iiiJaU±u.
Fast ships
Comparative Evaluation of Seakeeping
Prediction Methods
Volker Bertram, Hidetsugu IwashitaMöglichkeiten und Grenzen veschie-dener Methoden zur Berechnung des Seeverhaltens schneller Schiffe wer-den anhand eines schne!lcn Fischcr boots untersucht. Die Methoden sind Streifenmethoden, "high-speed strip theory" HSST, 3-D "Green Function Method" und Rankine-Singularitäten-Methode. Experimentelle und rechne-rische Zwischenergebnisse (hydro-dynamische Koeffizienten, erregende Kräfte) werden verglichen, um Fehler-quellen zu identifizieren. Streifen-methoden erwiesen sich in ihrer Anwendbarkeit begrenzt auf Froude-Zahlen unter 0,4.
Atechnical
Seakeeping Qualities of High-Speedcommittee on "Study of Vessel" was organized under the Marine Dynamics Research Committee of Japanto study the validity and limitations of
strip theory to estimate seakeeping
quali-ties of high-speed boats, This is a digest
of the Japanese report, Takaki and
Iwash-ita (1994), with some additional inter-pretations. The figures shown here are only representative samples. We omitted results for a second high-speed boat as they gave hardly any additional insight.
Die Autoren:
Dr.-Ing. VolkérBertram arbeitet am
Institut für Schiffbau der Universität Hamburg, Hidetsugu Iwashita ist Pro-fessor an der Universität Hiroshima.
Notation follows ETTC standard unless
specified otherwise.
Modei Experiments
Ship motion experiments were
perform-ed in a towing tank for a model of a
typical Japanese hard-chine fishing boat.
Fig. 1. Tables I and II. Table III gives
sinkage and trim (positive for bow im-mersion) for conventional resistance test ("still water") and for the ship in waves
(time-averaged value). The model's
eigenperiods of heave and pitch were
both 0.9 s. The experiments determined heave and pitch motions and surge force in regular head waves for F = 0.4. 0.6. and 0.8. Wave lengths were varied from
2JL I (L = L) to 4. The towing point was at the longitudinal position of the
Rg. 1: Body plan of fishing boat
Table I: Offset data of fishing boat (full-scale, L = L ' 9.90 m; A -0.05543 L, B -0.06086 L)
í' Laboratorium
'oor
Sheep3hydr3rne,ti
Mskeq Z 2628 CD !Ofr
- .- -lIt' center of buoyancy. 17.8 cm (= 9.7% L)above the base line. For high speeds.
con-siderable spray formed in the forepart of
LII uudci. Siajidaid wuvc ilcigili was
4.5 cm. Experiments with 9 cm wave height determined the degree of
non-linearity in some cases. None of the
mo-tions were affected by nonlinearity. Only
heave acceleration showed a weak
non-linear influence for F = 0.8.
Forced oscillation and exciting force ex-periments were performed for F,, = 0.4 and F,, = 0.6 in a model basin to supply
intermediate results necessary to evaluate the accuracy of the calculations methods. However, unlike in the motion
ex-periments, trim and sinkage were sup-pressed. The amplitude of the forced
oscillations was 2 cm, the frequency pa-rameter w 11g ranged from 1 to 25. The
exciting force experiments were
perform-ed in regular head sea of wave
steep-ness = 1/50. The wave length was
varied from XJL = 0.5 to 1.5. Added mass
coefficients are defined here A,1 = A
-C/w, where A,
are the usual addedmass coefficients and C1 are the restoring coefficients.
