CHAPTER 46
David Andrews
Multi – hulled Vessels
46.1
DESCRIPTION
46.1.1
Mission
Rather than a specific ship type configured to
undertake a transportation or operational mission,
multi-hulled configurations are options to be explored
by the ship designer in the early exploratory phases of
the ship design process. However it is clear that
certain multi-hulls have particular capabilities that
make them highly attractive options to be considered
in comparison with the ubiquitous monohull. Thus if
the design has to meet a requirement for a fast short
ferry route the designer may well choose a catamaran,
possibly with a wave piercing configuration. This form
which was first introduced on an economic basis in
Australia has become an extremely successful ship
type with an increasingly large portion of the fast ferry
market around the world and with an ever increasing
size range. Given this a separate chapter ( Chapter 46 )
is devoted to the high speed catamaran ferry type. This
chapter therefore focuses on the wider range of multihulls, and in particular the Small Waterplane Twin
Hull (SWATH) vessel and the Trimaran ship type,
namely a slender monohull with small displacement
outriggers. There are in addition variants of these two
basic configurations, such as the ( SLICE ) and the
Pentamaran, which will be considered as sub-variants
of the two main configurational types. Beyond them
are certain of the hybrid advanced hull forms, such as
the Surface Effects Ship (SES), which is really a
variant of the high speed Air Cushion Vehicle (ACV)
but with some buoyancy based lift from the two side
hulls is strictly a multi-hull, however it is being
covered in Chapter 44 as a High Speed Surface Craft.
There are also more exotic hybrids, such as the
(SWASH) and the TriSwash, which rather like the
(HYSWAS), can be considered currently as advanced
concepts yet to achieve even the maturity of the
SWATH and, more recently, the Trimaran.
46.1.2
These will be dealt with separately with any variant of
each type covered when and where it is deemed
appropriate.
The SWATH has had a much longer history,
both as a concept and as a practical ocean going ship
form, with many examples having been built and
operated around the world. The Trimaran is a much
more recent configuration, proposed as a seagoing ship
type only a decade ago and with currently (2001) only
one example, a 1,200 tonne vessel at sea as a
technology demonstrator for the surface combatant
naval vessel. Given that these two multi-hulled vessel
types have quite distinct characteristics, it is sensible to
deal with each in turn.
46.1.2.1 SWATH
The origins of a twin hulled ship
configuration with two deeply submerged hulls, struts
piercing the water air interface and being connected by
a box structure some distance above the still waterline
could be said to go back to a Captain Beadon of the
British Royal Navy, who proposed in 1860 such a
basic configuration, see Figure 46.1. This, as so much
technical exploration in the nineteenth century, came
to little due to the general explosion in commerce and
technology. It was also not developed at that time
because of a basic lack of understanding in the
burgeoning scientific knowledge related to the
engineering discipline of naval architecture, which was
fully employed in servicing the rapid developments in
steam powered monohulled vessels. The next
significant SWATH like proposal was by Creed (194?)
at the end of World War II. He produced a concept of a
twin hulled aircraft carrier, see Figure 46.2, which
exhibited the main features of the current SWATH
configuration, but did so primarily to achieve a long
wide and stable flight deck for aircraft operations
without the commensurate large monohull to support
it.
Unique Features and Capabilities
The defining characteristic of any multihulled vessel is clearly having more than one hull that
provides weight support, usually but not inevitably,
through buoyancy. As already stated in the previous
section, aside from the high-speed catamaran ferry
type covered in Chapter 44, the two principal multihulled vessel types are the SWATH and the Trimaran.
46-1
Figure 46.1 - Beadon's ICHYON of 1860
Figure 46.2 - Creed's 1946 Aircraft Carrier
Proposal
It could be said with the acquisition of a
scientific understanding of the nature of a ship's
seakeeping ability and the analysis of the ship's motion
in a seaway, made possible by the use of electronic
computers, that the inherent advantages of a form with
a very small waterplane ( and deeply submerged main
hulls providing buoyant lift ) was perceived in the
1960s. Thus several patents were filed in the U.S.A.
and research commenced on the SWATH form,
particularly at the David Taylor Model Basin (now the
Naval Surface Warfare Center, Carderock ). The
configuration that was seen to be the most suitable in
the quest for the ideal seakeeping form exhibited the
main characteristics shown in Figure 46.3.
