Thesis (PhD)--Stellenbosch University, 2002. / 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52756 |
| Date | 03 1900 |
| Creators | Migeotte, Gunther |
| Contributors | Hoppe, K. G., Thiart, G. D., Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 240 p. : ill. |
| Rights | Stellenbosch University |
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