Estimation results by
strip theory
12 strip methods were compared. Table 1V. (NSM = New Strip Methods. STE = Salvesen-Tuck-Faltirisen. OSM =
Ordi-nary Strip Method; LF = Lewis form. CF
= Close-Fit; w0 = method follows
Mizo-guchi (1982) using w0 to compute
ex-10XIL
Y Z Y Z Y Z Y Z Y Z Y Z Y Z B 0.0 0.34 1.04 0.41 1.04 1.02 1.19 1.10 1.19 1.73 A 0.0 0.32 1.05 0.42 1.05 0.98 1.20 1.04 1.20 1.68 AP 0.0 0.32 1.06 0.42 1.06 0.945 1.20 1.00 1.21 1.625 0.0 0.28 1.07 0.39 1.08 0.87 1.23 0.93 1.22 1.56 2 0.08-0.55
0.08 0.20 1.09 0.325 1.10 0.81 1.23 0.89 1.23 1.50 3 0.15-0.49
0.15 0.13 1.12 0.26 1.12 0.77 1.25 0.83 1.25 1.43 4 0.22-0.42
0.22 0.055 1.14 0.21 1.14 0.73 1.28 0.79 1.28 1.41 5 0.22-0.35
0.22 0.0 1.15 0.16 1.16 0.70 1.29 0.75 1.30 1.39 6 0.22-0.28
0.22-0.06
1.12 0.13 1.15 0.70 1.30 0.75 1.32 1.45 7 0.22 -0.21 0.22-0.08
1.07 0.16 1.12 0.76 1.28 0.80 1.32 1.45 8 0.17-0.14
0.18-0.10
0.92 0.26 0.99 0.87 1.12 0.92 1.24 1.44 1.24 1.56 8.5 0.14-0.10
0.15-0.08
0.75 0.37 0.84 0.945 0.97 1.00 1.14 1.52 1.14 1.63 9 0.11-0.07
0.48 0.53 0.66 1.05 0.73 1.12 1.02 1.61 1.03 1.74 9.5 0.03 0.48 0.15 0.703 0.38 1.18 0.44 1.24 0.86 1.71 0.87 1.85 FP 0.0 1.34 0.70 1.81 0.70 1.96Table Il: Principal particulars of fishing boat model (model scale
1:5.38), with respect to center of gravity
citing forces, o regular approach). 6
methods accounted for end effects by
in-cluding "end terms". (Strip theory
ex-presses the forces (and moment) on the ship as integral over the ship lengths
in-volving derivatives with respect to x.
Partial integration can transform this
in-tegral yielding terms which have to be
evaluated at the ends. These terms con-tain the added mass which is zero at the bow. For transom sterns with separating flows, a non-zero contribution remains
called "end term".) All results shown
here are for the design draft, i.e. not
con-sidering trim and sinkage. Trial
computa-tion with experimental trim and sinkage showed no significant change which is probably due to the wall-sided geometry
of the ship.
Heave added mass A33 and pitch added
mass A55 were generally underpredicted,
Fig. 2a. Only methods E and G predicted A55 well. Damping for heave B33 and
pitch B55 was generally overpredicted
es-pecially for long waves. The coupling
terms A35, A53, B, and B5- agree better
with experiments than the main terms,
especially for methods including end ef-fects. Fig. 2b. Surge exciting forces were
typically predicted to only half the
meas-ured value, Fig. 3. However, the surge
exciting force is small compared to heave and pitch exciting forces making large
re-lative errors more acceptable. Also, it is
difficult to measure surge forces and
measured values may therefore also be wrong. Heave and pitch exciting forces
agreed well with experiments for F5 =
0.4. Only pitch exciting moment was somewhat underpredicted for XJL > I.
For F5 0.6 the heave exciting force
disagreed for XJL< 1 and pitch exciting
moment for AIL> 1.
Heave and pitch motions agreed well
with experiments for close-fit methods.
Fig. 4. For F,, = 0.6. Lewis form methods
sometimes strongly overpredicted heave
and pitch. Added mass measurements
showed considerable scatter for AIL < i
making comparison in this range not
sen-sible. Close-fit methods generally
pre-dicted added resistance better than Lewis
form methods but also tended to
overpre-diction for i < AIL < 2.5. Takaki and
Iwashita (1994) compare the distribution
of 2-D heave added mass coefficients
over the length of the ship as predicted by
the various strip methods. Differences become pronounced towards the ship
ends. The effect of these differences on
'J I 1.! I (. f/...jJ.j') i " 'I l
Table Ill: Trim and sinkage (model sca e), values indicated by* are for 9 cm instead of 4,5 cm wave height
A,.
