Design and optimization of hydrofoil-assisted catamarans

Migeotte, Gunther (2002-03)

Thesis (PhD)--Stellenbosch University, 2002.

Thesis

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 and application of these to identify the most important geometric parameters of catamaran hulls and hydrofoil configurations that influence efficiency and performance. These goals are pursued by firstly gaining a thorough understanding of the governing hydrodynamic principles involved in the design process. This knowledge is then applied to develop new and improved experimental techniques and theoretical methods needed for design. Both are improved to the extent where they can be applied as design tools covering the important semi-displacement and semi-planing speeds, which are the focus of this study. The operational speed range of hydrofoil-assisted catamarans is shown to consist of three distinct hydrodynamic phases (displacement, transition and planing) and that different hydrodynamic principles govern vessel performance in each phase. The hydrodynamics are found to differ substantially from that of conventional high-speed craft, primarily due to the interaction between the hull and the hydrofoils, which is found to vary with speed and results in the need for more complex experimental procedures to be followed if accurate predictions of resistance are to be made. Experimental predictions based on scaled model tests of relatively small hydrofoilassisted catamaran models are found to be less accurate than that achievable for conventional ships because of the inability to correct for all scaling errors encountered during model testing. With larger models scaling errors are encountered to a lesser degree. The most important scale effect is found to be due to the lower Reynolds number of the flow over the scaled foils. The lower Reynolds number results in higher drag and lower lift coefficients for hydrofoils compared with those achieved at full scale. This effect can only be partially corrected for in the scaling procedure using the available theoretical scaling methods. Presently available theoretical methods commonly used for the design of conventional ships were found to be ill adapted for modeling the complex hydrodynamics of hydrofoil-assisted catamarans and required further development. Vortex lattice theory was chosen to model the flow around hydrofoil-assisted catamarans as vortex theory models the flow around lifting surfaces in the most natural way. The commercial code AUTOWING is further developed and generalized to be able to model the complex hull-hydrofoil interactions that change with speed. The method is shown to make good predictions of all hydrodynamic quantities with accuracies at least as good as that achievable through model testing and therefore fulfills the requirements for a suitable theoretical design tool. The developed theoretical and experimental design tools are used to investigate the design of hydrofoils for hydrofoil-assisted catamarans. It is found that the main parameter needing consideration in the hydrofoil design is selection of a suitable hydrofoil lift fraction. A foil lift fraction in the order of 20-30% of the displacement weight is needed if resistance improvements using hydrofoil assistance are to be obtained over 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 should then be given to directional and pitch-heave instabilities. The Hysuwac hydrofoil system patented by the University of Stellenbosch is found to be hydrodynamically optimal for most hullforms. The hullform and in particular the curvature of the aft buttock lines of the hull are found to have an important influence on the achievable resistance improvements and behaviour of the hydrofoil-assisted hull at speed. Hull curvature is detrimental to hydrodynamic performance as the suction pressures resulting from the flow over the curved hull counter the hydrofoil lift. The hullform best suited to hydrofoil assistance is 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 indeed suitable for improving the performance and efficiency of catamarans. The design and optimization of such vessels nevertheless requires careful consideration of the various resistance components and hull-foil interactions and in particular, how these change with speed. The evaluation of resistance for design purposes requires some discipline between theoretical analysis and experimental measurements as the complexity of the hydrodynamics reduce the accuracies of both. Consideration of these factors allows hulls and hydrofoils to be designed that are efficient and also free of dynamic instabilities.

