Propeller technology and hydrodynamics
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 has a positive effect on the additional speed induced by the propeller. Multiplied by each other, the propellant flow and the induced additional speed result in the propulsive power of the propeller. On the other hand, an increasing circulation speed of the propeller results in an increasing loss of efficiency.
Conventional outboards in the low-power range fail at using highly-efficient propellers: Either they do not have enough torque to move large sloped 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 covert these into superior efficiency, the Torqeedo propeller has been carefully adapted to the torque characteristic of the motor.
  Lattice structure for calculation of the propeller characteristics of the Variable-Pitch-Variable-Camber (VPVC) Propeller from Torqeedo |
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 concretized in general construction principles and are used by rule of thumb.
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 (almost) constant across all segments of the propeller. Instead, the pitch and camber are optimized based on a vortex grid calculation for each single segment of the propeller in a stepwise optimization over many thousand iterations. The additional scope for design resulting from this allows the additional speed to be induced by the propeller at the highest rate of efficiency. Due to these characteristics, the corresponding propeller is designated as a Variable-Pitch- Variable-Camber (VPVC) Propeller.
No compromises: hydrodynamic shaft
Torqeedo outboards are uncompromisingly trimmed to efficiency. This also applies to all fluidic-sensitive components such as the shaft and the pylon.
 Section of the calculated propeller jet (red for high speeds, blue of low speeds) |
Background knowledge on propeller geometries
In addition to the important parameters such as the diameter of the propeller and the number of blades (wing vamps), 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 with the aid of the tilt angle of its wing vamps. For propellers in which the pitch varies along the wing (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 cavitations
Cavitations are the phenomena caused by the formation and closing of cavities within fluids. Cavitations are caused in particular by fast moving objects within the water such as, e.g. propellers. Due to the fast movement, underpressures result in which the water starts to boil and evaporate 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, cavitations 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:
The standard propeller shows signs of fluctuating cavitations on the „suction“ side of the wing tip.
On the other hand, the Torqeedo VPVC-Propeller only shows signs of light Tipp.
In the video the standard propeller is enlarged. The Torqeedo propeller is larger and turns more slowly. Both propeller have the same propulsive power.