Addcd o,,,., od doo,png cooffi,o,*,., ro, I,o,.o, Add,d m.. ond d*m5fl onthcno, o,
Add,d,00,odd,.mp,oco,th.00fo,p*o
07.
Add,d n,o .od d**p,og co ic,00, io, p,o,h
L
Fig. 2a: Hydrodynamic coefficients (main terms),F,, = 0.6:
A33/pV, B3:,IpVüi,. (top), A55IpV, B55IPVWe(bottom)
Lewis form method Close fit method
8,. 0-08
.8>
Fig. 2b: Hydrodynamic coefficients (coupling terms),F5 = 0.6: A35IpV, B35IpVü. (top),A5.IpV,B53IpVw. (bottom)
Fn sinkage (cm) trim (deg)
still averaged still averaged
water in waves water in waves
0.4 1.18 0.60 0.80 0.72 0.6 1.65 1.52 - 1.98 - 1.91 1.54* - 1.76° 0.8 0.77 0.88 - 1.59 - 1.54 1.04* 1.30* LOA 2.543m D 0.158m A 52.3 kg 1.840m 01180m LCB 01632m B 0.428 m Tf 0.0586 m 0.275 .0
Lewis form method
Add,d,00,.*odd*A0p*co.thc,'ocn b,cocn o,.od p,c.rO Add m,,.0 ,nd d.oping co,.lc,o,o,
bnSo,*o h,000 .,d p,nob
Add,d ,o,.*o ood doc*pio co,th*.nco
bnccc., pAcb o,d b.00,
Add,00**ddoop,0000ffic,,n,o
global values. e.g. motions, will be small
because the 2-D added mass coefficients are multiplied by relatively small values
(cross section area or square of the water-line width) at the ship ends in the
integra-tion to global values. Lewis form
me-thods generally predicted smaller added
mass and larger exciting forces than close-fit methods in the middle of the
ship. The OSM differed largely from the other methods in the prediction of 2-D added mass but performed not worse in
the prediction of 3-D added mass and
damping.
The end terms were found to be
impor-tant in computing the hydrodynamic
coefficients for this ship due to the tran-som stern and the high speed. Computed hydrodynamic coefficients including end terms agree well with experiments.
Sub-sequently, motions also agreed better
especially around XJL 2, Fig. 5. For
XJL = 2. time-domain calculations based on Chin and Fujino (1991) gave virtually
the same results as strip methods. So in
this case, nonlinear effects play no role as
the ship is wall-sided over most of its
length.
Table IV: Compared strip methods
ooI
fo, pb
we xc 'wfo,c fo, so's.
W,.oex'w oo,,coo fo,
Close t method
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Fig. 3: Exciting forces,F5= 0.6:F/pg Ç5BL. FJpgÇ,BL (top). M0/pgÇ5BL (bottom)
xc,,, ,g fo rc fo, b
So,s..00,00fb.,bpohe.doe. Hcc,.moo,o,,of,h.,bjp,,b,4o,.oes
A,ldcd oOoe ,o'ec,c,, berd
Fig. 4: Motion amplitudes and added resistance, F,, 0.6: X/Ç,,, Z/Ç5 (top). 91k-Ç,, and
R,,,,/(pg Ç (B2/L))(bottom):
white circles denote added resistance without ship motions (diffraction only)
Strip method 2-D repr. aft append. mcl. (A+B) frequency exc.force End effect
ANSMLF
B STF LF
+ a +CNSMLF
w,. +DNSMLF
ENSMLF
+ w,.FNSMLF
w,,GNSMLF
HSTFCF
w.INSMCF
+ o +JNSMCF
+ o +KNSMCF
+ w,. +LOSMCF
+ w,. +Fig. 5: Effect of end terms on motions,F,, =
0.6: Z/Ça (left). 6/k Ç (right):
with end terms, - - without;
white circles denote results using
experi-mental hydrodynamic coefficients and excit-ing forces
Estimation results by other methods
Strip theory neglects most forward-speed
and 3-D effects. Therefore. High-Speed Strip Theory (HSST). 3-D Green
Func-- 'N...L..4 (r'Ffl\ _._.-J 0._i.:..., C.-...__.-....