AFRIKAANSE OPSOMMING: Hierdie studie is gerig op die hidrodinamiese ontwerp van hidrovleuel-gesteunde katamarans. Daar word gefokus op die ontwikkeling van nuwe en geskikte ontwerpmetodes, asook die toepassing van hierdie metodes om die belangrikste geometriese parameters van katamaranrompe en hidrovleuel-konfigurasies wat 'n invloed op doeltreffendheid en werkverrigting het, te identifiseer. As aanloop tot die studie is 'n deeglike begrip van die onderliggende hidrodinamiese beginsels bekom. Hierdie kennis is toegepas om nuwe en verbeterde eksperimentele en teoretiese tegnieke te ontwikkel wat nodig is vir die ontwerp van hidrovleuel-gesteunde katamarans in die belangrike deels-verplasing en deels-planering spoedbereike. Daar word getoon dat die bedryfspoedbereik van 'n hidrovleuel-gesteunde katamaran uit drie onderskeibare hidrodinamiese fases bestaan, naamlik verplasing, oorgang en planering, en dat verskillende hidrodinamiese beginsels die vaartuig se werkverrigting in elke fase bepaal. Daar is ook gevind dat die hidrodinamika wesentlik verskil van dié van konvensionele hoëspoed-vaartuie, hoofsaaklik as gevolg van die interaksie tussen die romp en die hidrovleuels wat wissel na gelang van die spoed. Hierdie interaksies moet in ag geneem word gedurende die ontwerpproses en beide eksperimentele en teoretiese metodes is nuttig om die omvang daarvan te bepaal. Daar is gevind dat die eksperimentele voorspellings gebaseer op toetse met relatief klein skaalmodelle van hidrovleuelgesteunde katamarans minder akkuraat is as dié wat bereik kan word met konvensionele skepe. Dit is omdat al die skaalfoute wat tydens die toetsing met die model ontstaan, nie gekorrigeer kan word nie. Die belangrikste skaaleffek is as gevolg van die laer Reynoldsgetal van die vloei oor die afgeskaalde vleuels. Groter modele Die laer Reynoldsgetal lei tot hoër sleur- en hefkoëffisiënte in vergelyking met dié vir die volskaal-hidrovleuels. Wanneer die beskikbare teoretiese metodes gebruik word, kan daar slegs gedeeltelik vir hierdie effek in die skaalprosedure gekorrigeer word. Daar is ook vasgestel dat die skaaleffekte op die Reynoldsgetal verminder word wanneer die hidrovleuels baie nabyaan die vrye oppervlakte is. Dit lei daartoe dat eksperimentele voorspellings van werkverrigting meer akkuraat is vir die ontwerpe waar die hidrovleuels nie so diep onder die water is nie. Daar is gevind dat die teoretiese metodes wat tans beskikbaar is en algemeen vir die ontwerp van konvensionele skepe gebruik word nie die komplekse hidrodinamika van hidrovleuel-gesteunde katamarans kan modelleer nie. Die werwelroosterteorie is gekies om die vloei om hidrovleuel-gesteunde katamarans te modelleer aangesien dié teorie die vloei om hefvlakke op die natuurlikste manier weergee. Die kommersiële kode AUTOWING is verder ontwikkel en veralgemeen om ook die komplekse spoed-afhanklike interaksies van die romp en hidrovleuel te kan modelleer. Hierdie metode lewer goeie voorspellings van al die hidrodinamiese maatstawwe met akkuraathede wat ten minste so goed is soos di wat met modeltoetsing bereik word en voldoen daarom aan die vereistes vir 'n geskikte teoretiese ontwerpmetode. Die teoretiese en eksperimentele ontwerpmetode wat ontwikkel is, word gebruik om die ontwerp van hidrovleuels vir hidrovleuel-gesteunde katamarans te ondersoek. Daar is gevind dat die belangrikste parameter wat in die hidrovleuel-ontwerp in ag geneem moet word, die keuse van 'n geskikte hidrovleuelhefverhouding is. Om in rompe met hidrovleuelsteun verbeterings in die weerstand te kry in vergelyking met rompe sonder vleuels, is 'n vleuel-hef-verhouding van 20-30 persent van die verplasingsgewig nodig. Dit is dikwels beter om hoër vleuel-hef-verhoudings (van 50 persent of meer) te gebruik omdat die verbetering in weerstand dan groter is. Daar moet dan egter gewaak word teen rigtings- en hei-hef-onstabiliteite. Daar is gevind dat die Hysuwachidrovleuel- stelsel wat deur die Universiteit van Stellenbosch gepatenteer is, hidrodinamies optimaal is vir die meeste rompvorms. Daar is gevind dat die vorm van die romp en veral die kromming van die lyne gevorm deur vertikale snitte deur die romp (Engels: "aft buttock lines") van die romp 'n belangrike invloed het op die bereikbare weerstandsverbeterings en die gedrag van die hidrovleuel-gesteunde romp wat op spoed is. Die kromming van die romp is nadelig vir die hidrodinamiese werksverrigting aangesien die suigdruk as gevolg van die vloei oor die gekromde romp die hefkrag van die hidrovleuels teenwerk. Die rompvorm wat die geskikste is vir hidrovleuel-ondersteuning is 'n romp met relatiewe reguit lyne en skerp hoekige diep- V seksies. Die belangrikste gevolgtrekking waartoe tydens die studie gekom is, is dat hidrovleuelondersteuning wel geskik is vir die verbetering van die werkverrigting en die doeltreffendheid van katamarans. Die ontwerp en optimering van sodanige vaartuie verg nogtans die noukeurige oorweging van die verskeie weerstandskomponente en rompvleuel- interaksies en veral hoe hierdie interaksies verander met spoed. Die evaluering van die weerstand vir die doeleindes van ontwerp verg dissipline tussen die teoretiese analise en die eksperimentele metings aangesien die kompleksiteit van die hidrodinamika die akkuraatheid van die algemeen-gebruikte teoretiese en eksperimentele metodes vir die hidrodinamiese ontwerp verminder. As hierdie faktore in ag geneem word, kan rompe en hidrovleuels ontwerp word wat doeltreffend is en ook vry is van dinamiese onstabiliteite.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/52756
This item appears in the following collections: