Experimental and Numerical Study of Steady and Unsteady Multi-hull Systems Moving in Calm Water

A multi-hull system can be a vessel made by multiple component hulls bound together by decks and superstructures, or several individual vessels traveling in the vicinity of each other with different speeds and courses. The former can be considered as a steady-state multi-hull system with the absence of relative motions among component hulls, while the latter is called an unsteady multi-hull system./nThe wave resistance and wave pattern of each component hull in a steady-state multi-hull system, e.g. multi-hull vessels, are significantly affected by the interference of ships waves generated by the moving bodies. To study such steady-state multi-hull wave interference, towing-tank tests of asymmetric steady-state di-hull systems were performed, with the resistance, sinkage and trim of each component hull measured in the process. Then, the experimentally-validated Multi-hull Simple-source Panel Method (MSPM) is utilized for an extensive study of di-hull systems with a large number of hull configurations to extract a basic understanding of the wave-interference phenomenon. Significant wave-resistance reduction was shown for configurations that were evaluated experimentally, especially for the component hull traveling at the rear of the formations with high forward speeds. Furthermore, the wave patterns and wave resistances of di-hull systems were discovered to be considerably affected by the interference of waves with the strongest free-wave spectral density of the component hull as a mono-hull in isolation, and the beneficial hull configurations with reduced wave resistance can be qualitatively determined with a knowledge of mono-hull free-wave spectrum and simple wave-angle alignments. Moreover, for di-hull systems with small spacing between component hulls, the near-field interaction, which is normally missing in the far-field analyses, was found to have non-negligible effects over the wave resistances and wave patterns of the individual component hulls as well as the di-hull systems they constitute./nFor the unsteady modeling of multi-hull systems, a group of independent vessels traveling in the vicinity of each other during overtaking maneuvers with relative motions among component hulls was studied. The MSPM computation introduced in the steady-state study was generalized to simulate the moderate-speed overtaking maneuvers, and the results were validated by experimental measurements of towing-tank model tests. The influence of overtaking speed and the speed difference between vessels on the hydrodynamic loads were studied. It was found that the deformed free surface caused by the blockage of hull bodies, wave-making of the vessels, and the interference of unsteady wave patterns considerably affect the hydrodynamic interaction forces and moments between the vessels. In addition, the slower ship being overtaken generally experiences larger loads with stronger variations than the passing ship. The loads on both hulls become more similar to those of a steady-state di-hull system when the speed difference between vessels is small./nLastly, with the abundant computational results provided by the GPU-accelerated MSPM computation developed in this work, a Deep Neural Network (DNN) was trained and tested to efficiently evaluate the calm-water transportation efficiencies of a large number of different trimaran configurations. In this manner, the global understanding of the influences of different design parameters, such as the sizes and shapes of the component hulls, hull spacing, and forward speed can be established, and the optimal configuration with the best transportation efficiency can be quickly determined at the early-design stage. It is discovered that with sufficient computational results, the DNN can be trained to accurately evaluate a large quantity of trimaran configurations of various design parameters with very little computing cost.

Advisor: Yeung, Ronald W.