11th International Conference on Fast Sea Transportation
FAST 2011, Honolulu, Hawaii, USA, September 2011
Tools for Analysing and Comparing Tri-SWACH, Monohull and Trimaran
Hullforms
Tim P. McDonald, Richard W.G. Bucknall, and Alistair R. Greig
Marine Research Group, Dept. of Mechanical Engineering, UCL, London, UK
ABSTRACT
Alternative hullforms such as multihulls may allow small
vessels to deliver levels of performance normally associated
with a far larger and more costly monohull vessel. This
paper describes a research programme on alternative
hullforms being undertaken by a consortium of six
universities and academic institutions. As well as reviewing
the current research programme, this paper discusses in
detail the contribution being made by UCL: Tools for
undertaking the comparison and exploration of the
Monohull, Trimaran and TriSWACH alternatives.
These tools combine the modelling and analysis capabilities
of an existing naval architecture design package with a
varied range of search and optimisation techniques to enable
the exploration of alternatives ship design solutions. This
paper presents the initial results obtained from tools that are
currently under development at UCL.
Such tools can be applied to identify and explore key topics
related to the Trimaran and TriSWACH hullforms when
compared to a baseline Monohull configuration. The
Trimaran hullform has significant potential for small fast
combatants; as demonstrated by the US Navy’s LCS
programme. The TriSWACH hullform (with a small
waterplane area centre hull stabilised by two outriggers) is
of interest as it offers the potential to improved seakeeping
performance.
UCL is applied to examine a specific issue that arise with
the TriSWACH designs. Conclusions and an indication of
intended future work are then provided.
2.0
ACCeSS PROJECT
The research presented in this paper has evolved as part of a
project undertaken by the Atlantic Center for the Innovative
Design and Control of Small Ships (ACCeSS). ACCeSS is a
university/industry consortium that includes: Stevens
Institute of Technology; US Naval Academy; Webb
Institute; US Naval Postgraduate School; Florida Atlantic
University; and University College London (UCL);.
ACCeSS is currently engaged in collaborative research
activities across three areas:

Unmanned Surface Vessels;

TriSWACH Studies;

Educational Collaboration.
The TriSWACH studies consist of development and
validation of a trimaran variant featuring a centerhull with
minimal area at the waterplane and two small outriggers (to
provide necessary stability). The work includes design
studies, hydrodynamic testing and performance analysis.
This hullform is seen to offer the potential to improve
seakeeping performance. Fig. 1 provides definitions of the
elements of a TriSWACH hullform as used in this paper.
KEY WORDS
Design comparison, Trimaran, TriSWACH, Ship, Hullform.
1.0
INTRODUCTION
Examining the potential of any novel hullform style requires
a significant expenditure of effort if radical design
alternatives are to be considered at the outset of the design
process (McDonald, 2010). This paper details one approach
to exploring alternatives that is currently being examined.
After introducing the context in which the research is taking
place, this paper then summarises prior Trimaran Small
Waterplane Area Center Hull (TriSWACH) research and
identifies risk areas. Resistance data, derived from model
tests, highlighted that favourable residual resistance could
be obtained up to a Froude number of 0.5 (corresponding to
a displacement of 1000 tonnes for a typical TriSWACH
geometry). Initial numerical sizing studies highlight the
gross characteristics of Monohull, Trimaran and
TriSWACH patrol craft with a displacement in the region of
1000 tonnes. Next, a software tool under development at
© 2011 American Society of Naval Engineers
Fig. 1. TriSWACH hullform definitions
One part of the UCL contribution to the ACCeSS group’s
work will be a comprehensive exploration of the
TriSWACH hullform and compare it to alternative hullform
solutions such as the monohull and trimaran. This work is
intended to identify the requirements and missions for
which the TriSWACH could provide a superior option to
these other hullforms.
3.0
TriSWACH RESEARCH
Considerable research and design guidance is available for
SWATH hullforms (Kennel, 1992). Comprehensive reviews
747
of prior research on the design and performance prediction
of three hulled hullforms (concentrating on the trimaran)
have been conducted by several authors (Dubrovsky, 2001,
2004; Andrews, 2004). These authors also introduce some
research related to the TriSWACH hullform which Andrews
(2004) highlights as “more nearly another type of multihull
rather than a trimaran variant since it lacks the essential
characteristic of the trimaran, namely being a slender
monohull with small displacement side hulls.”
3.1
Prior UCL Design Studies
UCL has conducted a number of concept design studies into
the application of the TriSWACH hullform to a variety of
different design requirements. Three significant UCL
studies are described below and illustrated in Fig. 2:

