HYDRODYNAMIC PERFORMANCE COMPARISON
BETWEEN TWIN HULLS
Campana E.F.; Peri D. ; INSEAN
Via di Vallerano, 139 – 00128 Rome, ITALY
Zotti, I. ; University of Trieste, D.I.N.M.A.
Via A. Valerio, 10 – 34127 Trieste, ITALY
Abstract
When designing a twin hull, resistance and seakeeping are operative
requirements which often clash each other. It very well known that SWATH
hulls, having classical lines, display good seakeeping features, but bad
resistance performance. Conversely catamaran hulls, when appropriately
designed, display satisfactory resistance features, but the seakeeping qualities
are less satisfactory, unless additional submerged flaps are fitted.
To improve the SWATH resistance of a new design hull, the lower hulls
and the struts geometry were optimised at INSEAN research centre in Rome,
by using numerical methods, obtaining a more slender and faired configuration.
This new geometry significantly modifies the general features of a classical
SWATH hull, converting it to a MWATH hull, the resistance characteristics of
which was optimised.
The resistance, trim and sinkage and the wave pattern resistance were
measured and compared for two models, a catamaran and a MWATH, having
the same length, width and demihull distances; the most appropriate speed
intervals were defined for every tested condition. This investigation also permits
defining the most suitable operative configuration for a specific design by using
these hulls.
Introduction
The choice of the multi hulls found an appropriate utilization in those
fields where the use of large spaces on board and the transportation of
cumbersome, but light weights are required. Passenger transportation on short
or mean distances or the use of research or support ships are some specific
fields where the twin hull geometry application found its largest attainment. The
availability of areas on board for passenger and car transportation in the first
case, and the need of spaces for laboratories and equipments, such as the
opportunity of an helicopter deck installation, in the second case, are operative
factors which determine the choice of these hulls. Their use for goods transport,
such as containers or pallets, is very limited, although some possible
applications were found for ships used in fast feeder and coastal transport on
short and mean distances.
The most common types of twin hulls can be classified in Catamaran and
SWATH (Small Waterplane Area Twin Hull) hulls. From these basic hulls new
lines were derived for the Wave Piercing Catamarans and the MWATH
(Medium Waterplane Area Twin Hull) hulls, to improve their efficiency at higher
speeds (Figure 1). It is very well known [1, 2] that SWATH ships respond less to
waves than conventional displacement ships, also because a large partition of
their displacement is well below the water surface, so that the wave exciting
force are lower for SWATHs than for conventional displacement ships.
However, in the traditional SWATH hulls, having a geometry with small
waterplanes and long cylindrical parallel displacing bodies, with circular or
elliptical sections, the resistance increases very much with the speed.
Figure 1 : Twin hull types
An ocean-going catamaran is, on the contrary, more vulnerable to wave
impacts, specially on the bottom of the cross deck in severe sea conditions,
which may cause speed reduction, local structural damages and transient hull
vibrations. But its resistance is more favourable and depends strictly on its
speed and separation to length ratios (S/L) between the demihulls. The Wave
Piercing Catamarans constitute an important development of the catamaran
concept, in which the distance between the hulls has been enlarged, the hull
length increased and the hull lines sharpened, to reduce the resistance and
improve the seakeeping features.
On the contrary, the SWATH hull features can be improved by minimizing
the calm-water resistance, the wave resistance and the seakeeping
performance by contouring the hull lines. The geometric results obtained
consist, in general, in more faired hull forms. A description of an interesting hull
optimisation method for SWATH hulls is reported in [3]. In this specific case a
similar procedure was developed at INSEAN Centre, by using numerical
methods for the resistance and wave pattern resistance calculation.
The tested hulls
With the experimental survey, the most suitable application range of the
investigated hulls was searched. Particulars of the models used for the tests are
given in table 1. The lines of the hulls are shown in figures 2a and 2b.
Table 1 : Particulars of the tested demihull models
Model
LWL (m)
LOA (m)
BWL (m)
BMAX (m)
T
(m)
∇ (dm3)
WS (m2)
CB
Catamaran
MWATH
1.250
1.314
0.100
0.130
0.088
5.060
0.231
0.451
1.250
1.344
0.107
0.142
0.127
8.440
0.362
0.497
The models were built in fibreglass, owing to its low weight, and tested at
the towing tank of the DINMA Department of the University of Trieste. All the
tested models were fitted with turbulence stimulators. Five testing series were
made for the condition given in table 2 and defined by the separation/length
ratio (S/L).
Table 2 : The tested conditions
Condition
1
2
3
4
5
S/L
0
0.18
0.23
0.28
0.33
S (m)
0
0.225
0.288
0.350
0.413
Figure 2a: The catamaran hull lines.
Figure 2b : The MWATH hull lines.
