TECHSCNE UNVER$1TEJT .aboratorjum voor Scheepshydromeej,pJ Archief Mekelweg 2,2628 CD beift TeL' 016-786873.. Foc018. 781838
Seakeeping performance assessment of planing hulls
G.J.Grigoropoulos & T.A.Loukakis
Department of Naval Architecture and Marine Engineering,
National Technical University ofAthens, 15773 Zografos, Greece
Abstract
A new systematic series of planing hull fOrms, with good characteristics with
respect to both resistance and seakeeping
is under development at the
Laboratory for Ship and Marine Hydrodynamics of the National Technical University of Athens. Before the selection of the parent hull, five "equivalent" models with the same main particulars and different hull forms have been designed and tested for resistance and seakeeping. The seakeeping performance of three of these models, both in regular and random waves, is presented and discussed in the paper.
1
Introduction
The seakeeping performance of any sea-going vessel should be assessed with respect to the sea states its mission requires to be able to operate in. Hence, describing, comparing and assessing the seakeepmg characteristics of a ship on
the basis of limited-range regular wave results can be misleading and/or
inadequate and the same is true when trying to decide if analytical results can beuseflul for prediction purposes. Thus, appropriate random wave testing is
necessary for prediction purposes, either per se or for validation of analytical prediction methods.
Planing hull forms possess some additional characteristics, which make their seakeeping testing more difficult: they move at high Froude numbers and their kinematic responses are known to be non-linear with respect to the wave
amplitude, at least in some wavelength ranges. In addition, they experience dynamic lift underway, which makes their underwater displaced volume to vary with respect to speed and, hence, further complicates any attempt for analytical prediction of their seakeeping responses.
In view of the above, the title of this paper is certainly overambitious and
only some interesting, we hope, pieces of information with respect to the
seakeeping performance of planing hull forms are going to be presented in the sequel.2
Description of the hull forms
The five "equivalent" hull forms used for the selection of the parent hull form of the new series have been described in Grigoropoulos and Loukakis [1]. In the present paper, part of the test results in both regular and random waves for three of these hull forms will be presented. Two of the hull fonns have narrow transom, the first being a normal L/B=5.5, Series 62 hull form (Clement and Blount [2]) and the second a deep-V single chine variant of the above (Keuning and Gerritsma [3]). The third hull form is a wide transom, same L/B, double chine hull form (Savitsky et al. [4]). The body plans of the three hull forms are shown in Figures 1 a to 1 c. All tests have been conducted with the models at the same displacement (29.7 kp), the same static trtm (nearly even-keel) and the same radius of gyration (0.25 LWL). All models were wooden, had an overall length of 2.2 in, were appended with appropriate spray rails and have been tested in various Wave conditions for model speeds up to 4.5 rn/s or Froude number 1.032.
3
Some results of the testing program
As it is very well known in the seakeeping community, seakeeping results have a sinister way to vary, even when all test conditions are kept constan see for example the Seakeeping Committee Report of the 18th I.T.T.C. [5]. This real life phenomenon is demonstrated in Figure 2, where the regular wave results for the deep-V hull form are shown. The response shown is the vertical acceleration at the C.G. and at Froude No. = 1.032 .and at each wavelength, the experiment
was performed three times. In the context just mentioned, the results are
pronounced very satisfactory and, after testing in a similar manner for all
responses, the rest of the testing program was performed with single runs.The linearity test for the same model, the same response and the same Fr.No. is shown in Figure 3. The results are substantially non-linear about the peak of the R.A.O. and less so for the lower frequencies of encoUnter (longer wavelengths).
As it will be discussed shortly, strip theoiy predictions for the responses of
planing hull forms are substantially higher than the experimental values,
especially near resonance. The situation is better for long wavelengths (e.g. high sea states with large peak spectral periods). This fact is demonstrated in Figure 4 for the same hull form, ship response and speed and again in Figures 13 to 16.Nevertheless, the fact that strip theory [6], which is a high frequency
theory, with the hull forms at their static waterline, gives reasonable results at the lower frequencies, with the hulls at the planing condition, is yet another curiosity associated with this theory. The predictions might even become better if an older version of the strip theory is used [7]! Unfortunately no theoretical prediction method exists for the seakeeping behaviour of planing hull forms at any Froude nwnber of practical interest. In this context, the theory of Martin [8] is valid for Froude Nos above 1.3 and at these speeds the operability of a vessel is in doubt for any sea state of serious magnitude.
In Figures 5 to 8 and 9 to 12 the experimental response amplitude
operators of pitch, bow acceleration, stern acceleration and added resistance are shown, for model speeds 3.0 m/s (Fr.No.=0.688) and 4.5 rn/S (Fr.No.=1.032) respectively and for the three hull forms. The theoretical predictions for the double chine hull form are also shown in all cases, with the added resistance computed as in [9].
Two conclusions can be drawn from these figures : that the theoretical predictions are much higher near resonance and the seakeeping behaviour of the double chine hull form is overall better than that of the two others. This is an interesting result if one takes into account that the calm water resistance of the doublechine hull form is overall lower than the resistance of both the deep-Vee and the series 62 hull forms.
However, to the experience of the authors, carefully conducted random wave experiments are a much better yardstick for the comparative study of seakeeping behaviour. Thus, in Figures 13 to 16, the rms values for pitch, bow acceleration and stem acceleration and the mean added resistance of the three
hull forms are shown at Froude No =0.688 and for sea states with modal
periods 6 to 10 sec. These results cOrrespond to ships of 22 m length overall (scale ratio of 10) and the full scale testing was about 16 mm.
From these figures, the overall superiority of the double chine hull form is verified.