TriSWACH Anti-Submarine Warfare (ASW)
Frigate – Integrated Full Electric Propulsion twin shaft
design capable of 30kts. With a total displacement of
7700te this ship was designed to Royal Navy standards
and provided a highly capable ASW and land attack
payload. (Boulby et al. 2000)

TriSWACH Offshore Patrol Vessel (OPV) – A low
cost and low risk OPV featuring a largely Commercial
Off The Shelf (COTS) single shaft propulsion system
able to achieve 14.5kts. With a total displacement of
1000te this small vessel provided both a boat and nonorganic helicopter capability. (Smith et al. 2004)

Persistence Unmanned Aerial Vehicle (UAV)
Carrier – This design study demonstrated a large
11000te TriSWACH intended to carry six large fixed
wing surveillance UAVs. The TriSWACH hullform’s
geometry allowed a short take-off and landing
capability through a flight deck approximately 200m
long in a relatively low displacement ship (c.f. a
monohull). The vessel employed IFEP propulsion to
achieve 18kts. (Andrews & Pawling, 2009)
3.2
Hullform Risk and Research Areas
A review of the wider research related to the TriSWACH
hullform identified key risks across five areas: Speed;
Stability; Seakeeping; Strength; and Style.
3.2.1 Speed
There is currently little published data on the applicability of
different resistance estimation techniques for TriSWACH
hullforms. There is limited availability for data arriving
from model tests (Min-tong et al., 2004) and few studies of
the validity of resistance prediction tools to these hullforms.
On-going work by other members of the ACCeSS
consortium is expected to improve the available knowledge
in these areas. The lack of appropriate information for early
stage design is judged to pose significant risk. The physical
constraints of the submerged TriSWACH hull and narrow
strut could cause considerable difficulties to the
arrangement of prime movers and hence the choice of
overall propulsion system architecture and the total system
performance (e.g. efficiency, carbon foot print, removal
routes, e.tc.). At higher speed providing sufficient “power
into the water” may also be problematic.
(b) OPV (Smith et al. 2004)
(a) Anti-submarine warfare frigate (Boulby et al. 2000)
(c) Persistence UAV carrier (Andrews & Pawling, 2009)
Fig. 2. TriSWACH hullform definitions
748
© 2011 American Society of Naval Engineers
3.2.2 Stability
The intact and damage stability standards that would be
applicable to a conventional hullform must be carefully
interpreted to ensure the vessel complies with the intent of
the regulations. Additionally, other potential risk areas must
be examined to ensure no unforeseen issues arise (i.e. UCL
studies highlighted the possibility of parametric resonance
arising from the vessels GM changing in waves – Smith et
al. (2004) reported GM variations between 3.2m and 8.0m).
3.2.3 Seakeeping
linked propulsion system sizing and performance evaluation
tool able to explore large variations in the total vessel size
and its resultant powering requirements.
4.1
General Trends
Using the numerical sizing tools general trends in total
displacement and weather deck area were examined as the
payload capacity was changed. The payload mass was
changed from 0-500 tonnes and the payload volume was
changed from 0-2000 m3. The trends for the three different
hullforms are presented in Fig. 3.
The TriSWACH hullform is assumed to provide seakeeping
advantages compared to other hullforms. While significant
experimental and full scale seakeeping data now exists for
the trimaran hullform, there is limited experience in
predicting the seakeeping performance of the TriSWACH
hullform. Other members of the ACCeSS consortium are
currently undertaking studies in this area. Earlier research
by two of the ACCeSS partners has identified the possibility
of heave-roll coupling in three hulled vessels (Grafton 2007,
Onas 2009).
3.2.4 Strength
Given the uncertainties regarding TriSWACH seakeeping, it
is currently difficult to quantify likely local and global
structural loads arising from combined vessel and wave
motions. However, the small waterplane area of the centrehull should lead to low longitudinal bending moments
compared to a monohull. Furthermore, the TriSWACH will
experience smaller prying loads than a comparable vessel
adopting a SWATH hullform. The potential applicability of
existing design methods to the TriSWACH (e.g. Lloyd’s
Register’s Rules for Trimaran Ships (Cheng, 2004; Lloyd’s
Register, 2006)).
(a) Monohull
3.2.5 Style
This paper has suggested that adopting a TriSWACH
hullform would allow a smaller vessel to achieve
comparable seakeeping performance to a larger vessel with
a conventional monohull. Any comparison of alternative
hullform options must consider designs able to perform an
equivalent role. In fulfilling the role other criteria related to
the operability of the vessel become important. Two areas of
concern identified at this early stage in the study are launch
and recovery operations (of the boats, aircraft and
unmanned craft required as part of the vessel’s operation)
and draught sensitivity (especially for TriSWACH vessels
with large fuel loads). The TriSWACH is seen to provide
opportunities related to layout and signature reduction.
4.0
(b) Trimaran
EARLY VESSEL SIZING STUDIES
Initial tool development efforts commenced with the
creation of a suite of numerical sizing tools for Monohull,
Trimaran and TriSWACH vessels. The procedure and data
underlying these tools were adapted from the guidance
given to UCL postgraduate students as part of their ship
design exercise (UCL 2011a, 2011b). These tools were
constructed in a manner that allowed the exploration of
potential alternatives through high-level comparative studies
between the three different options. The tools featured a
© 2011 American Society of Naval Engineers
(c) TriSWACH
Fig. 3. Trends in total displacement and weather deck area
with changing payload mass and volume
749
The irregularity seen in some of the steps from Fig. 3 arises
from the complex interaction of the changing payload with
the vessels overall size, geometry, resistance, power and
propulsion machinery. The nonlinearities found within the
different stages in this sizing process lead to nonlinear steps
between different options. It is clear that the different
hullforms provide radically differing amounts of upper deck
space as the payload is changed.
4.2
Indicative Solutions
Three of the design options produced by the numerical
sizing tool are detailed in Sections 4.2.1 to 4.2.3. These
point designs are indicative of the monohull, trimaran and
TriSWACH solutions the tool generated. Common design
specifications adopted across the three hullforms are given
below:

Accommodation for a crew of 30;

Installed propulsive machinery and fuel able to provide:

o
A loitering capability of 8 knots for 400
nautical miles (equivalent to ~2 days);
o
Slow fleet operations capability of 18 knots for
1000 nautical miles (equivalent to ~3 days);
o
Fast fleet operations capability of 24 knots for
1000 nautical miles (equivalent to ~1.75 days);
o
Fast intercept mission 30 knots for 500
nautical miles (equivalent to ~16.5 hours).
A modular payload space sized to support a payload
with a mass of 125 tonnes and a volume of 500m3.
4.2.1 Monohull
Table 1 contains the key characteristics of the monohull
solution designed to the common design specifications
given in Section 4.2.
Table 1. Monohull key characteristic
Property
Total Displacement (m3)
Length (m)
Beam (m)
Depth (m)
Draft (m)
Power for 30 knots (MW)
Mass Grp 1 – Structure (tonnes)
Mass Grp 2 - Personnel (tonnes)
Mass Grp 3 - Systems (tonnes)
Mass Grp 4 - Propulsion (tonnes)
Mass Grp 5 - Electrical (tonnes)
Mass Grp 6 - Payload (tonnes)
Mass Grp 7 - Variables (tonnes)
Value
1291
99.6
9.2
8.6
2.8
10.24
452.9
25.3
185.0
176.0
43.4
133.8
242.0
Table 2. Trimaran key characteristic
Property
Total Displacement (m3)
Mainhull Length (m)
Mainhull Beam (m)
Mainhull Depth (m)
Mainhull Draft (m)
Sidehull Length (m)
Sidehull Beam (m)
Sidehull Draft (m)
Total Beam (m)
Power for 30 knots (MW)
Mass Grp 1 – Structure (tonnes)
Mass Grp 2 - Personnel (tonnes)
Mass Grp 3 - Systems (tonnes)
Mass Grp 4 - Propulsion (tonnes)
Mass Grp 5 - Electrical (tonnes)
Mass Grp 6 - Payload (tonnes)
Mass Grp 7 - Variables (tonnes)
Value
1478
118.3
6.9
7.0
3.5
34.5
0.73
1.9
20.6
9.74
664.0
25.3
203.6
160.8
52.4
133.8
237.4
4.2.1 TriSWACH
Table 3 contains the key characteristics of the TriSWACH
solution designed to the common design specifications
given in Section 4.2.
Table 1. TriSWACH key characteristic
Property
Total Displacement (m3)
Mainhull Length (m)
Mainhull Beam (m)
Mainhull Depth (m)
Sidehull Length (m)
Sidehull Beam (m)
Sidehull Draft (m)
Bulb Length (m)
Bulb Diameter (m)
Strut Length (m)
Strut Beam (m)
Total Beam (m)
Power for 30 knots (MW)
Mass Grp 1 – Structure (tonnes)
Mass Grp 2 - Personnel (tonnes)
Mass Grp 3 - Systems (tonnes)
Mass Grp 4 - Propulsion (tonnes)
Mass Grp 5 - Electrical (tonnes)
Mass Grp 6 - Payload (tonnes)
Mass Grp 7 - Variables (tonnes)
Value
1556
96.1
4.2
2.1
50.8
0.55
2.7
96.3
4.4
67.3
2.2
35.3
14.30
653.1
25.3
224.6
138.3
70.4
133.8
310.5
4.2.2 Trimaran
Table 2 contains the key characteristics of the trimaran
solution designed to the common design specifications
given in Section 4.2.
750
4.3
TriSWACH Stability
During the development of the initial design options for
both the trimaran and TriSWACH the geometry of the three
© 2011 American Society of Naval Engineers
hulls and the side hulls separation were modified to ensure
sufficient stability. At the very early stage this only
consisted of consideration of the vessels intact metacentric
height. Clearly for a novel vessel shape such as the
TriSWACH ensuring that large angle stability results are
also acceptable is of upmost importance. However, when
the TriSWACH options were examined their GZ curves
were found to be unsatisfactory, with a significant re-entrant
feature that occurred above six degrees of heel (associated
with sidehull emergence) that led to negative values of
righting lever. Fig. 4 shows the GZ curve for one of the
TriSWACH point designs that was developed. Fig. 5 shows
how, for this TriSWACH point design, side hull emergence
occurs at these relatively low angles.
5.0
PARAMETRIC TOOL UNDER DEVELOPMENT
The difficulty presented by the TriSWACH stability issue
