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trials location may be such that a small speed loss is incurred. The measured trial speed may then be augmented by a speed correction, and the prescribed way so far was by a formula from Lackenby (1963). However, this formula was often found inadequate.

A new trial correction procedure has now been proposed [3]

. From the main

dimensions and block coefficient, the viscous resistance of the ship and its increase in shallow water are estimated. The wave resistance is supposed unaffected as long as the depth Froude number is limited; but an additional correction for the effect of the increased dynamic sinkage is applied. The procedure thus estimates the power increase in shallow water at equal speed; for which the trial measurement may be corrected.

ITTC procedure With the support of the ‘Sea Trial Analysis’ (STA) Group, speed trials have been done for three ships at full scale in several water depths. Invariably the new procedure estimated the shallow-water speed loss much better than Lackenby’s method. It is now being considered by the ITTC for general acceptance [4]

.

With all these steps, a much better understanding has been obtained of what happens in shallow water [5]

Significant changes of the wake field for decreasing water depth

. For the same

model tests, the figure shows how we can explain the large difference between the measured model resistance in deep and shallow water. We added the two empirical contributions from the shallow-water correction method to the deep-water resistance curve: the increase of the viscous resistance, and the increase of resistance due to the additional dynamic sinkage. Thus we come quite close to the actual measurements in the Shallow Water Basin, corrected for the tank wall effect; the remaining discrepancy being the shallow- water increase of the wave resistance.

While a decade ago these model test results just had to be taken for granted, now we understand precisely what is going on. An understanding reflected in improved and consistent solutions for the long- standing problem of the power and speed of ships in shallow water.

Measured model resistance in deep and shallow water, and decomposition of the difference

[1] Raven, H.C., A method to correct shallow-water model tests for tank wall effects. Jnl Marine Science Techn., Vol.24-2, June 2019.

[2] Raven, H.C., A computational study of shallow-water effects on ship viscous resistance. 29th Symp. Naval Hydrodynamics, Gothenburg, Sweden, 2012.

[3] Raven, H.C., A new correction procedure for shallow-water effects in ship speed trials, PRADS 2016 Symposium, Copenhagen, Denmark, 2016.

[4] ITTC Recommended Procedures and Guidelines, Preparation, Conduct and Analysis of Speed/Power Trials, Procedure 7.5-04-01-01.1, 2017.

[5] Raven, H.C., Shallow-water effects in ship model testing and at full scale, 5th MASHCON conference, Oostende, Belgium, 2019; to appear in Ocean Engineering.

report 23

Page 1 | Page 2 | Page 3 | Page 4 | Page 5 | Page 6 | Page 7 | Page 8 | Page 9 | Page 10 | Page 11 | Page 12 | Page 13 | Page 14 | Page 15 | Page 16 | Page 17 | Page 18 | Page 19 | Page 20 | Page 21 | Page 22 | Page 23 | Page 24

A new trial correction procedure has now been proposed [3]

. From the main

dimensions and block coefficient, the viscous resistance of the ship and its increase in shallow water are estimated. The wave resistance is supposed unaffected as long as the depth Froude number is limited; but an additional correction for the effect of the increased dynamic sinkage is applied. The procedure thus estimates the power increase in shallow water at equal speed; for which the trial measurement may be corrected.

ITTC procedure With the support of the ‘Sea Trial Analysis’ (STA) Group, speed trials have been done for three ships at full scale in several water depths. Invariably the new procedure estimated the shallow-water speed loss much better than Lackenby’s method. It is now being considered by the ITTC for general acceptance [4]

.

With all these steps, a much better understanding has been obtained of what happens in shallow water [5]

Significant changes of the wake field for decreasing water depth

. For the same

model tests, the figure shows how we can explain the large difference between the measured model resistance in deep and shallow water. We added the two empirical contributions from the shallow-water correction method to the deep-water resistance curve: the increase of the viscous resistance, and the increase of resistance due to the additional dynamic sinkage. Thus we come quite close to the actual measurements in the Shallow Water Basin, corrected for the tank wall effect; the remaining discrepancy being the shallow- water increase of the wave resistance.

While a decade ago these model test results just had to be taken for granted, now we understand precisely what is going on. An understanding reflected in improved and consistent solutions for the long- standing problem of the power and speed of ships in shallow water.

Measured model resistance in deep and shallow water, and decomposition of the difference

[1] Raven, H.C., A method to correct shallow-water model tests for tank wall effects. Jnl Marine Science Techn., Vol.24-2, June 2019.

[2] Raven, H.C., A computational study of shallow-water effects on ship viscous resistance. 29th Symp. Naval Hydrodynamics, Gothenburg, Sweden, 2012.

[3] Raven, H.C., A new correction procedure for shallow-water effects in ship speed trials, PRADS 2016 Symposium, Copenhagen, Denmark, 2016.

[4] ITTC Recommended Procedures and Guidelines, Preparation, Conduct and Analysis of Speed/Power Trials, Procedure 7.5-04-01-01.1, 2017.

[5] Raven, H.C., Shallow-water effects in ship model testing and at full scale, 5th MASHCON conference, Oostende, Belgium, 2019; to appear in Ocean Engineering.

report 23

Page 1 | Page 2 | Page 3 | Page 4 | Page 5 | Page 6 | Page 7 | Page 8 | Page 9 | Page 10 | Page 11 | Page 12 | Page 13 | Page 14 | Page 15 | Page 16 | Page 17 | Page 18 | Page 19 | Page 20 | Page 21 | Page 22 | Page 23 | Page 24