SWATH will give rise to specific issues which will
need to be approached with care and resolved in a
manner appropriate to the particular application being
considered. These differences from the conventional
ship derive from the quite different overall proportions
of the SWATH, being greater in Beam, Depth and
Draught, whilst shorter in overall Length than the
equivalently sized monohull. What actually constitutes
a monohull equivalent of a given SWATH design is
not actually straight forward since for a given
performance, dependent on the mission the design is to
meet, different SWATH design solutions are likely to
apply. All that can be said at a generic level is that a
direct displacement comparison or even an enclosed
volumetric equivalence is unlikely to be a meaningful
comparison. This issue is considered in more depth in
Section 46.1.2 under seakeeping performance. The
differences between SWATH and monohull have been
identified and discussed by Kennell in 1985 and are
summarised below:( Brief summary of Kennell 1985)
46.1.2.2 TRIMARAN
( Evolution of Outriggers
ILAN VOYAGER
UCL/MOD Studies plus
Trimaran features
R.V.
TRITON
with illustrations
)
46.1.3
General Arrangements
46.1.3.1 SWATH
( Survey of a range of existing SWATH
designs. Photos and G.As
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

Kaimalino
Kaiyo
Halcyon
Victorious
Patria
Radison Diamond
Stena HSS 1500
Sea Shadow
SLICE
)
46.1.3.1 TRIMARAN
Figure 46.3 - The Main Features of a SWATH
Vessel
( Survey of a range of Trimaran designs
including studies with Photos and G.As
where available
 Cable and Wireless
 R.V. TRITON
 FSC concept
 Vosper Corvette
The SWATH vessel is considerably different
from the conventional monohull, requiring the
designer to appreciate that the different features of the
46-2
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46.2
SYSTEM DESIGN for SWATH
VESSELS
46.2.1Hull Form, Propulsion and Performance
46.2.1.1
rotational speed due to position aft of
elliptical hull, so high propulsive eff. Like
submarine (Kennell '92 p58)
Endurance depends on speed regime
appropriate to mission.)
UCL fast ferry model
UCL studies, including Frigates, small
Aircraft Carrier, large Cruise Liner
Pentamaran
Triswach
)
Hull Form
( Hull form essentially defined in Fig 46.3
with cross structure, struts, hulls (ellipitcal)
and haunch and wet deck.
Hull designed with either high prismatic or
low prismatic form, elliptical configuration
1.5 horiz by 1.0 vert.
Issue of strut length relative to hull length
for rudder arrangement.
Issue of one or two struts.
Issue of stabilisers.
Issue of wet deck with deflectors
(Kaimalino) )
46.2.1.2 Resistance and Propulsion
( SWATH suffers from calm water
resistance penalty cfompared
with
monohull of the same displacment.
Resistance made up of skin friction,
residual drag (wavemaking), appendage
drag.
Powering usually as monohull, that is,
openwater propeller eff., wake factor,
thrust deduction, RRE, and propulsive eff.,
but geometry results in quite different flow
regime into prop so resistance elements in
quite different proportions and powering
can be noticeably different.
Larger wetted area and have to consider
struts separately from hulls with different
Reynolds number.
Residuary resistance leads to complicated
picture with hump based on Froude
number on hull length:- Low speed vessels
displ >2000 t , Fn < 0.3
High speed vessels displ < 500 t
, Fn >0.46
Issue of prismatic forms summarise
Kennell '92 p51-54
Typical model to full scale correlation
coefficient 0.0005
Plus appendages
(Kennell '92 p 55)
Issue for the designer is lack of a
methodical series
Propeller design process as monohull but
scope for large diameter and lower
46.2.1.3Seakeeping
( SWATH primary attribute long natural
periods in heave, pitch and roll . ( E.g.
3000 t SWATH 11-13 sec Heave, 14-17
sec Pitch, 18-20 sec Roll , see Kennell '92
p65)
Steady platforming motion in moderate
seas (wave periods less)
High seas waves following contouring
motion ( wave periods more), so reduced
likelihood of wet deck slamming.
