Experimental investigation on propeller and blade loads during off-design conditions

The present thesis wants to take a step forward towards the understanding and quantification of the complex hydrodynamic mechanisms which characterize the propeller performance during real operative scenarios: a novel set–up, based on the measure of three forces and moments exerted by the single blade of the propulsor, has been developed, installed and successfully tested on a twin screw scaled model at the CNR-INM maneuvering basin and towing tank. At first, the new setup is presented, reporting the results of the extensive experimental activity performed on the selected test case, dedicated to quantify, for the first time by means of free-running model test, the entity of the single blade forces and integrated propeller loads in different operating conditions. Although free running model tests still represent the primary approach for a reliable performance assessment, they require facilities and devices that are not commonly affordable; alternatively, rectilinear towing tank can be used for manoeuvring investigations by static or dynamic tests and can be a valid alternative to investigate propeller performance in off-design. Considering this, another experimental campaign has been performed at CNR-INM towing tank. In the experiments, the drift angle and the advance speed of the model were varied systematically, to focus on the relation between propeller operating conditions and loads. Moreover, the averaged and periodic blade and propeller loads are compared, in terms of the equivalent drift angle, to the measurements obtained by free running model tests, in order to demonstrate the feasibility of simulating turning circle stabilized phase with pure oblique flow tests, providing a preliminary quantification of the off-design loads developed by the propeller. The research ends with the comparison of single blade loads that arise during transient motions of the turning manoeuvres at weak and tight rudder angles, performed at the same reference speed of the captive model test. This section allows to analyse the possibilities and limitations of the characterization of propeller performance under time varying inflow by means of quasi-steady towing conditions. The availability of cycle–resolved blade loads further highlighted their fluctuating nature with respect to the averaged ones and opens new paths for the investigation of hydrodynamic phenomena that characterize the performance of the propeller and blade(propeller)/wake interaction in behind hull, both during design and off-design conditions.