Propeller technology

[adapted from Torqeedo website]

Top propellers require top motors

Basically, propellers which slowly turn in the water and have a high pitch and a large diameter, have the highest degree of effectiveness. A large propeller diameter results in a high propellant flow, while a high propeller pitch increases the speed generated by the propeller. Multiplied by each other, the propellant flow and the additional speed generated result in the propulsive power of the propeller. On the other hand, a faster rotation of the propeller results in an increasing loss of efficiency.

Lattice structure for calculation of the propeller characteristics of the Variable-Pitch-Variable-Camber (VPVC) Propeller from Torqeedo

Conventional outboards in the low-power range fail at using highly-efficient propellers: Either they do not have enough torque to move high pitched propellers or they do not have enough elasticity (availability of torque over a large speed range). Combustion engines are particularly susceptible to a lack of elasticity. This is because they only have an extremely low torque at small speeds. Propellers that would normally have a good rate of efficiency within the efficient range of the motor stall the motor when within low speed ranges. The rates of efficiency for propellers that can be used for low-power class combustion engines are therefore limited to 20-30%.

To ensure that the Torqeedo motor can fully exploit its strengths in the maximum torque and in elasticity, and then convert these into superior efficiency, the Torqeedo propeller has been carefully adapted to the torque characteristic of the motor.

Low eddy, high thrust:
The Torqeedo propeller from large shipbuilding

The majority of propellers used in recreational activities are based on series tests that were carried out in the 40’s to 60’s of the 20th century in the Wageningen test facility in The Netherlands as well as by the US Navy. The results of these tests have been embodied in propeller design principles and are used as rule’s of thumb.

Section of the calculated propeller jet (red for high speeds, blue for low speeds)

On the other hand, the most modern large ships have been equipped for some years now with propellers that are the result of multi-dimensional optimization calculations. In contrast to standard propellers, the pitch and camber of the propeller are not kept constant across all segments of the propeller.  Instead, the pitch and camber are optimized based on the vortex lattice method for each single segment of the propeller in a stepwise optimization over many thousand iterations. The  additional scope for design resulting from this allows   additional speed to be generated by the propeller at the highest rate of efficiency. A propeller designed using these methods is known as a Variable-Pitch-Variable-Camber (VPVC) Propeller.

Background knowledge on propeller geometries

In addition to the important parameters such as the diameter of the propeller and the number of blades, propellers can also be described by the radial course of the following parameters: Pitch, chordlength, skew, rake as well as the profile parameters of thickness and camber.

“Pitch” describes the distance covered by a propeller during each complete turn without any slip. Since this idealized size cannot be established on a moving boat (in practice, slip always occurs), the slip of a propeller is determined by measuring the tilt angle of its blades.  For propellers in which the pitch varies along the blade (Variable-Pitch Propeller), the pitch is measured on a circle that is drawn around the middle of the propeller at 70% of the propeller diameter.

Loss of efficiency due to cavitation

Cavitation is the phenomena caused by the formation and closing of vapour cavities within fluids. Cavitation is caused in particular by fast moving objects within the water such as propellers. Due to the fast movement, areas of low pressure result in which the water starts to vaporise at normal temperatures. The energy used for this is not converted into propulsive power and is lost as inefficiency. Depending on the quality of the drive system and its propeller, cavitation of various severities may occur. The two pictures taken with a high-speed camera at a shutter speed of 1/8,000 second show the difference between the Torqeedo VPVC-Propeller and a standard propeller at comparable operating points:

click to watch video

The standard propeller shows signs of fluctuating cavitation on the “suction” side of the wing tip.

On the other hand, the Torqeedo VPVC-Propeller only shows signs of light tip cavitation.

In the video the standard propeller is enlarged. The Torqeedo propeller is larger and turns more slowly. Both propeller have the same propulsive power.

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