In between resonance probably following
seas, basic SWATH form poor cf.
Monohull but easily ameliorate with active
fin stabliser control, fins relatively large (
Kennell '92 p.64)
Hull form design for seakeeping then key
considerations : Probable sea conditions
 Operational speeds and headings
 Seakeeping criteria
 Hull parametrics
 Fin and control systems
 Avoidance of multiples of periods
 Adequate damping to limit heave and
pitch resonance
 Adequate wet deck clearance
 Acceptable performance in realistic
survival seas
(also see Betts '88 on operability index)
SWATH characteristics e.g. TAGOS 19 :Roll gyration 46% Beam cf. 40%
monohull
Pitch gyration 31% LBP
cf. 24%
monohull
Issue of Munk moment and pitch stability
Issue of active control
Comparative seakeeping
Kaimalino
- Tagos 19
- Kennell SNAME '85 plots )
46.2.1.4 Manoeuvring and Control
( SWATH has very good directional
stability.
Means turning circle is poor cf. Monohull
e.g. TD/L 1 at zero speed , 5 at 10 kts, 10
at 25 kts cf 4-7 for monohull, but
remember "equivalent ship" is shorter than
monohull, so TD can be the same value
absolutely.
46-3
Rudders usually ahead of propellers, spade
on aft of strut.
Or can adopt control fins cantered 20-30
deg. Downwards as in TAGOS 19. )
46.2.2
Payload Related Systems
( No specific systems for SWATH in
general, though if specialist mission then
there would be specific to mission
equipment e.g. TAGOS 19 as towed sonar
vessel has special winches as well as data
processing outfits. The SWATH relevant
issue is that the SWATH provides a very
stable platform together with widely
separated hulls making operation of arrays
in very high sea states very effective.
For certain research ship uses ( e.g. Kaiyo)
then incorporation of a moon pool is
facilitated by the short aspect ratio of the
cross deck box structure and the widely
separated hulls. The is still a need to check
that the insertion of a moon pool does not
infringe the structural integrity of the
transversely limited structural design
philosophy ( see section 46.2.3).
For comparison of payload abilities for
three main applications of the SWATH
concept see Kennell '92 p 81-82 ( Frigates,
TAGOS, Fast Ferries).
Question on validity of payload
comparison as SWATH justification more
likely to be one of operability (e.g.
TAGOS/PATRIA justified on excellent
seakeeping over straight payload carrying
ability).
46.2.3
Hull Structure
(Significant section - draw on Kennell' 92,
ISSC '88 and Betts)
( Typical structure Kennell fig 4.1.
Material usually steel - except small high
speed vessels< 1000 t typically.
Most loads on SWATH as for monohulls
except main hydrodynamic loads arising
from pressure forces which give both
primary and secondary loads.
Wave induced pressure distribution but
where vertical shear force and longitudinal
BM is dominant in monohulls they are
small in SWATH due to hulls slenderness
and section shape.
However in Beam seas the differential
distribution on the hulls leads to the
"prying" action, squeezing hulls together
and apart giving bending action on the box
structure transversely (see Kennell '92 fig
4.3
46-4
Thus alternating transverse side forces
applied near mid draught, these reach a
maximum value since the higher waves
with low probability of occurrence are too
long to cause the differential loading on the
two hulls (e.g. SWATH 3-4000 t then peak
side load at Sea state 6 ).
For reliable structural design assurance
need to perform model tests, but worth
earlier
analysis
using
essentially
seakeeping theory programs :- force induced on hulls by incident
seas not distorted by ship
- force due to diffraction of waves
about the statioary ship
- the effect of the ship's motion
Betts '88 suggests initial value of
design transverse load as F = K x Displ0.77 ,
where K
is 7.94 for single strut and 4.26
for double strut configuration based on
regression analysis.
Sikora '83 gives the results of
parametric variation in SWATH dimensions
for transverse force, but only for large
SWATHS >3000 t.
There is also an issue with
quartering seas where the torsional load can
the design of the strut bulkheads at the ends
of the vessel.