joli ISICLIIOU t'.J11i) ciilU i'e.Ui!ro.ILIL )LJULL..t..
Method (RSM) were investigated. Vari-ous authors have contributed to the de-velopment of HSST. e.g. Faltinsen and Zhao (1991 a. b). Ohkusu and Faltinsen (1990). HSST formulates the problem in terms of linear potential flow theory in
the frequency domain. The numerical
so-lution for the flow around the hull starts at the bow. The free-surface conditions are used to step the solutions on the free-surface elevation and the velocity
poten-tial on the free surface in the longitudinal
direction of the hull. The velocity poten-tial for each cross section is found by a two-dimensional analysis. Transom stern
effects are accounted for by assuming that the flow leaves the transom stern
tangentially in the downstream direction
so that there
is atmospheric pressureat the transom stern and the transom
stern is dry. The method is applicable for
F,, > 0.4. when the transverse wave sys-tem of the ship becomes less important.
because the linearized solution of the HSST accounts only for the divergent wave systems generated by the ship.
Takaki et al. (1995) give a summary of
the involved equations.
Fig. 6a. b show HSST and GFM
predic-tions at F, = 0.6. Strip method K is
in-cluded for reference. HSST divided the ship into 100 strips. Each strip used 40 segments on the hull and 100 on the free surface. The GFM (direct method, con-stant strength per panel, line integral and rn-terms neglected) used 292 panels on the hull. All 3 methods predict the
cou-pling terms(A35. A53. B35. B53) accurately, but underpredictA33andA55.On average,
HSST performs best, strip method worst
also for B33 and B55. All 3 methods underpredict heave exciting forces for X./L < 1. roughly by a factor of 4. On
average, again HSST performs best, strip
method worst. GFM overpredicts heave and pitch motions by up to 50% around
?JL = 2. This is mainly due to poor predic-tion of added masses. HSST predicts
mo-tions better than GFM and strip method,
but still overpredicts pitch for long waves
by some 20%. This is probably due to
poor prediction ofB55.HSST gives better
results than GFM but is twice as fast. For
2.0
1.5
1.0
0.5
0.0
11eue IflotlollS of hc .slii ii head 'tICS
Fig. Sa: Hydrodynanlic coefficients,F,, = 0.6:
A33IpV,Bs3/PVw. A35IpV.B35IpVw (top). A55IpV, B55IPVco. A 53/pV, B5-,/pV ú (bottom)
o
O Fn0.0
Fig. Sb: Exciting forces and motion amplitudes,F,, = 0.6:
F8/pgÇ5BL. F/pg Ç5BL, M0/pgÇ5BL2(left),X/Ç5, Z/Ç5, 0/k Ç5(right)
Pitch rootiolts of the ship in head wav
0,,
0.01-Add,d oo od do,optog co,ifetoo,a lo, hopo,
505.0 oe00000dOotoptopoo,ffic,o,t,lo,pttch
t, Lo l0
HSST fOg,,,oFM. Stop Method
Add,,, m,t ond d..,opng co,dlotop., be.o.eo h... ocA path
Atoo,00,00,o,tddo,op,Oioo,iiiae000 b,to.enpath..ndh..o. ton, F. -dt 3D g,.. , F M. Stop MomoS Oc. /40 "t
Path otto,, of th. nhtp io booS
'-s 'eDT 3D genee F M. Sn,p f.to0000 2.0 1.5 1.0 0.5 0.0 0.0 180 .<6 -t
Stai, notton.. 01,5, ha e 50.5 0LO.0 5c, 001000 of the nbtp e hoed SVtoeoottOtOOloOColOClt000C
practical purposes. HSST seems to be the best choice. However, the relatively poor
performance of the GFM could be due to the neglect of the rn-terms and the line
integral. Unfortunately. no results includ-ing these two contributions are available.
