Design and optimization of hydrofoil-assisted catamarans

[摘要] ENGLISH ABSTRACT:This work is concerned with the hydrodynamic design of hydrofoil-assisted catamarans.Focus is placed on the development of new and suitable design methods andapplication of these to identify the most important geometric parameters of catamaranhulls and hydrofoil configurations that influence efficiency and performance. Thesegoals are pursued by firstly gaining a thorough understanding of the governing hydrodynamicprinciples involved in the design process. This knowledge is then applied todevelop new and improved experimental techniques and theoretical methods neededfor design. Both are improved to the extent where they can be applied as designtools covering the important semi-displacement and semi-planing speeds, which arethe focus of this study.The operational speed range of hydrofoil-assisted catamarans is shown to consist ofthree distinct hydrodynamic phases (displacement, transition and planing) and thatdifferent hydrodynamic principles govern vessel performance in each phase. The hydrodynamicsare found to differ substantially from that of conventional high-speedcraft, primarily due to the interaction between the hull and the hydrofoils, whichis found to vary with speed and results in the need for more complex experimentalprocedures to be followed if accurate predictions of resistance are to be made.Experimental predictions based on scaled model tests of relatively small hydrofoilassistedcatamaran models are found to be less accurate than that achievable forconventional ships because of the inability to correct for all scaling errors encounteredduring model testing. With larger models scaling errors are encountered to a lesserdegree. The most important scale effect is found to be due to the lower Reynoldsnumber of the flow over the scaled foils. The lower Reynolds number results in higherdrag and lower lift coefficients for hydrofoils compared with those achieved at fullscale. This effect can only be partially corrected for in the scaling procedure usingthe available theoretical scaling methods.Presently available theoretical methods commonly used for the design of conventionalships were found to be ill adapted for modeling the complex hydrodynamicsof hydrofoil-assisted catamarans and required further development. Vortex latticetheory was chosen to model the flow around hydrofoil-assisted catamarans as vortextheory models the flow around lifting surfaces in the most natural way. The commercialcode AUTOWING is further developed and generalized to be able to model thecomplex hull-hydrofoil interactions that change with speed. The method is shown tomake good predictions of all hydrodynamic quantities with accuracies at least as goodas that achievable through model testing and therefore fulfills the requirements for asuitable theoretical design tool.The developed theoretical and experimental design tools are used to investigate thedesign of hydrofoils for hydrofoil-assisted catamarans. It is found that the main parameterneeding consideration in the hydrofoil design is selection of a suitable hydrofoillift fraction. A foil lift fraction in the order of 20-30% of the displacement weightis needed if resistance improvements using hydrofoil assistance are to be obtainedover the hull without foils. It is often more favorable to use higher foil lift fractions(50%+) as the resistance improvements are better, although careful attention shouldthen be given to directional and pitch-heave instabilities. The Hysuwac hydrofoilsystem patented by the University of Stellenbosch is found to be hydrodynamicallyoptimal for most hullforms.The hullform and in particular the curvature of the aft buttock lines of the hull arefound to have an important influence on the achievable resistance improvements andbehaviour of the hydrofoil-assisted hull at speed. Hull curvature is detrimental tohydrodynamic performance as the suction pressures resulting from the flow over thecurved hull counter the hydrofoil lift. The hullform best suited to hydrofoil assistanceis found to be one with relatively straight lines and hard chine deep- V sections.The main conclusion drawn from this study is that hydrofoil-assistance is indeedsuitable for improving the performance and efficiency of catamarans. The design andoptimization of such vessels nevertheless requires careful consideration of the variousresistance components and hull-foil interactions and in particular, how these changewith speed. The evaluation of resistance for design purposes requires some disciplinebetween theoretical analysis and experimental measurements as the complexity ofthe hydrodynamics reduce the accuracies of both. Consideration of these factorsallows hulls and hydrofoils to be designed that are efficient and also free of dynamicinstabilities.

[发布机构] Stellenbosch University