The Influence of Catamaran Main Characteristics on Ship Motions in Regular Waves

Costa, Joao Ricardo Centeno da (1998) The Influence of Catamaran Main Characteristics on Ship Motions in Regular Waves. MSc(R) thesis, University of Glasgow.

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Abstract

The aim of this thesis is to develop a practical approach for calculating the catamaran motions in waves and to analyse the influence of hull separation and other main characteristics on the motions. An experimental program with two different catamaran models is used to help validate the theory. The theoretical work developed in this thesis is mainly based on strip theory extended to twin-hull configurations in which viscous force components are introduced. The two-dimensional theoretical model is developed with a cross flow drag term and its predictions are compared with the ones of a three-dimensional theoretical model. Both theoretical results are compared with the experimental results. The thesis is divided in six chapters. The first one is introductory and presents the importance of the study of the seakeeping behaviour of catamarans in waves. It also introduces the history of ship motions in waves and the latest work done concerning catamaran motions in waves. The second chapter formulates the problem of motions of a vessel in waves. The basic fluid properties and assumptions used in the theory are explained in this chapter. Both two and three-dimensional theories used in this work are based in the potential flow theory. The coordinate systems most suited to each different kind of situation are defined and explained. The boundary conditions necessary to solve this particular problem of ship motions in waves are also defined. Since the exact boundary conditions lead to a very complex non-linear problem, the theoretical formulation is simplified through a linearisation process. Chapter 3 develops the hydrodynamic forces associated with the catamaran motions in regular waves. All the usual forces in conventional mono-hull ships are developed as well as the viscous forces, which are particularly important in the motion responses of twin-hull ships. The viscous effects are modelled by a cross-flow drag approach developed from aerodynamic theories and are then added to the basic two-dimensional potential flow theory. The numerical implementation of the method is described. Comparisons with published results are made for simple hull forms, like twin-cylinders and Wigley forms, as well as with a realistic catamaran model. In chapter 4 the experimental investigation performed with two catamaran models at the Hydrodynamic Laboratory of the University of Glasgow is presented. This chapter presents the statistical data analysis performed to choose the experimented model characteristics. The instrumentation set-up used for the experiments is described. The heave and pitch experimental results are presented and compared with the two- dimensional calculations. This study shows how well both heave and pitch results are improved when the viscous forces are added to the theory. At the end of the chapter an error analysis is performed to analyse the confidence of the heave and pitch experimental results as well as to analyse the quality of the incoming wave. In chapter 5 the catamaran hull interaction and resonance frequencies are studied and analysed in order to understand what kind of interactions occur and what is the influence of the distance between the hulls in the ship motion responses. In order to better understand the hull interactions, the resonance frequencies and the influence of the hull distance in the body responses, the twin cylinder results calculated in chapter 3 are reanalysed to explain the results near the resonance frequencies at zero and forward speeds. The results from the experimental work are analysed and compared with two and three-dimensional theoretical calculations. The three-dimensional results do not give better results than the two-dimensional results especially near the resonance frequencies where the viscous effects are important and are not modelled by the three-dimensional theory. The experimental results from the two tested models are compared and the influence of the distance between the hulls is investigated. The experimental model results are also compared with mono-hull motion responses and the results analysed. Two parametric studies with two different hull forms are made. One uses a hard chine hull type like the one used in the experimental work while the other uses a round displacement hull. The effect of speed, hull separation, displacement and ship main dimensions are analysed. Finally, in the last chapter the main conclusions are drawn and some recommendations and suggestions for future work are made.

Item Type: Thesis (MSc(R))
Qualification Level: Masters
Additional Information: Adviser: C Guedes Soares
Keywords: Naval engineering
Date of Award: 1998
Depositing User: Enlighten Team
Unique ID: glathesis:1998-76008
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 17:07
Last Modified: 19 Nov 2019 17:07
URI: http://theses.gla.ac.uk/id/eprint/76008

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