Feature 1 | GEARS AND PROPELLERS
Aspects of the design procedure for propellers
In this article, Dr-Ing Paul Mertes, Schottel, and Dipl-Ing Hans-Jürgen Heinke, Schif au-Versuchsanstalt Potsdam GmbH, discuss design procedures for improving bollard pull.
T
he number of tugs, as well as their size and power, is increasing rapidly. The most important
design and operation criteria of the tug is the available bollard pull. Propeller manufacturers are trying to increase the delivered power and the system diameter of the ducted propellers to meet these requirements. This has led to a higher power density
of the ducted propeller, and to a greater risk of cavitation. An important aspect regarding the bollard pull of highly loaded ducted propellers is the cavitation behaviour. This is why the design process is complex and has to rely on calculations and model tests. Cavitation tests have been part of the
development of thrusters and ducted propellers for the last 15 years. Model tests showed that the cavitation behaviour must be taken into consideration in the design and optimisation process, especially for ducted propellers with high thrust loading coefficients. To study the influence of cavitation
on the thrust of a ducted propeller at bollard pull condition, a research and development project was initiated. Systematic tests with ducted propellers showed that the thrust breakdown of
ducted propellers is caused mainly by the reduction of the nozzle thrust. The propeller model VP 1303 represents
a propeller with a Kaplan blade type. The model propeller VP 1305A is a propeller with a diminishing chord length at the blade tip. Both propellers were tested in the nozzle D 221, type Wageningen 19A. The investigations show that the chord
length at the blade tip is very important for influencing the cavitation behaviour and thereby the thrust breakdown. The analysis of the cavitation gives the indication that the thrust breakdown of the nozzle starts if cavitation appears over the whole revolution at the blade tip profile. During the cavitation tests with the
two propellers it could be observed, that at the cavitation number for the inception of nozzle thrust breakdown, the degree of cavitation was smaller for the propeller VP 1305A. Nevertheless the thrust breakdown starts distinctly earlier on the ducted propeller with a blade outline with diminishing chord length at the blade tip. Extensive CFD calculations have
been carried out to find the reason for the nozzle thrust breakdown due to the relatively low cavitation. The calculations
have shown that it is possible to predict the cavitation behaviour and the thrust breakdown due to cavitation. In addition, the CFD calculations provide an opportunity to study flow details as shown in Fig 2. The cavitating tip vortex disturbs the
flow in the diffuser of the nozzle. The result is flow separation and backwards flow. The thrust breakdown of the nozzle is connected with the flow in the diffuser area. It can be shown that the nozzle thrust decreases if the outflow area is reduced by flow separation. The flow separation in the diffuser also has the effect of increasing the propeller thrust and torque. The results of
the cavitation tests
with different ducted propellers are summarised in Fig 3 by means of the cavitation number σn and the torque coefficient 10KQ.
Germany’s most powerful tugs MAN Ferrostaal AG and Schottel have supplied the CP propellers for the
Fig 1: Blade outlines and test arrangement. 20 Ship & Boat International September/October 2008
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