The most important design and operation criteria of tugs is the available bollard pull. The efforts of propeller manufacturers to increase the delivered power leads to a larger power density of the propulsion system and thus to a higher risk of cavitation. This makes the design process more complex, and propeller designers have to rely on both calculations and model tests.
Tugs with ducted propellers
An important aspect regarding the bollard pull of heavily loaded ducted propellers is the cavitation behaviour and the risk of a thrust breakdown due to cavitation. Systematic tests with ducted propellers showed that the thrust breakdown of ducted propellers is affected mainly by the reduction of the nozzle thrust. The figure shows the influence of the blade outline on the ducted propeller coefficients during a variation of the cavitation number at high thrust loading (low advance coefficient).
It could be shown that the chord length at the blade tip is very important for the cavitation behaviour and thereby the thrust break down. The analysis of the cavitation indicates that the thrust break down of the nozzle starts when cavitation occurs at the blade tip profile for a whole revolution. The pictures show the blade tip cavitation of two ducted propellers at the cavitation number of the inception of the nozzle thrust breakdown.
Extensive CFD calculations have been carried out to find the reason for the nozzle thrust break down due to the relatively small cavitation. The calculations gave results that were similar to the experimental observations. Additionally, numerical simulations provide the opportunity to study flow details as shown in the following figures.
The cavitating tip vortex disturbs the flow in the diffuser of the nozzle. This results in flow separation and reversed flow. Moreover it can be shown that the nozzle thrust decreases if the outflow area is reduced by flow separation, which also causes increasing propeller thrust and torque. The results of the cavitation tests with different ducted propellers are summarised in the following diagram. Diagrams of this kind allow the prediction of the risk of thrust breakdown in an early design stage.
Voith Water Tractor (VWT)
Voith-Schneider Propellers (VSP) are used primarily for ships that have to satisfy particularly demanding safety and manoeuvrability requirements. Unique to the Voith Schneider Propeller is its vertical axis of rotation. The thrust is generated by separately oscillating, balanced propeller blades.
Voith Schneider Propellers operate at a comparably low number of revolutions. For better investigation of VSPs, particularly under bollard pull condition, SVA Potsdam developed a special thrust measurement system for VSP model drives. The VSP balance allows the measurement of the longitudinal and transverse forces of the VSP during propulsion and cavitation tests. In the cavitation tunnel single blades as well as whole VSPs can be investigated experimentally with high speed cameras and thrust and torque measurements. The forces and power of a Voith Water Tractor with two VSPs can be measured in the large circulating and cavitation tunnel UT2 of the Technical University of Berlin, which holds whole models in the test section. The test section of this tunnel has a length of 11.0 m, a width of 5.0 m and a depth of 3.0 m which allows tests at bollard pull conditions. Left you find a photo showing a VWT model during the installation in the UT2.