Calculations were also performed by a
Rankine source method. The radiation
condition was fulfilled following
Sciavou-nos and Nakos (1988). The free-surface
grid extended 0.25L before and 0.75L
behind the ship and L in transverse
direc-tion using 80 x 20 panels . The ship was discretized rather coarsely by 292 panels giving a total of 1892 panels. For com-parison. Sciavounos and Nakos (1993) claim that 4000 panels are necessary to
compute seakeepings flow around
yachts. The transom stern of the ship re-quires special care in panel arrangement on the free surface near the aftbody. The
chosen grid featured a rapid change in
width for the panels on the free-surface at
the transom stern. Otherwise it was
smooth. The steady disturbance of the
free surface and in the rn-terms was
ig-nored. There are no experimental data for
F = 0.2 but results at this Froude number
appear plausible compared to results for a
surface-piercing ellipsoid of same Frou-de number. Attempted computations for
F = 0.4 and 0.6 could not obtain any
reasonable results. Calculations were
very grid-sensitive near the stern. These
problems do not occur for usual ships
without transom sterns. Obviously tran-som sterns require special treatment. e.g.
following the approach of Sciavounos
and Nakos (1993).
Conclusions
1. The benchmark test showed that strip methods were valid only to roughly
F<0.4.
For ships with transom sterns, end
ef-fects are important and should be con-sidered.
HSST ("high-speed strip method")
is at present the most suitable
me-thod for fast ships for practical
pur-poses.
CPU time requirements.
time-consum-ing grid generation and difficulties
in modelling transom sterns make 3-D
Green function method and Rankine source methods at present unsuitable
for practical applications.
References
Chin. F. C.; Fujino. M. (199!). Nonlinear predic-tion of vertical mopredic-tions of fishing vessel in head
sea. J. Ship Research 35/1
Faltinsen. O.; Zhao. R. (1991a). Numerica! predic-tions of ship mopredic-tions at high forward speed. Phil.
Trans. Royal Soc., Series. A, Vol. 334
Faltinsen. O.; Zhao. R. (1991b), Flow prediction
around high-speed ships in waves. Math.
JJnnI
Approaches in Hydrodynamics. SIAM. ed. Miloh Mizoguchi. S. (1982). Exciting forces on a high
speed containership in regular oblique waves
-Frequency selections for calculating exciting forces by the strip method. Kansai Soc. Nay. Arch. 187 (in Japanese)
Ohkusu, M.; Faltinsen. O. M. (1990). Prediction
of radiation forces on a catamaran at high
Froude number. 18. Symp. Naval Hydrodyn.. Ann
Arbor
Sclavounos. PD.; Nakos. D. E. (1993). Seakeeping
Die EU-Kommission hat jetzt ein internationales Konsortium mit einer umfassenden Untersuchung zur
Verlage-rung von Gütertransporten auf die Binnenwasserstraßen beauftragt. Das
Forschungsprojekt ,,Shifting Cargo to
Inland Waterways" wird über das 4. Rah-menprogramm für Forschung,
Ent-wicklung und Demonstration (FTD) der
EU finanziert. Das Konsortium be-steht aus dem Europäischen
Ent-wicklungszentrum für die Binnenschiff-fahrt e.V. (EBD). Duisburg, dem
Oster-reichischen Institut für Raumplanung (OIR) und dem Architecture Navale
Analyse des Systèmes de Transport
(ANAST). Das EBD ist Koordinator des
Projektes ..Shifting Cargo" (Kurz-bezeichnung).
