CHALLENGE - The researchers of University of Genoa, in collaboration with SISTEMAR SA, proposed a study to design and analyze a tip loaded propellers to improve full-scale propeller efficiency (lower fuel consumptions and lower emission levels to comply with environmental pollution regulations). The modified tip loaded propeller (a mix between a tip rake and a contracted and tip loaded propeller) is designed via an optimization strategy using a Boundary Elements Method (BEM), a custom parametric description of the unconventional blade geometry and an optimization algorithm within the modeFRONTIER environment.
SOLUTION - The design case considered in the present work exactly resembles the design of the conventional Contracted and Tip Loaded (CLT) propeller. The reference geometry was a six bladed propeller, designed to operate in the hull wake of delivering an average thrust coefficient of 0.215 at a cavitation index of 3.5. With respect to the reference propeller, the CLT geometry optimized on the basis on these assumptions demonstrated superior performance for all the considered merit functions. The newly designed propeller had improved efficiency with a sensible reduction of cavitation, verified by both BEM and RANSE calculations.
Objectives of the design were the maximization of the average (mean inflow condition) efficiency, together with the minimization of the backside cavitation area and the avoidance, in any functioning conditions under investigation, of face cavitation. These results were constrained by the delivering of an average propeller thrust equal to that of the original CLT propeller, with a tolerance of ±1.5% accepted to speed up the Pareto convergence of the designs. The optimization chain was built using the modeFRONTIER environment selecting the Multi Objective Genetic Algorithm MOGA-II. An initial population of 200 members served to feed the calculations from which derive the optimal geometries. A total of 10,000 configurations (50 generations) has been calculated. The results of the design process are summarized, as usual in the case of multi-objective optimizations, in the Pareto diagram. The reference performance, marked with a cross, are those of the reference CLT propeller.
BENEFITS - The effectiveness of the optimization, at first sight and only based on the BEM calculations, is outstanding. The optimization of the blade shape in the case of conventional CLT propellers resulted, for instance, in a reduction higher than 70% of the cavity area while facing a slightly reduction of the propeller efficiency.