A cycloidal propeller simulation model and its application to low and high speed ship manoeuvres

The aim of this work is to study the application of a cycloidal propulsion system to a hull whose behaviour, when equipped with a bow thruster and traditional propellers and rudders at the stern, is known. Starting from the kinematical model for each single blade, a simulator of an epicyloidal propeller has been developed on a Matlab/Simulink platform. This kind of simulation involves a reliable representation of the cycloidal propellers, the manufacturers of which unfortunately do not publicly share their performance maps for confidential reasons. The numerical modelling presented is based on a mixture of theoretical and empirical considerations: in particular, the propeller thrust and torque evaluation is based on the kinematics of the blades, taking into account suitable correction factors in order to properly consider “dissipative” phenomena (such as interference between blades, the shielding induced by the half of the rotor which receives the incoming flow, and the slight reduction of back thrust). The calibration of the so obtained simulator has been carried out by comparing simulation outputs with real data found in open source. The result is a simulation approach able to represent the performance prediction of an epicycloidal propeller, avoiding demanding computations (e.g. CFD methods) that would not allow an effective simulation of the whole DP system. As a first application of the implemented simulation platform, different thrust allocation logics of a DP system for the surface vessel equipped with a bow thruster and two cycloidal propellers at stern have been presented. The examined ship for this work is the same unit with conventional propulsion system, for which a dynamic positioning system has already been developed and installed on board. A comparison between the performances of the two distinct propulsion configurations has been then possible and proposed. The first results of the static analysis (simply obtained by balancing forces and moments generated by environmental disturbances) have been very optimistic and they have been confirmed by the dynamic analysis. A dynamic model for the simulation of the manoeuvrability, both at low and high speed, of the unit equipped with cycloidal propellers have been carried out and presented together with some results: the outcome of the simulation shows and confirms the better performance of cycloidal propulsion during dynamic positioning manoeuvres in different operating situations. The same response has been obtained for manoeuvres at cruise speed: when equipped with cycloidal propellers, the ship gives better results in terms of times and distances needed to carry out the manoeuvre. Summing up all the obtained results, it can be stated that the developed simulator can reproduce the dynamic behaviour of the ship in a reliable way.