Figure 2. Stern wave systems of the candidate and original vessel


MARIN’s decades of experience is a big advantage because it can add new basis hull forms into the process.


Bare hull In essence, PARNASSOS calcu- lates the viscous flow around the bare hull. The estimate of the required power requires not only the nominal resistance, but also the thrust deduction fraction and the achievable performance of the propeller. It is essential to estimate the latter, as this may vary significantly between design variations. Propulsive performance could be evaluated by a coupling with a propeller code or by incorporating the propeller in the RANS computation. In these cases only one propeller design would be used in order to minimise the parameter space and compu- tational effort. This would mean the hull form is optimised for this particular propeller only, instead of optimising the combination of the optimum hull with the optimum propeller design. In order to improve the estimate of the achievable performance, the open water efficiency obtained from the optimum B-series propeller selection for a given resistance and corrected for thrust


deduction is used. The thrust deduction fraction and thrust T required for self- propulsion can be estimated from the resist- ances computed without propeller thrust and with a fixed imposed thrust T0


, which is reasonably near T at self propulsion.


Techniques extended Recently, PARNASSOS Explorer techniques have been extended with the possibility of taking the ship’s wave making into account. This shows how modifications of the hull form near the water line influence wave resistance, propeller inflow and required power. MARIN demonstrated this in a first systematic variation of the aft part of a single screw chemical tanker, which is one of the test cases used in the 7th Framework EU project STREAMLINE. The design space was set up with six distinct hull shapes verifying the gondola, prame or V-type frames, transom width and buttocks slope. Figure 1 shows the required power relative to the initial hull form on the vertical axis and the quality of the wake on the horizon- tal axis. Each point gives the computed values for one hull form variation. There is


an envelope, a ‘Pareto front’, indicating the best that can be achieved. A compromise between both objectives is clearly required. The candidate hull form has a deeper transom immersion, a reduced waterline curvature near the transom and a more slender gondola. The reduced waterline curvature and higher transom immersion result in a less pronounced stern wave system (Figure 2). These results show that even a first syste- matic variation with only a limited number of hull-form evaluations already results in a significant decrease in required power (5.8%), without a decrease in the quality of the inflow to the propeller. Based on the insight obtained, the design space can be extended, leading to a sharper Pareto front and more detailed knowledge about the optimal hull-propeller combination.


References Ploeg A. van der and Raven H.C. (2010), “CFD-based Optimi- zation for Minimal Power and Wake Field Quality”, Proceed- ings 11th International Symposium on Practical Design of Ships and other Floating Structures, Rio de Janeiro, pp. 92-101.


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