For combined loads Betts '88 gives guidance (
Table V )
Slamming is an issue for both the wet deck
and the strut panel design.
Given the importance of the transverse
bulkheads in resisting the primary transverse
loads great care needs to be taken in the panel
design against shear failure especially at the
bulkhead outboard most panels.
The structural design process is :- define the operational envelop
- define the load spectrum
- design structural scantlings for
selected design criteria
- conduct stress analysis
- conduct fatigue analysis
The scantlings will be determined
by the design pressures assumed for the slam
and hydrostatic loads when combined with
the framing and subdivision selected. In this
regard the shear lag effect limiting the
effective breadth of the deck structure acting
with the bulkheads in bending must be
allowed for in choosing the transverse
bulkhead spacing. ( Kennell '92 Fig 4.2).
A good approximation can be achieved by
using the panel and grillage design to resist
these slam and hydrostatic secondary loads.
To estimate the maximum load on the
transverse bulkheads mid bay to mid bay need
to resort to F.E.A. ( see Kennell '92 p. 34 ).
Structural weight fraction for a SWATH will
be greater than its "equivalent" monohull due
to a greater surface area ("wall paper") with
more bulkheads and greater loads likely. This
can mean the SWATH is typically some 25 30 % greater than the monohull and this can
be quite penalising if there is not an
overriding requirement for the SWATH's
superior seakeeping ability.)
46.2.4
Ship Outfitting
( Issues associated with boat handling given
the relatively high weather deck, but this is
mitigated by the greater degree of motion
stability.
This is also the reason why SWATHs , even
quite small in displacement are seen as
excellent as personnel ferries (eg Royal
Navy's two ferries to take inspection teams
out ships riding out beyond harbour.
The rough weather trials on TAGOS 19 (
USNEJ '9? ) revealed that in beam seas
plumes of water could rise vertically and
result in damage to boats and life rafts placed
at the ships side even with the
characteristically high freeboard on a
SWATH.)
46.2.5
concommitent increase in outfit weight and
cost. Against this must be weighed the fact
that the SWATH hull shape enables the motor
to be placed as far aft as possible to virtually
eliminate the shaft as a major cost and layout
feature.
As far as ship auxiliary systems are
concerned, the move to the all electric ship
technology will further simplify the ship
systems design task. It is the case that a
SWATH is likely to demand a greater hotel
load due to its larger exposed area to the
elements than it monohull equivalent. This
can be seen specifically with regard to
HVAC.
Another ship service that clearly is more
demanding in the case of the SWATH is the
ship's Trim and Ballast system, where there is
a clear need to maintain the waterline at the
design level (otherwise the seakeeping
performance will be off design and structural
loadings exceed design levels). In this regard
the SWATH is more akin to the military
submarine with a significant requirement for
trim and ballast tanks with the associated
pipe work and pumping capacity, albeit not to
submarine levels of pressure hardening. )
46.3
DESIGN ISSUES for SWATH VESSELS
46.3.1
Stability and Damage Stability
Machinery Arrangements
( A wide range of propulsion plant was fitted
in the early and relatively small SWATHs
from gas turbines and diesels using bevelled
gears, chain drives and direct drive
transmission.
Now that electric ship technology is
becoming the standard choice for cruise liners
in the commercial world and is the declared
choice for future naval combatants ( McCoy
'88, Newell 2000), this is providing
propulsion technologies that are ideal for
SWATH ships in terms of high density
motors that can more readily fit into the
SWATH's hulls and distributed electrically
generating prime movers that can be located
to exploit the more rectilinear internal
arrngement provided by the cross deck box
structure.
There may still remain the issues that there
have been in the current SWATH vessels at
sea, associated with motor cooling when
deeply located well below the normal
monohull machinery bilge level. The depth
issue is also relevant to the need for main
power cable runs of greater lengths with
46-5
( Given the small waterplane and the
constant value of the Tonnes Per Metre
Immersion (TPI) until either the haunches or
the top of the hulls are at the waterplane, then
the
ship
hydrostatics
are
boringly
uninteresting , see fig 46. xx. Again with the
two widely spaced struts, even with their
small waterplanes they provide sufficient
second moment to mean that the GZ curve for
a SWATH vessel is inherently good, see Fig
46. xx.
