SHIP MOTIONS 



155 



225 



" 180 



135 



90 



45 







1.5^ 





 1. 5 



• Experiment 



• Coupled Mo+lon 



. Uncoupled Mo+ion 



2.00 



Fig. 3(<«) Comparison of computed and observed motion 

 amplitudes and phases of Series 60, 0.60-block-coefficient 

 model: a wave amplitude, Z„ amplitude of heaving motion, 9„ 

 amplitude of pitching angle, a maximum wave slope 5 phase 

 lag of heaving after pitching motions (from Gerritsma, 1958) 



Z25 



180 



S 135 



i 45 







E xperiment 

 Coupled Motion 

 Uncoupled Mo+ion 



2.00 



Fig. 3(A) Comparison of computed and observed motion 

 amplitudes and phases of Series 60, 0.60-block-coefficient 

 model: a wave amplitude, Z,, amplitude of heaving motion, 9o 

 amplitude of pitching angle, « maximum wave slope, b phase 

 lag of heaving after pitching motions (from Gerritsma, 1958) 



the sectional added masses (derived by F. .M. Lewis, 1929) 

 were eorrected for the free surface effect on the basis of 

 Ursell's data (see Section 2-3.12). The damping-force 

 coefficients \> and B were estimated by the methods of 

 Holstein (1933) and Havelock (1942) in the 1955 paper 

 and by the method of Grim (1953) in the 1957 paper. 

 The Froude-Kriloff hypothesis, often used in the past for 

 evaluation f)f the wave force F and moment M, was 

 abandoned and these forces and moments were calculated 

 instead, taking into account the interference of the ship's 

 hull with the water flow in waves. The use of the strip 

 theory permits calculations to be made for ships of 

 arbitrary form and also provides the data for sulise(|uent 

 calculation of bending moments 



2.12 Comparison of computed and observed ship 

 model motions. The regular long-crested sea ajipears to 

 be a theoretical concept. It can be well a])pro.\iniated in 

 towing tanks but it is not found in nature. The records 

 of even "regular" swells are found in rcalit v to consist of a 



spectriun of wa\-t's of many fr(M|ueucies. Any compari- 

 son of the calculated and observed motions of a ship at 

 sea must be deferred, therefore, to Section 3 on irregular 

 ship motions. The present section must necessarily be 

 limited to C(5mparison of calculated and observed motions 

 of ship models in towing tanks. 



It appears that ])rior t<i 1955 no comparison of calcu- 

 lated and measured ship model motions had been made. 

 The first such comparison (for two ship models) was 

 given by Korvin-Kroukovsky and Lewis (1955); com- 

 parative data for eight ship models were given by Kor\'in- 

 Krouko\'sky and .Jacobs (1957). Figs. 1 and 2 are taken 

 from the latter work and refer to a destroyer model and a 

 series 60, 0.()0 block coefficient model. The experimental 

 data and calculations'' were compared o\-er a range of 

 model speeds and at several wave lengths. The com- 

 parison covers the amplitudes of the heaving and pitching 



' Calculated by methods described in Appendix C. Additiiin;il 

 test data were published by Gerritsma (1960). 



