Simulation of turbulent effective wakes for propellers in off-design conditions by a correction factor approach

This paper presents a procedure for the estimation of propeller effective wakes in oblique flows. It shows how a recently developed method for controlling coupling errors can be applied to analyze propellers operating in off-design conditions. The approach allows the use of fast potential flow methods for the representation of the propeller in the context of viscous flow solvers and works accurately for a wide range of advance numbers and incidence angles with a minimum computational cost. The new method makes it possible to disclose flow phenomena on the effective wake that were hidden in conventional approaches of effective wake simulation. Different application cases are analyzed, such as a propeller-shaft configuration in inclined flow, a pod propulsor in an oblique inflow, and a ship hull advancing at a yaw angle. A dipole-like distortion on the effective wake is unmasked for a uniform flow incident to a propeller mounted on an inclined shaft. The flow component perpendicular to the axis is found to be responsible for the distortion. The effect of the direction of propeller rotation on the effective wake is illustrated for a single-shaft ship moving at a yaw angle. In particular, keel vortices are either attracted to or repelled from the propeller disk depending on the sign of the yaw angle or alternatively on that of the propeller rotation.

Design Method for Contra-Rotating Propellers for High-Speed Crafts: Revising the Original Lerbs Theory in a Modern Perspective

The main theoretical and numerical aspects of a design method for optimum contrar-rotating (CR) propellers for fast marine crafts are presented. We propose a reformulated version of a well-known design theory for contra-rotating propellers, by taking advantage of a new fully numerical algorithm for the calculation of the mutually induced velocities and introducing new features such as numerical lifting surface corrections, use of an integrated modern cavitation/strength criteria, a modified method to consider different numbers of blades among the two propellers, and to allow for an unloading function in the search for the optimal circulation distribution. The paper first introduces the main theoretical principles of the new methods and then discusses the influence of the main design parameters on an emblematic example of application in the case of counter rotating propellers for a pod propulsor designed for fast planing crafts (35 knots and above).

Design and Testing of a Hybrid Shaft-Pod Propulsor for a High Speed Sealift Ship

The hybrid shaft-pod contra-rotating propulsor configuration has been demonstrated to offer increased efficiency and maneuvering performance over conventional propulsor configurations. The design and testing of this propulsor concept for a large 39 knot high speed sealift application is described here and compared with the performance of a baseline shaft and strut mounted propulsor. This hybrid contra-rotating configuration uses twin tractor propulsion pods to power the aft propellers of the contra-rotating set, where the forward propellers are powered by conventional shaft lines. The design process used a number of design and analysis tools to develop a propulsor intended to operate at 39 knots while avoiding thrust breakdown. The propulsor geometry was manufactured and tested in open water at two different scales and used in powering and cavitation tests. Software predictions against the open water data demonstrated that performance predictions can accurately be made for this type of propulsor. The absence of thrust breakdown at speeds above the 39 knot design speed was demonstrated. The powering performance of the hybrid pod configuration will be discussed relative to experimental data obtained for a conventional shaft and strut design for the same hull form.