Research for evaluation of underkeel clearances in finnish waterways

O. Fluuska

RESEARCH FOR EVALUATION OF UNDERKEEL CLEARANCES IN FINNISH WATERWAYS.

2ND TEMPORARY REPORT:

MEASUREMENTS OF THE VERTICAL MOTIONS IN TRANSIENT SHALLOW WATER WAVES

M-38 _TT-36/72

INTRODUCTION..

This work represents one part in a research programme for the evaluation of underkeel clearances in Finnish waterways. The Finnish Board of Navigation finances this work. This

interim report suminarises some first experiments done at the Ship Hydrodynamics Laboratory of the Helsinki University of Technology. The purpose of the first tests was to get experimental material for the comparison with the theore-tical results given by Beck and Tuck /1-3/, by Kim /14/ and by Guliev /51. For this reason the ratio h/T was chosen equal to 1,5. So far the tests have been carried out only at zero speed. in head waves.

EXPÉRIMENTAL DETAILS.

2.1 General arrangement.

The model used in the experiment was Series 60 hull form with the following particulars:

Length between perpendiculars 3,50 in

Beam 0,50 "

Draft 0,20

Block coefficient 0,70

Longitudinal radius of gyration 0,25 x

The tests were run in the manoeuvring basin, whiôh .hasthe dimensions ¿f 140 x 140 m. The plain smooth

area, which has the accuracy of ±1 mm, contains 32 x 32 in. For other details, see fig. 1. Notice that small surge movements are possible. The mooring arrangement shown in fig.1 was chosen because the cross-coupling of surge with pitch änd heave is small. As the wave generating, element acts the front panel of the wave generator. The panel can be set in piston, in flaplike or ii combined motion.

2.2 Instrumentation..

difficult to produce harmonic waves. Nearly the same procedure as outlined in /6-7/ has been used.

Converging transient wave packet was produced by controlling the revolutions of the driving motor of the wave generator through the output from an apparatus of the type "Data-Trak't. The stroke of the panel of the wave generator was constant (: 50 nun) during these tests.

The wave form wa measured with a capasitive wave gauge. The gauge consists of two teflon coated thin wires.

The vertical movements of the model were measured w.ith two ultrasonic detectors installed at both ends of the model. The detectors measured the distance

to the bottom of the basin.

The signais from these devices were transmitted via cables to the instrument room, where they were

amplified and registered on a paper tape and on a plotter. (See fig. 3)

3. DATA PROCESSING

Due to some limitations in our dataiogg'ing system the sampling rate was times /second/ channel. According to the éxamples in /6/ this may be too small a sampling rate. As the wave spectrum in question is band limited, we do not think this has any serious effect on results.

The. digitized signals were analysed later in UNIVAC.1108 computer. The calculations included were:

- multiplication of the signals with calibration coefficients

calculation of the pitch, heave and vertical

movements of the bow and stern at perpendiculars from the ultrasonic signals

Fourier transformation of ali these five things mentioned above

smoothing of the Fourier transfOrms (Manning procedure five times)

- calculation of the amplitude response function.

. RESULTS

The results from four runs are shown in fig. 2, , 5. In

fig. 6 the mean results are shown together with the theoretical predictions. The theoretical curves are

composed by putting together the predictions of Feck /3/ and of Elm /14/ The results of Guliev /5/ are also shown.

(Guliev makes some simplyfying assumptions in order to reduce calculation work.) As Tuck and Eeck indicate ii

Il-31 their calculations would give best results when Lpp/Lw is nearly one. On the other hand, Kim's theory would give the best results when L/L is about two or more. Tests done so far support this remark.

5.

The most difficult thing in the measurements has been the function of the wave gauge. So far the results from this device are not so reliable as it was hoped. This

can be se&n from fig. 2 and 4. In fig. 2 the harmonics of the wave form are shown. As it can be seen, the curves differ quite a lot in spite of the fact that the waves were produced by the same Data-Trak programme. On the other hand, the harmonics of the motion of' the bow do not differ

so much in separate runs. The scattering of the results

of pitch and heave in fig. 5 is mainly due to the improper s

function of the wave gauge.

As it is mentioned earlier, the low sampling rate ( c/s) may affect the results. However, this sampling rate is

sufficent for analysing the most crucial harmonics from the .poiht of view of a ship's movements. Moreover, the smoothing (Hanning five times) process used reduces the 'Tpike effect" of' the low sampling rate.

The length of the samples had only a slight effect on the results when one saw to it that the sample did not include reflected waves.

6. CONCLUSION

These are thé results of the first tests run in our

research programme. Experiments have been already carried out at hIT equal to 1,25 and 1,10, but the analysing work has not been completed yet. When water comes shallower the linearity assumption mày be violated more and more.

Another question is the effect of viscosity when trans-ferririg results of model tests to full scale. This effect must also be considered when comparing theoretical

predictions with model tests.

Thèse tests indicate that the theories of Tuck and Beck and Km and Guliev, when used together, give a reasonable approximation of the vertical motion of a ship in head seas at least when h/T = 1,5 and. at zero speed.

REFERENCES

/1/ Beck, R.F., Tuck, E. O., Heave and Pitch of Ships in Shallow Water, Fourth Austral-Asían Conference on Hydraulics and Fluid Mechanics at Monash University Melbourne, Adelaide 1971

/2/ Beck, R. F., Tuck, E. O., Computation of Shallow Water Ship Motions, Paper presented at 9th SNH in Paris

1972, Adelaide 1972

/3/ Beck, R. F., Present Status of the Slender-Body Theory

for Ship Motions in Shallow Water, University of Michigan, 1973

/14/ _{Kim, C. H., The Influence of Water Depth on the Heaving}

and Pitching Motions of a Ship Moving in Longitudinal Regular Head Seas, Schiffstechnik, Band 15, Heft 79, 1968

/5/ Guliev, Ju M., Heaving and Pitching Motions of Ships

in Shallow Water, Paper to be presented at 13th ITTC, 1972

/6/

Research and Development Center, Report 2960, Washington, April 1969

/7/ Takezawa, S., A Practical Method for Testing Ship Models in Transient Water Waves, Selected Papers from the

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