Predicting the propulsive power of ships with high accuracy still remains a challenge. Well established practices in the 1978 ITTC Power Prediction method have been questioned such as the form factor approach and its determination method. This paper investigates the possibility to improve the power predictions by the introduction of a combined CFD/EFD Method where the experimental determination of form factor is replaced by double body RANS computations. Following the Quality Assurance Procedure proposed by ITTC, a best practice guideline has been derived for the CFD based form factor determination method by applying systematic variations to the CFD set-ups. Following the verification and validation of the CFD based form factor method in model scale, the full scale speed-power-rpm relations between large number of speed trials and full scale predictions using the CFD based form factors in combination with ITTC-57 line and numerical friction lines are investigated. It is observed that the usage of CFD based form factors improves the predictions in general and no deterioration is noted within the limits of this study. Therefore, the combination of EFD and CFD is expected to provide immediate improvements to the 1978 ITTC Performance Prediction Method.
Abstract. For a sailing yacht, depowering is a set of strategies used to limit the sail force magnitude by intentionally moving away from the point of maximum forward driving force, potentially reducing the ship speed. The reasons for doing this includes among others; reduction of quasi-static heeling angle, structural integrity of masts and sails and crew comfort. For a wind powered cargo ship, time spent on a route is of utmost importance. This leads to the question whether there is a performance difference between different depowering strategies and if so, how large. In this research, a wind-powered cargo vessel with rigid wings is described in a Velocity Prediction Program (VPP) with four-degrees of freedom, namely surge, sway, roll and yaw, with a maximum heel angle constraint. The resulting ship speed performance for different depowering strategies are investigated and the implications in roll and pitch-moments are discussed. The wind conditions when depowering is needed are identified. A statistical analysis on the probability of occurrence of these conditions and the impact of the different depowering strategies on the required number of days for a round-trip on a Transatlantic route is performed.
Speed and power (S/P) trials are most important to guarantee the ship's propulsive performance. However, it was pointed out that the existing procedures often give a good guideline, but are not specific and can introduce inconsistent results. Recently, ITTC and ISO have improved their S/P trials procedures and harmonized the two procedures. During the harmonization process, we have verified the 'Mean of Means' (MoM) method and the 'Iterative' method which are used as the current correction methods and the 'Direct Power Method' and the 'Extended Power Method' which are applied for the evaluation of the acquired data. The results of verification are presented in this paper. The results show that using the 'MoM' method for each power setting, two double runs should be made to keep the accuracy of S/P trials, and the 'Iterative' method leads to less errors in average of the tested cases when 1 ? 2 ? 2 double runs are used in the 'MoM' method, although the methods are equally adequate if the time periods between the runs are short enough. In specific cases, e.g. in case of large speed range and/or humps and hollows within the speed-power curve, the 'MoM' method has advantages over the 'Iterative' method. In case of current time history deviating from the assumed parabolic/ sinusoidal trend and the change of the current within the time span of two double runs is very high, neither of the methods are applicable. Summarizing the results, the 'Iterative' method is fully compatible with the simple 'Direct Power Method'.
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