prompted a recognition that a parametric, geometry based
design tool able to rapidly explore a range of possible
options would be necessary to better examine future
alternatives. The initial concepts used to develop this tool
were derived from an existing ship design tool framework
developed by the UK Ministry of Defence that uses a
combination of ship design/analysis tools linked to
optimisation and concept exploration techniques (Cooper et
al. 2007; Horner, 2009). These tools aim to support early
stage design exploration by allowing a designer too quickly:

explore the design space for a given set of capabilities;

identify the
capabilities;

enable informed decisions to be made during
subsequent, more detailed design stages.
5.1
cost
implications
of
the
different
Implementation Details
The implementation presented in this early stage does not
have a complete ship synthesis capability (as presented by
Cooper et al. (2007) and Horner (2009)). It is envisaged that
this capability will be added in a later stage of the research.
The specific implementation relies upon two separate
computer programs:
Fig. 4. Initial TriSWACH GZ curve.

Paramarine – which provides a geometric modelling
capability together with a range of naval architectural
analysis capabilities (i.e. large angle stability analysis);

Matlab – which was employed as an external control
interface able to drive changes to the Paramarine ship
model.
These two tools are considered to provide the potential for
future development. The current ship model contained
within Paramarine could be enhanced to represent other
areas of interest (e.g. sizing, layout, structures). Matlab
provides diverse optimisation and analysis techniques
should these features be required in any later version of the
control interface.
6.0
Fig. 5. TriSWACH sidehull emergence.
It should be noted that this is not a general result for all
TriSWACH vessels. Rather, the particular geometry style
adopted within the initial numerical sizing model fails to
produce a solution with acceptable large angle stability
behaviour, even though its small angle behaviour satisfies
an applicable criterion (albeit one intended for monohull
ship).
Clearly, this behaviour is undesirable and options for
exploring possible amelioration strategies were studied. This
led to the development of a parametric tool able to examine
the impact of modifying different aspects of the hullforms
geometry.
© 2011 American Society of Naval Engineers
EXAMPLE APPLICATION OF PARAMETRIC
TOOL - INITIAL STABILITY STUDY
An initial demonstration of the tool was conducted to
explore the tool’s application to resolving the transverse
stability issues associated with the TriSWACH. In this case
the amount of side hull flare/tumblehome and the haunch
width/box width ratio were explored using the proposed
tool. The impact of these variations upon several key design
parameters was examined.
The side hull flare/tumblehome angle (β) was varied
between ten degrees of flare (β=-15deg) and forty degrees
of tumblehome (β=40deg). The haunch width to box width
ratio (Hw) was varied between minimal haunches (Hw=0.10)
and haunches extending almost the full width of the
underside of the box (Hw=0.99). The range of options that
were considered are illustrated in Fig. 6.
751
β=-15 deg
β=12.5 deg
Side hull flare / tumblehome
β=40 deg
Hw=0.10
Hw=0.55
Hw=0.99
Haunch width / Box width
Fig. 6. Illustration of variation in haunch and box width.
Using the range of geometries illustrated in Fig. 6
a matrix of intermediate points were examined to
determine how different design parameters varied.
In this initial study the parameters considered
included:

structural mass;

vertical centre of gravity;

righting lever up to an angle of heel of
30 degrees.
Fig. 7 shows an example of initial outputs
generated by this tool. Fig. 7(a) indicates how
structural mass changes with the overall geometry
while Fig. 7(b) shows how the change in
structural mass would lead to a shift in the vessels
overall vertical centre of gravity. As would be
expected, increasing tumblehome reduces the
breadth of the upper decks, which causes a
considerable decrease in their structural weight.
The removal of this weight from high in the ship
also causes a reduction in position of the vessels
vertical centre of gravity.
Next, Fig. 8 shows the variation in GZ against
heel angle for a range of values of side hull
flare/tumblehome and at different haunch
width/box width ratios. It is important to note that
the options in Figure 8(a), representing low values
of haunch width, all experience a significant reentrant GZ curve. This is denoted through the
darkly shaded region on the graph. Increasing the
size of any haunch significantly diminishes the
extent of this region, leading to more acceptable
stability
characteristics.
Side
hull
flare/tumblehome angle has a less pronounced
effect.
752
(a) Structural weight (te)
(b) Vertical centre of gravity (m)
Fig. 7. Example of initial outputs.
© 2011 American Society of Naval Engineers
(a) Haunch width / box width = 0.1
Fig. 9. Improve GZ curve (with sidehull flare of 15 degrees,
haunch width and box width ration of 0.99).
7.0
CONCLUSIONS
The early vessel sizing studies presented in Section 4 have
illustrated that the TriSWACH hullform could present an
alternative to existing hullforms. However, the issues arising
with the assessment of the TriSWACH large angle stability
highlight the importance of ensuring all aspect of any design
be reassessed to ensure unexpected issues do not arise
(b) Haunch width / box width = 0.52
The proposed parametric tool introduced in Section 5
provides a mechanism for rapidly exploring the effect of
variations to a number of different design variables. With an
appropriately constructed ship model, this provides the
designer with a tool able to develop variations from a point
design allowing the rapid development of an acceptable
solution. For a radical alternative, like the TriSWACH, such
a tool allows examination of the wider consequences of
design choices.
7.1
Future Work
Further work is required to better quantify the powering and
seakeeping performance of the TriSWACH hullform,
compared to monohull and trimaran alternatives.
Two key areas of future work at UCL have been identified:
(c) Haunch width / box width = 0.99
Fig. 8. Example of initial outputs.
Figure 9 shows the GZ curve for one improved solution,
further work is required to fully remedy the re-entrant GZ
curve. It would be interesting to investigate the impact of
deeper sidehulls and/or larger displacement sidehulls on
these results.
© 2011 American Society of Naval Engineers

Exploration of hydrostatic and hydrodynamic trade-offs
in the TriSWACH design space;

Exploration of marine engineering systems options on
the TriSWACH design space.
It is envisaged that these elements of work will be
undertaken as part of a continued research programme
undertaken by UCL and the other ACCeSS partners. Further
development of the explorative parametric tool presented
here will be conducted to assist meeting these research
goals.
753
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ACKNOWLEDGEMENTS
This work forms part of the ACCeSS project which is
supported by a grant from the US of Naval Research (ONR
Award No. N00014-10-1-0652). The assistance and support
of Dr Paul Rispin and Ms Kelly Cooper is gratefully
recognized.
The authors would also like to acknowledge the input of the
other ACCeSS partners to the study described in this paper.
Particularly Raju Datla and the other researchers from
Stevens Institute of Technology who provided experimental
TriSWACH resistance data that was employed in the vessel
sizing studies.
© 2011 American Society of Naval Engineers