If the same criterion is applied for damage
stability as would be applied to a monohulled
warship (e,g, Sarchin-Goldberg or equivalent
) then typically a 15% opening is likely to
bring the box structure of a typical SWATH
vessel to the waterline. This is unlikely to be
a problem as shown by a design study for a
SWATH frigate ( Betts '87) where this
damage incident leads to a condition with a
21 metre hole in one hull resulting in the ship
heeling to 14 degrees, which would not
hazard the ship. Further more with the
generous provision of ballast tankage the ship
cold be readily restored to the upright, a
facility not available to the monohull
designer. The only proviso to be made in
exploiting this feature of the SWATH is that
the box structure and sensibly its internal
subdivision must be designed to be
watertight. )
46.3.2
Fire Safety
( This is not a major issue in large SWATH
vessels as they are not different from their
conventional monohull equivalents with
regard to susceptibility to fire and to the level
of fire fighting facilities required - always
assuming that the SWATH vessel is not being
used for the carriage of cargoes with
significant fire hazards. The issue for small
high speed SWATH craft and ferries is likely
to be more significant, as they are more likely
to be of aluminum construction and carry
automotive vehicles with flammable fuels.
However, again the design standards are
unlikely to be any different to those for their
monohull, and indeed catamaran equivalent.
As far as SWATH specific fire safety
considerations are concerned, it is likely that
the rectilinear short aspect ratio of the cross
decked box structure will facilitate both the
installation of fire fighting features and the
separation of escape routes in the case of
evacuation. The same can not be said for the
accessibility of the hulls via tall narrow struts,
however these are not going to be heavily
manned spaces, if occupied at all.)
46.3.3
and other multihulls, to the extent that
helicopter operations can continue in higher
sea states. Other evolutions such as towed
array operations on the US Navy TAGOS 19
Class vessels are possible well beyond the sea
states
that
limited
their
monohull
predecessors effectiveness. Again for military
vessels the deeply located propellers in
excellent flow regimes, combined with
electric drives, ensure the acoustic signatures
are optimised for the vessel's sonar
performance.)
46.3.5
( There are no major specific issues
associated with human factors as far as
SWATH vessels are concerned, as they are
not that different in their operations from their
conventional equivalents. Clearly the motion
stability and the separation of the working
and living areas from the services in the
deeply submerged hulls, provides an
essentially well designed environment for life
at sea. )
46.3.6
Environmental Issues
( Aside from any hazardous operations or
cargoes associated with a specific role or
mission there are no major environmental
issues specific to SWATH vessels as such.
The relatively deep draught of the SWATH
could in certain confined coastal waters be a
potential issue but this is more likely to limit
the choice of a SWATH form for certain
shallow ferry routes. On the other hand with
the very small waterplane and deeply
submerged propellers the SWATH might be
attractive where concerns over the wake
produced by more conventional displacement
craft is a major environmental concern. )
Comfort, motion, noise, vibration
(It is probably sensible to consider these
aspects for the small high speed SWATH
ferries separately from consideration of the
larger, generally ocean going specialist
SWATHs. Having said that, all SWATHs are
characterised by their superior seakeeping
qualities over other ship configurations, they
are certainly far superior to the basic fast
catamaran ferries, which suffer from
slamming and slam induced whipping leading
to fatigue damage in quite moderate
quartering seas and a resultant unpleasant
motion. SWATH ferries are thus less likely to
be weather limited as has been shown by the
success of the PATRIA on the mainland to
Balaerics route. Vibration and noise for the
smaller SWATHs is dependent on the
propulsion system adopted, but in general
deep propellers and excellent flow conditions
into the propellers eliminate vibration. For the
larger SWATH vessels motions are greatly
reduced, relative to conventional monohulls
Human Factors
46.3.7
46-6
Producibility
( Given the SWATH is largely a conventional
ship in technology terms, it is just the
configuration
that
is
different,
the
producibility considerations are also largely
similar to conventional ships. Thus SWATHs
are built in normal shipyards appropriate to
small high speed ferries or larger
conventional steel ships. In fact with
essentially a prismatic box, strut and hull
shape it ought to be cheaper and easier to
build SWATH ships than the more curvacious
monohull equivalent as ththe shpe is more
conducive to modular constuction. There is
some evidence for this in the one class of
sizeable SWATH ships built for the US Navy,
TAGOS 19 Class. These ships were built in a
Gulf Coast yard specialising in basic support
vessels for the offshore industry, and were
able to build the SWATH ship quickly and
cheaply as a class. Having said that larger
SWATHs become problematic to launch
unless the yard has a very large ( and wide )
dry dock, or in the case of the 3000 t TAGOS
ships, the use of a massive floating crane. )
46.3.8
-
-
Preliminary Design Methods
( Draw on Betts Shaghai'88 and UCL MSc
procedure.