Zielsetzung
Um den Verkehr nicht zum Hemmnis der wirtschaftlichen Entwicklung Europas
werden zu lassen, ist es dringend
er-forderlich, Transportalternativen zu
finden. Eine wichtige Alternative bildet
die Binnenschiffahrt. die noch über
erhebliche Kapazitätsreserven verfügt.
Erklärtes politisches Ziel ist daher eine verstärkte, ökonomisch und ökologisch sinnvolle Einbindung dieses
Verkehrs-trägers.
Bisher hat jedoch eine
nennens-werte Verlagerung auf das Binnenschiff
kaum stattgefunden. Vielmehr
bevor-zugen Verlader und Spediteure
weiter-hin den Lkw. der schon auf vielen
Magistralen an die Grenzen seinerKapa-zität gestoßen ist. Hauptziel von .,Shifting Cargo" ist es deshalb, nach
den Ursachen dieser Entwicklungen zu fragen und hieraus Konzepte und
Strate-gien für eine stärkere Nutzung der
Binnenwasserstraßen abzuleiten. Damit
leistet das Forschungsvorhaben einen
grundlegenden Beitrag, um das von der
Europäischen Union angestrebte Ziel
einer auf Dauer tragbaren Mobilität zu
erreichen.
and added resistance of IACC yachts by a
three-di-mensional panel method. Il. Chesapeake Sailing
Yacht Symp. Annapolis
Takaki. M.; Iwashita. H. (1994). On the Estimation Methods of the Seakeeping Qualities for the High
Speed Vessel in Waves. Applications of Ship
Moti-on Theory to Design. 11th Marine Dynamics
Symp.. Soc. Naval Arch. of Japan
Takaki, M.; Lin. X.; Gu, X; Mori. H. (1995).
Theo-retical predictions of seakeeping qualities of high-speed vessels. FAST '95. Travemünde '
Inhalte
Um das angestrebte Ziel zu erreichen,
gilt es zunächst, das
Verlagerungspoten-tial zugunsten der Binnenschiffahrt zu er-mitteln und Möglichkeiten seiner
Bewäl-tigung im Rahmen der vorhandenen Ka-pazitäten aufzuzeigen. 1m Vordergrund stehen jedoch die Fragen, welche Fakto-ren bisher eine nachhaltige Verlagerung auf die Wasserstraßen behindert haben und welche Voraussetzungen erfüllt sein müssen, damit das Binnenschiff dort, wo
es sich anbietet, problemlos in die
Trans-portkette integriert werden kann. Ein
wichtiger Baustein zur Beantwortung
dieser Fragen ist eine ausführliche
Befra-gung der Verlader und Spediteure. Den
Abschluß der Untersuchung bildet die
Erarbeitung von Verlagerungskonzepten
und -strategien, die einmal auf die
Anbie-ter der Verkehrsdienstleistungen. zum anderen auf die politischen
Entschei-dungsinstanzen zugeschnitten sind.
Das Projekt wird zwei Jahre dauern und beinhaltet im einzelen folgende
Arbeits-schritte
1. Verlagerungsmöglichkeiten
- Analyse des Verkehrsaufkommens - Abschätzung des
Verlagerungspo-tentials
- Vergleich von
Verlagerungspoten-tial
und Kapazität der
Binnen-wasserstraßen 2 Verlagerungshemmnisse - schriftliche Befragung - ausführliche Interviews - case studies Verlagerungssvoraussetzungen füreine verstärkte Integration der
Bin-nenschiffahrt in die Trarisportkette
- technische Optimierung - operative Optimierung - marktliche Optimierung
Verlagerungskonzepte und -strategien
- Szenarien möglicher Entwick-lungspfade
- Handlungsempfehlungen für
Poli-tik und Gewerbe
- Auswahl von Pilotprojekten. 'i
