How do propellers work?


Let’s define first a Propeller which is a propulsion device that converts most of the power from the engine into a thrust force to propel a vessel. The screw propeller is the most common form of marine propulsion device.

The motion of the screw is a combination of a rotation with a translation along the axis of rotation. Consists of a number of identical twisted blades (usually 3, 4 or 5) equally spaced around a hub mounted on a driving shaft. 

From the desktop of

Leonardo Montoya


The twist (pitch) and inclination of a blade are defined by the angles between a datum plane normal to the axis of rotation and a number of datum lines fixed relative to the blade. A set of pitch datum lines are taken at a series of constant radii from the axis of rotation, and the angle between the datum plane and a pitch datum line is named the pitch angle.

 

 
The operation of a screw can be illustrated by considering an annular element of one of the blades. The figure below shows the forces acting on an annular blade element of width dr  at a radius r from the axis of rotation. A torque Q is applied to the screw by driving the shaft, and the propeller and shaft rotate at a rate of rotation . Owing to the reaction of the fluid in which it operates, the blade element experiences a resultant force d R which has two components, a tangential torque force dQ/rB, where B is the number of blades, acting opposite to the direction of rotation, and a thrust force dT/B acting ahead, parallel to the axis of rotation.

 

 

 

 

The sum of the moments of the tangential torque forces on all the blade elements is balanced by the torque Q applied to the propeller, and the sum of the thrust forces on all the blade elements is equal to the thrust force T which propels the screw in a forward direction at a speed of advance.

 
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