-
The initial design procedure for a SWATH
vessel is more complicated than for its
conventional equivalent, as there are many
more variables that need to be taken into
account from the beginning in order to get a
first estimate of ship size. This does have the
advantage that that the geometry of each
separate part, namely box structure, struts and
hulls, can to some degree be considered
separately, although they must then be
sensibly brought together, as their final
selected dimensions are interactive with each
other in achieving a balanced design. The
complication is further compounded, as with
all novel hull forms, by a basic lack of design
data and in particular sufficient parametric
guidance.
Betts '88 describes the existing design tools
for SWATH preliminary design and Betts and
Kennell
'92
outline
the
important
considerations in the choice of the various
features :- A clear preference for a single strut
per side hull, rather than twin
configuration, for reasons of greater
stability, better access to the hulls,
greater space available in the struts
and
improved
structural
arrangements
- Additionally a single strut per hull
gives reduced drag by elimination of
wave interference effects between
the struts, improved survivability
and better behaviour in extreme seas,
where a twin strut configuration will
not contour as well, inducing wet
deck slamming
- Choosing the strut length to be less
than the hull, reduces resistance and
pitch excitation, whilst easing
alignment of LCF and LCB,
46.3.9
46-7
minimising coupling of pitch and
heave, however a short strut means
the rudder arrangement is likely to
be of the hull rather than the simpler
spade arrangement behind the strut.
Strut separation is likely to be
chosen to suit layout concerns and
achieving the requisite transverse
statical stability
Box clearance is likely to be driven
by the need to clear the design
significant wave height when
platforming, although this can then
be the driver on structural weight
fraction
Flare on the strut sides can be
helpful against the effects of weight
growth but can impair seakeeping in
moderate seas, hence is restricted
often to well above the waterline and
then assists in plume deflection in
beam seas. )
Future Developments
( The priority areas seen as requiring to be
addressed if the currently research dependent
nature of SWATH design and operation, are
seen as those that have been identified in the
several international conferences that have
been held on SWATH vessels specifically or
on advanced and unconventional craft
including SWATHs as a particular theme. In
particular the following have been highlighted
:- accurate predictions, validated by
full scale trials, of motions
- development of realistic operability
criteria and the evaluation of
performance against these criteria
- accurate and validated structural load
prediction
- rational structural design tools
- refinement of control system design
- realistic costing methods, including
through life cost
- improved CAPD tools for principal
parameter selection
- refined damage stability criteria,
including
for moderate heel
conditions
- improved auxiliary system design,
linked to the all electric ship concept
It has to be said that consistent with the
general comment that SWATHs
are
essentially unconventional hulls applied to
conventional ship technology, that many of
these issues also apply to improving
conventional ship design and performance.
Finally it is worth listing the range of ship
types where the SWATH configuration is
seen as having distinct potential :- small and medium ferries
- hydrographic and surveying vessels
- mine counter measures ships
- luxury yachts and small cruise ships
- Anti- Submarine Warfare escorts
- Offshore Patrol Vessels
- Small and moderate speed helicopter
carriers )
46.4
SWATH SHIP CHARACTERISTICS
( Table I Betts '88 amended by Kennell '92
and any more recent vessels - SEA SHADOW
etc )
46-8