A spectrogram of a ship wake is a heat map that visualises the time-dependent frequency spectrum of surface height measurements taken at a single point as the ship travels by. Spectrograms are easy to compute and, if properly interpreted, have the potential to provide crucial information about various properties of the ship in question. Here we use geometrical arguments and analysis of an idealised mathematical model to identify features of spectrograms, concentrating on the effects of a finite-depth channel. Our results depend heavily on whether the flow regime is subcritical or supercritical. To support our theoretical predictions, we compare with data taken from experiments we conducted in a model test basin using a variety of realistic ship hulls. Finally, we note that vessels with a high aspect ratio appear to produce spectrogram data that contains periodic patterns. We can reproduce this behaviour in our mathematical model by using a so-called two-point wavemaker. These results highlight the role of wave interference effects in spectrograms of ship wakes.
A novel CFD-based approach is presented that is used in conjunction with model test experiments to predict ship resistance at full-scale Reynolds and Froude numbers. It relies on verification using model scale experiments, including an agreement of integrated shear force with established model-ship correlation lines at model and full-scale, and includes surface roughness effects. One major advantage of the method is that the geometric dimensions of the CFD modelling remain at model scale. CFD simulation results were successfully verified considering the drag of two different catamarans at 1:22 and 1:50 model scale. Furthermore, it is shown that an identical near-wall mesh can be used for both model and full-scale simulations without compromising the accuracy of the shear force. At full-scale the deviation of resistance between CFD prediction, model test extrapolation and full-scale measurements of a 98 m catamaran was as low as 5% at F r = 0.40 and 0.43. For a novel 130 m catamaran variations in full-scale drag for a smooth hull were also less than 5% when comparing extrapolated model scale experiments and CFD predictions. However, at such large Reynolds numbers CFD predictions for correlation and roughness allowance were significantly higher compared to estimates proposed in ITTC guidelines.
Maintenance scheduling for naval vessels and ships requires ongoing improvement to manage rising maintenance costs within availability constraints. Existing maintenance scheduling approaches are not optimal as maintenance costs continue to rise without an improvement in vessel availability. This paper reviews the Risk-Based Maintenance Scheduling (RBM) framework as applied to ships and naval vessels, and provides a critical analysis of Risk Assessment and Maintenance Scheduling techniques used. Further, objectives and considerations are defined for future applications for ships and naval vessels, and the framework evaluated as an improvement on existing Preventative Maintenance (PM) and Reliability Centered Maintenance (RCM) methods. A probabilistic approach supported by condition monitoring data in combination with Decision Theory is suggested for the Risk Assessment and Maintenance Scheduling elements comprising an RBM Scheduling framework. Implementation of this framework from both periodic PM and RCM is presented. Development of applications from the component level upwards is suggested. Availability and overall maintenance cost are suggested as evaluation metrics against existing methods. The development of an application is formalized within a proposed framework. The development of an application within the RBM Scheduling framework is expected to result in reduced maintenance costs while meeting availability requirements for ship and naval vessel applications.
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