Contrary to conventional rudder propellers, podded drives produce the lateral steering force not only by changing the propeller jet direction, but also by using a considerable area of the strut like a conventional rudder. Moreover, an inclined propeller inflow changes the flow conditions at the propeller blades compared to the often assumed axial flow. Angle of attack, resulting relative velocity at the blades, vary now with blade position during propeller revolution to an extent which cannot be neglected and results in additional side forces and moments. In this study, these steering forces and moments are calculated with an unsteady Reynolds-averaged Navier-Stokes solver. The vortex lattice method (VLM) is used to obtain steady forces distributed over the propeller blade to be applied in the RANSE calculation as body forces. A complete RANSE simulation and experiments are used to validate this hybrid VLM-RANSE approach. The results are almost the same as with the propeller modelled geometrically in detail. Measured and computed forces for the whole pod with working propeller also agree well. The calculated side forces produced by the strut and the gondola agree well with simple formulae of classification rules for a conventional rudder. The relative influence of the propeller on the total lateral steering force (produced by the components strut, gondola and propeller) decreases with increasing speed. However, a validation of contributions of the individual components is so far not possible due to lack of data. At design speed, the steering capability of a pod is not much superior to a conventional rudder, but at lower ship speeds, the pod offer much better steering. Different principles to determine an equivalent rudder stock diameter with respect to SOLAS requirements (e.g. equivalent torsion moment, equivalent moment of resistance against torsion, equivalent stress in the shaft) are still to be discussed.
Numerical analysis of the steering capability of a podded drive
Numerische Berechnung der Steuerkraft von Ruderpropellern
Ship Technology Research / Schiffstechnik ; 51 , 3 ; 134-145
2004
12 Seiten, 9 Bilder, 5 Tabellen, 6 Quellen
Article (Journal)
English
Numerical Analysis of the Steering Capability of a Podded Drive
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