efficient are small-diameter and fast-turning (such as on an outboard motor). Using [[Sir Isaac Newton|Newton's]] laws of motion, one may usefully think of a propeller's forward thrust as being a reaction proportionate to the mass of fluid sent backward per time and the speed the propeller adds to that mass, and in practice there is more loss associated with producing a fast jet than with creating a heavier, slower jet. (The same applies in aircraft, in which larger-diameter [[turbofan]] engines tend to be more efficient than earlier, smaller-diameter turbofans, and even smaller [[turbojet]]s, which eject less mass at greater speeds.)<ref>How propellers work - https://www.deepblueyachtsupply.com/boat-propeller-theory</ref>

===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

===Cavitation===
{{main|Cavitation}}
[[File:Cavitating-prop.jpg|thumb|left|Cavitating propeller in [[water tunnel (hydrodynamic)|water tunnel]] experiment]]
[[Image:Cavitation Propeller Damage.JPG|right|thumb|[[Cavitation]] damage evident on the propeller of a personal watercraft]]
[[File:Propeller & anti-cavitation plate & Schilling rudder.jpg|thumb|Bronze propeller & anti-cavitation plate, & [[Schilling rudder]] (on a river barge)]]

[[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 vibration and wear, and cause damage to the propeller. It can occur in many ways on a propeller. The two most common types of propeller cavitation are suction side surface cavitation and tip vortex cavitation.

Suction side surface cavitation forms when the propeller is operating at high rotational speeds or under heavy load (high blade [[lift coefficient]]). The pressure on the upstream surface of the blade (the "suction side") can drop below the [[vapor pressure]] of the water, resulting in the formation of a vapor pocket. Under such conditions, the change in pressure between the downstream surface of the blade (the "pressure side") and the suction side is limited, and eventually reduced as the extent of cavitation is increased. When most of the blade surface is covered by cavitation, the pressure difference between the pressure side and suction side of the blade drops considerably, as does the thrust produced by the propeller. This condition is called "thrust breakdown".  Operating the propeller under these conditions wastes energy, generates considerable noise, and as the vapor bubbles collapse it rapidly erodes the screw's surface due to localized [[shock wave]]s against the blade surface. 

Tip vortex cavitation is caused by the extremely low pressures formed at the core of the tip vortex. The tip vortex is caused by fluid wrapping around the tip of the propeller; from the pressure side to the suction side. This [https://www.youtube.com/watch?v=GpklBS3s7iU&feature=PlayList&p=218220F6C5BD650E&playnext_from=PL&index=18 video] demonstrates tip vortex cavitation. Tip vortex cavitation typically occurs before suction side surface cavitation and is less damaging to the blade, since this type of cavitation doesn't collapse on the blade, but some distance downstream.