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Propeller pitch
The pitch of the propeller can be compared to screw pitch: When making a turn of the screw in a nut, the screw advances a distance called the pitch of the screw.

pitch screw propeller

The propeller behaves almost the same way by screwing into the fluid, and First marine propellers, looked like screws:(see propeller history)

pitch marine propeller

A difference must be made ​​between the geometric pitch of the propeller, and the effective pitch of the propeller: The geometric pitch is the distance that traverses the propeller one turn if it was screwed into a nut::

Le pas de l'hélice  et le perimetre au rayon r donne l'angle apparent perçu par un profil de pale au rayon r

In a fluid the propeller does not screw like in a nut,The propeller can rotate without the boat or plane advance. The actual distance traveled by the propeller in one revolution the effective pitch) is the distance traveled by the aircraft or vessel.The effective pitch is related to the operating point. The effective pitch:

ex:

pas geometrique et pas effectif de l helice

The geometric pitch does not correspond at actual movement of the propeller, the difference between the geometric pitch of the propeller and the effective pitch of the propeller is the slip of the propeller.
Geometric propeller pitch is not constant along the blade. The blade is twisted to adjust the pitch angle of the profiles at the operating point. When we speak of geometric pitch of a propeller is actually talking about the profile pitch to a given point.

the geometric pitch varies according to the distance from the axis: the twisting of the blade

it may be useful to modify the geometric pitch of the propeller to fit the actual pitch imposed by the operating point. To make this adjustment geometric pitch, the systems of pitch variation can equip the propeller.