The experimental tests
Calm water total resistance, running trim and sinkage and wave pattern
analysis were carried out for the demihulls and the twin hulls of catamarans and
MWATHs. All the tests were performed, where possible, up to a speed range of
Fn = 0.7, which corresponds to Reynolds Numbers from 0.53 to 3.0 * 106. The
wave pattern analysis was carried out by using the longitudinal cut method of
S.D. Sharma [4]; multiple longitudinal cuts were made at different distances
from the models, by using five capacitive probes; one probe was placed
between the two hulls, to record the composite wave pattern and to investigate
on the wave breaking phenomena.
The total resistance of a catamaran hull, represented in dimensionless
coefficient form, can be expressed [5] as :
CT = (1 + βK) CF + τ CW
(1)
where β = 1 and τ = 1 are used for the demihull in insulation. The form factor
[(1+K) for the demihull; (1+βK) for the twin hulls] was calculated by using the
2.80
Prohaska method. Its value for the
demihulls was respectively 1.183 for
the catamaran and 1.04 for the
MWATH. The β viscous interference
factor is shown in figure 3, versus the
ratio S/L. The results for the total and
wave pattern resistance of the models
are
presented,
through
their
dimensionless coefficients CT and CWP,
in figures 4a,b and 5a,b. The wave
resistance interference factor τ, defined
as :
MWATH
2.40
ß
2.00
1.60
Catamaran
1.20
0.16
0.20
0.24
S/L
0.28
0.32
0.36
Figure 3 : The viscous interference factor β
τ = (CW Twin hull/ CW Demihull) = [CT – (1+βK)CF]Twin hull / [CT – (1+K)CF]Demihull (2)
is shown in figures 6a, b. In figures 7a,b the graphs giving the changes of bow
14
16
MWATH
Catamaran
Demihull
DEMIHULL
S/L = 0.18
S/L = 0.18
12
S/L = 0.23
S/L = 0.23
12
S/L = 0.28
10
1000*CT
1000*CT
S/L = 0.28
S/L = 0.33
S/L = 0.33
8
8
6
4
0.20
0.30
0.40
0.50
Froude Number
0.60
0.70
0.20
0.30
0.40
0.50
Froude Number
0.60
0.70
Figures 4a, b : Total resistance coefficients for the tested hulls
and stern levels are shown; they are given by using the ratios FP/L and AP/L,
where FP and AP are the fore and aft perpendicular levels, measured on the
tests. From these graphs the trim and sinkage variations are easily obtained as
:
tg θ = FP/L + AP/L and MSV = (FP/L + AP/L) * L/2
(3)
where θ is the trim angle and MSV is the mean sinkage variation.
Discussion of results
The following consideration can be deduced from the figures.
A) On both the twin hulls the resistance coefficients increase up to a speed
corresponding to Fn ≅ 0.45; then they decrease.
B) By increasing the ratios S/L, the resistance coefficients decrease, specially
for Fn > 0.45.
C) The largest resistance coefficients correspond to MWATH hulls, whereas the
catamaran hulls present smaller values.
6
8
Catamaran
MWATH
Demihull
Demihull
S/L = 0.18
S/L = 0.18
6
S/L = 0.23
S/L = 0.23
S/L = 0.28
S/L = 0.28
1000*Cwp
1000*Cwp
4
S/L = 0.33
4
S/L = 0.33
2
2
0
0
0.20
0.30
0.40
0.50
Froude Number
0.60
0.70
0.20
0.40
Froude Number
0.60
0.80
Figures 5a,b : Wave pattern resistance coefficients for the tested hulls.
2.00
Wave resistance interference
factor - MWATH
2.50
Wave resistance interference
factor - Catamaran
S/L = 0.18
S/L = 0.18
S/L = 0.23
S/L = 0.23
1.60
S/L = 0.28
2.00
S/L = 0.28
S/L = 0.33
S/L = 0.33
1.20
1.50
0.20
0.80
0.30
0.40
0.50
Froude Number
0.60
0.70
1.00
0.20
0.30
0.40
0.50
Froude Number
0.60
0.70
Figures 6a,b : Wave interference factors for the tested hulls.
D) The largest CT coefficients are found for the catamaran hulls at Fn = 0.475
and for the MWATH hulls at Fn = 0.50; when the S/L ratios increase, the
maximum values shift towards slightly smaller Fn numbers.
E) The Cwp coefficients are larger for MWATH hulls, but for these hulls a
minimum value is found for Fn ≅ 0.35. For this condition also the wave
interference factors are < 1.0, both for the catamaran, as for the MWATH
hulls.
F) When the differences CW – CWP = CWB are calculated, the wave breaking
phenomena can be examined. It is very easy to verify that these coefficients
are larger for the MWATH hulls. The wave breaking phenomena are
generated mostly by the interference effects due to the waves between the
hulls. These phenomena were recorded by the central probe and the largest
wave breaking effect was noticed for the MWATH hulls.
Figures 7a,b: Bow and stern levels for the tested hulls.
G) The trim and sinkage variations present the largest excursions for Fn ≅ 0.55
for both the hulls, but the MWATH hulls generate also a large trim by had at
Fn ≅ 0.40 and a deep sinkage at θ = 0°, when Fn ≅ 0.45.
