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A High Froude Number Time Domain Strip Theory Applied to the Seakeeping of Semi-SWATHs


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Holloway, DS (1998) A High Froude Number Time Domain Strip Theory Applied to the Seakeeping of Semi-SWATHs. PhD thesis, University of Tasmania.

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In recent years there has been a rapid growth in the fast passenger ferry industry. Initially speed
was the main selling point for designers, builders and operators, but as competition and choice
have increased passengers are demanding better seakeeping performance. In addition designers
and builders are starting to see the benefits of better seakeeping not only in terms of passenger
comfort but in terms of structural strength and loading, allowing reduced structural weight and
its many associated advantages. Two aspects of the seakeeping of fast ships are addressed in
this thesis: response computation and the behaviour of semi-SWATH designs.

Motion and load prediction for the practising naval architect has traditionally been done
using strip theories, usually one closely related to the well known theory of Salvesen, Tuck
and Faltinsen. This is a low Froude number theory, and although it is still being used, often
successfully, for fast ships there is no rational justification for its validity in these cases. As
speeds are increasing it is becoming imperative that an equivalent analysis tool suitable for
higher Froude numbers be developed. This thesis proposes such a theory, based on calculation
of two dimensional hydrodynamic potentials in a fixed reference frame in place of the traditional
moving one. This strip theory of necessity is a time domain theory, which also allows the possi-
bility of introducing non-linearities, random sea input, and even slamming events (although only
the first of these is discussed in any detail in the thesis). Validation has involved comparison
with traditional theory and tank testing. Most notably pitch and coupling effects have shown
improved predictions, but heave tends to be over predicted. The main candidates for explana-
tion of this phenomenon are argued to be wake shedding, hull entry effects, steady-unsteady
interactions and three-dimensionality.

The majority of fast ferries being built at present have very conventional hull forms below
the calm waterline. These have poorer seakeeping than their slower equivalents because their
natural frequencies are encountered in longer waves, and traditionally designers have relied on
lifting surfaces to counteract the increased motions. As these vessels get faster this approach
will become less viable in terms of forces involved and appendage drag penalty. The type of hull
form that will reduce motion accelerations without too much sacrifice of drag is not obvious,
and a family of semi-SWATHs has been investigated as a possible alternative hull form. The
investigation shows that as speed is increased the advantages of SWATH like forms become much
greater if the criterion is to reduce accelerations.

Item Type: Thesis (PhD)
Keywords: Froude, Naval Architecture, SWATH, Small Waterplane Area Twin Hull Vessel,
Additional Information:

Copyright the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Date Deposited: 23 Mar 2006
Last Modified: 11 Mar 2016 05:55
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