Our propellers don't cause much turbulence but provide a great deal of thrust. An idea taken from commercial shipping.
Most propellers used in the field of leisure are based on series testing conducted in the 1940s to 1960s in the Wageningen test facility in the Netherlands as well as in the USA. The test findings were incorporated into general design principles and are applied in accordance with rules of thumb and design tables.
In contrast, very modern commercial ships have for a number of years been equipped with propellers that have their origins in multi-dimensional optimisation calculations. Unlike with standard propellers, the pitch and camber of these propellers is not kept (virtually) constant across all propeller segments. Instead, pitch and camber are optimised on the basis of a so-called vortex grid calculation and a stepwise optimisation over many thousand iterations for each propeller segment. The different design possibilities that result from this lead to additional speed induced by the propeller at maximum efficiency. Owing to these characteristics, these types of propeller are called variable pitch variable camber (VPVC) propellers, since the propeller pitch as well as the propeller camber are optimised in accordance with the radial flow conditions. VPVC propellers are a Torqeedo patent.
Background information on propeller design
Besides the important parameters such as propeller diameter and number of vanes, propellers can be described by the radial gradients of the following parameters:
- Chord length
- Rake as well as profile parameters
In this context pitch describes the path a propeller would take for on complete rotation without any slip. Since this idealized parameter cannot be determined on a moving boat (slip always occurs in practice), the pitch of a propeller is measured with the help of the angle of attack of its vanes. In the case of propellers where the pitch varies along the vane (variable pitch propeller), it is measured on a circle that is drawn at 70% of the propeller diameter around the centre of the propeller.
Efficiency loss through cavitation
Cavitation is the formation and breakup of cavities in liquids. It is caused in particular by fast-moving objects in liquids, e.g. boat propellers. The fast movement causes low pressure where water boils and evaporates at normal water temperature. The energy used in this process is not converted into propulsive power and is lost inefficiently. The degree of cavitation caused can vary widely depending on the quality of the drive system and its propeller. These two pictures taken with a high-speed camera with an exposure time of 1/8,000 s show the difference between our VPVC propeller and a standard propeller at comparable operating points:
>> The standard propeller exhibits fluctuating cavitation on the "suction side" of the tip of the vane.
>> In contrast, our VPVC propeller exhibits only slight tip vortex.