H) The wave interference effects are favourable only at low speeds, in particular
for Fn < 0.375; the positive wave interference interval is wider for the
catamaran hulls and for high S/L ratios.
I) The viscous resistance components are larger for the MWATH hulls, and
decrease by increasing the S/L ratio, both for the catamaran, as for the
MWATH hulls (figure 3).
Conclusions
This preliminary comparative investigation on twin hulls reveals that the
examined hulls present a number of favourable hydrodynamic characteristics
and their practical application is more promising when the suitable velocity field
is well defined. Assuming that the seakeeping characteristics are more
favourable for the MWATH hulls, certainly the most advisable hull is the
MWATH when navigating in open sea at Fn < 0.45. At higher speeds and in
calm water the catamaran solution is certainly the best. This statement can be
confirmed also by comparing the specific resistance RT/Δ or the total resistance
coefficient CT * 1000 for a catamaran and a MWATH hull, having the same
ratios S/L. In figures 8a, b two cases are shown for the extreme cases S/L =
0.18 and S/L = 0.33. Similar trends were verified also for S/L = 0.23 and 0.28.
When choosing a hull, it is very important to set in evidence that the MWATH
hulls have a greater displacement and a wider wetted surface, at the same
ratios S/L and at the same length LWL. It follows that MWATH hulls are more
suitable to be designed for the transportation of heavier cargoes at lower
speeds in open sea, whereas the catamaran design is more appropriate for
faster ships at lower displacements. In the first case research, exploration and
16
120
1000*Ct model comparison
1000*RT/Displ.
MWATH, S/L = 0.18
MWATH, S/L = 0.18
Catamaran, S/L = 0.18
Catamaran, S/L = 0.18
MWATH, S/L = 0.33
12
MWATH, S/L = 0.33
RT/Displ. * 1000
80
1000*Ctm
Catamaran, S/L = 0.33
8
Catamaran, S/L = 0.33
40
4
0
0.00
0.20
0.40
Froude Number
0.60
0.80
0.00
0.20
0.40
Froude Number
0.60
0.80
Figures 8a,b : Resistance components comparison between the hulls.
recovery ships for civil and military use can be suggested; also the use for long
range slow cruiser ships, for voyages near the Polar Oceans or around the
Pacific Ocean islands could be investigated. The fast transport of passengers in
protected areas is the most suitable use for the catamaran hulls; the installation
of submerged fins for the motion damping could be useful for open sea
navigation.
References
[1] Kathryn K. Mc Creigh; “Assessing the Seaworthiness of SWATH Ships”,
Trans. S.N.A.M.E., vol. 95, 1987, pp. 189 - 214
[2] Choung M Lee; Richard M. Curphy; “Prediction of Motion, Stability and Wave
Load of small Waterplane-Area, Twin-Hull Ships”, Trans. S.N.A.M.E., vol. 85,
1977, pp. 94 – 130.
[3] Nils Salvesen, Carl A. Scragg, Christian H. von Kerczeck, Carl A. Scragg,
Christopher P. Cressy, Michael J. Meinhold; “Hydro-Numeric Design of SWATH
Ships”, Trans. S.N.A.M.E., vol. 93, 1985, pp. 325 – 346.
[4] Eggers K.W.H., Sharma S.D., Ward L.W.; “An Assessment for Determining
the Wavemaking Characteristics of a Ship Form”, Trans. S.N.A.M.E., vol. 75,
1967, pp. 112 – 144.
[5] M. Insel, A.F. Molland; “An Investigation into the Resistance Components of
High Speed Displacement Catamarans”, Trans. R.I.N.A., vol. 133, 1991, pp. 1 20.
Nomenclature
Demihull :
AP :
BMAX :
BWL :
CB :
CF :
CW :
CWB :
CWP :
CT :
Fn :
FP :
LOS :
LWL :
MSV :
RT :
S:
T:
V:
WS :
1+K:
Δ:
β:
τ:
θ:
One of the hulls, which make up a Twin Hull
Aft Perpendicular
Maximum Breadth (m)
Water Line Breadth (m)
Block Coefficient
Frictional Resistance Coefficient (ITTC ’57)
Wave Resistance Coefficient
Wave Breaking Resistance Coefficient
Wave Pattern Resistance Coefficient
Total Resistance Coefficient
Froude Number
Fore Perpendicular
Length over Ship (m)
Water Line Length (m) ; L = LWL , in general
Mean Sinkage Variations (m)
Total Resistance (kg or N)
Separation Distance between centreline of Twin Hulls (m)
Draught (m)
Speed (m/s or knots)
Hull Wetted Surface (m²)
Form Factor
Hull Displacement (kN or tonnes or kg)
Viscous Resistance Interference Factor
Wave Resistance Interference Factor
Trim angle (°)
Acknowledgments
The Authors are grateful to Mr. S.T.V.G.N. Marco RIZZA, of the Italian
Navy, for his collaboration in the experiments, data processing and analysis of
the results and to Mr. S. BERGAMINI for his collaboration in the preliminary
design of MWATH hull.