On sway damping and added mass in shallow water

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TECHNISCHE HOGESCHOOL DELFT

AFDELING DER MARITIEME TECHNIEK

LABORATORIUM VOOR SCHEEPSHYDROMECHANICA

ON SWAY DAMPING AND ADDED MASS IN

SHALLOW WATER

W. Beukelman

17th International Towing Tank Conference

Proceedings Volume 2, 8-15 September 1984

Gótenborg, Sweden.

Report No. 603-P

Ship Hydromechanics Laboratory - Delft

Delft University of Technology Ship Hydromechanics Laboratory Mekelweg 2

2628 CD DELFT The Netherlands Phone 015 -786882

17TH INTERNATIONAL

TOWING

CON FERENCE

SSPA

PROCEEDINGS

VOLUME 2

8-15 SEPTEMBER 1984

GOTEBORG, SWEDEN

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W. BEUKELMAN - Ship Hydromechanics Laboratory, Deift University of Technology, Delft, The Netherlands.

ON SWAY DAMPING AND ADDED MASS IN SHALLOW WATER

I agree with the Recommendation of the Committee to continue the development of theoretical and empirical methods for the determination of hydrodynamic and aerodynamic forces acting on a

ship.

A suggestion would be to add and to stress the importance of the shallow

water case.

Recent work in this field has been carried out by the towing tank of the Delft University of Technology.

Forced oscillation tests with a segmented model have been performed

188 8520c 202.0127 o 0.26 Experiment f A w - 0.50 0 w 0.75 w 0.26 - - - Calculation ( £ u 0.50 w 0.75 + - Static 8 50

Fig. 1 Measured and calculated

distribu-tion of Y" for h/T = 2.4, 1.5

v and 1.15

to obtain added mass and damping distributions in shallow water ranging from 2.4 to 1.15 T. The experiment covered a low frequency range to be of interest for

manoeuvring including static tests.

Fig. 1 shows the distribution of sway damping forces for three

waterdepth ratios. The dynamic test results agree satisfactorily with the static values as well as with the results of a calculation based on a two-dimensional multipole

approxi-Experiment o = 0.26 A w = 0.50 o = 0.75 w 0.26

- - - caicuiationf £ w

Fig. 2 Measured and calculated

distribu-tion of Y for h/T = 2.4, 1.5 V

and 1.15

mation. It should be remarked that no viscosity effects are included in

the calculation. The comparison

indicated a rather important influence of viscosity in the aft part of the

model.

Fig. 2 compares the sway added mass as

found in the experiment with

corres-ponding calculated values.

Except for the smallest waterdepth ratio (1.15) the agreement of the distributions is satisfactory.

In general the agreement is better for larger waterdepth ratios and

higher frequencies of oscillation and further investigations are

required with respect to the

distribution of the damping

coeffi-cient in shallow water.

Reference

[11 Beukelman, W. and Gerritsma, J.,

"The longitudinal distribution of low frequency hydrodynamic deriva-tives for lateral motions in

shallow water",

Ship Hydromechanics Laboratory, Delft University of Technology, Report no. 562 A and no. 603 P,