4
Conclusions
The conclusions from the results presented can be summarized very briefly as follows:
o Testing for seakeeping in regular waves for planing hull forms is not very meaningful.
o On the contrary, testing
in sea states and at speeds where the vessel can operate can provide really useful information.o However, the cost of conducting the great numberof runs, necessary to get sufficient testing time for statistically reliable results, can be excessive.
o In this respect, a systematic series of hull forms tested for both resistance and (random waves) seakeeping, could be useful.
o The up to now results of our investigation point out that the double chine, wide transom hull form with warp is appropriate to be the parent hull for the
series to be created.
5
Acknowledgements
A significant portion of the tedious experimental work for the new series is being done by last (5th) year students in the framework of their diploma theses. In this respect, the authors are indebted to Mrs. S.Peppa and to Messrs. E. Latsos, C. Dousis and M. Pasalaris. The same holds true for the carriage crew Messrs. M. Nounos and F. Kasapis.
6
References
Grigoropoulos, G.J. & Loukakis, T.A. Effect of spray rails on the
resistance of planing hulls, 3rd Intl. Conf. on Fast Sea Transportation
FAST 95, Travemuende, 1995.Clement, E.P. & Blount, D.L. Resistance tests of a systematic series of planing hull forms, Trans. SNAME, Vol. 71, p. 491, 1963.
Keuning, J.A. & Gerritsma, J. Resistance tests of a systematic series of planing hull forms with 25° deadrise angle, Intl. Shipbuild. Progress, Vol. 29, No. 337, p. 222, 1982.
Savitsky, D., Roper, J.K. & Benen, L. Hydrodynamic development of a high speed planing hull for rough water 9th Symp. Naval Hydrodyn., ONR, Paris, p. 419, 1972.
Report of the Seakeeping Committee, Proc. 18th Intl. Towing Tank Conf, Vol. 1, p. 401, Kobe, Japan, Oct. 1987.
Salvesen, N., Tuck, E.O. & Faltinsen, O.M. Ship Motions and Sea Loads, Trans. SNAME, Vol. 78, 1910.
Blok, J.J. & Beukehnan, W. The high-speed displacement ship systematic series hull forms - Seakeeping characteristics, Trans. SNAME, Vol. 92, pp. 125-150, 1984.
Martin, M. Theoretical prediction of motions of high-speed planing boats in waves, Journal of Ship Res., Vol. 22, No. 3, pp. 140-169, Sept. 1978. Grritsma, J. & Beukelman, W. Analysis of the resistance increase in waves of a fast cargo ship, Intl. Shipb. Progress, Vol. 19, pp. 285-293, Sept.1972.
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Figure 1: Body plans of the models Figure 4: Analytical and
cxpenmen-tested. tal results for the Deep-Vee model.
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8.0Figure 7: Experimental stem vertical acceleration R.A.O.'s for the three models and analytical predictions for the double-chine model at 3.0 rn/s.
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Figure 5: Experimental pitch R.A.O.'s for the three models and analytical pre-dictions for the double-chine model at
3.0 rn/s.
Figure 6: Experimental bow vertical acceleration R.A.O.'s for the three
models and analytical predictions for the double-chine model at 3.0 rn/s.
50 4° a 30 l0 0 0.0 2.0 4.0 5.0 )./1 6.0 tOO 6.0 10.0 0.0 2.0 4.0 5.0 8.0 tOO )IL.
Figure 8: Experimental added resi-stance R.A.O.'s for the three models and analytical predictions for the double-chine model at 3.0 rn/s. 4Ô4 wa. SER DEEP DOua.E ooua.z 62 (Ocp.) VtE (tp. CI4N. CHIN: .3.0 Exp.) Strip m/, Theory) VmoI 4'4k1 e59 *4.aL,
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Figure 1! :Experimental stern vertical acceleration R.A.O.'s for the three models and analytical predictions for the double-chine model at 4.5 rn/s.
10.0
Figure 9: Experimental pitch R,A.O.'s
for the three models and analytical pre dictions fOr the double-chine model at
4.5 rn/s.
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Figure 1 Q:Experimental bow vertical
acceleration R.A.Q.'s for the three
models and analytical predictions for the double-chine model at 4.5 m/s.
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Figure 12: Experimental added
resi-stance R.A.O.s for the three models
and analytical predictions for the double-chine model at 4.5 rn/s. 4'4&. 000RD ta.*a SCRIS DEEP DOU1 62 (Ocp.) vtE (t.) CHIN CHIN - (Exp.) (Strip Theory) - DOU.E VmoI = 4. m/ 80 60 4A&A Q99I ta.ea, -SCRIL DEEP 00uE D0UZ 62 (Op.) VEE CII N CHIN - (Exp.) (Strip Theory) Vrnodel -4 .$ m/. 0.0 2.0 4.0 8.0 Afl in' 4'
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Figure 13:Experimental r.m.s. pitch
response for the three models and
ana-lytical predictions for the double-chine model at 3.0 rn/s.
100
MODM. PERiOD (eec)
Figure 15: Experim. r.m.s. stern vert, acceleration response for the three models and analytical predictions for the double-chine model at 3.0 rn/s.
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Figure 14:Experim. r.m.s. bow vert. acceleration response for the three models and analytical predictions for the double-chine model at 3.0 rn/s.
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Figure 16: Exper mental mean added resistance for the three models and analytical predictions for the double-chine model at 3.0 rn/s. e'A SERIES QQg, DE.EP t&&& DOUBLE tJ.it DOUBLE 62 VEE (Lip. CHII4( CiuWt Cip.) Exp.) Strip 'heory) vr 3.0 r/i ,wS;RIES 9099, DEP'i Va (Fip.)62 U8L( CHIWE UBLE CHIWE -3Dm/s xp) ((ip.) (Strip hcory)
