204 



THEORY OF SEAKEEPING 



10 ^:c 



Period o+ Encounter in Sec. 



Fig. 47 Relation between period of encounter of waves and 



period of oscillations of ship (from Experiment Tank Committee 



of Japan, 1954) 



90 

 X 



180 



Fig. 48 Effect of irregularity of waves on ship's rolling angle 

 (from Experiment Tank Committee of Japan, 1954) 



sea tests of three destro\'ers of the Royal A'^etherlands 

 Navy. The instrumentation used is shown in Fig. 49. 

 "Verimeter," not otherwise described, is apparently a 

 gyroscopic instrument used for recording pitching and 

 rolling motions. The destroj^ers, sailing abreast, went 

 through a series of maneuvers in different sea conditions. 



^ J, 



Al S 



H.N. M.S. EVERSTEN 



H.N.M.S.ZEELAND 



H.N.M.S. FRIESLAND 



A Accelerometer 



C Camera 



P Woter Pressure Goges 



S Strain Gage 

 V Verimeter 

 *""* Instrumentation Room 



Fig. 49 Locations of instruments (from Warnsinck and St. 

 Denis, 1957 NSMB Symposium) 



A novel feature of these tests was the taking of a rapid 

 sequence of photographs of each ship from the other 

 ships in the formation. The instant of each photograph 

 was identified on the motion record. 



A sample of a 15-sec record with a sequence of 20 

 photographs was presented in the paper. A repro- 

 duction of it will be found in Fig. .5-40 on page 289. 

 These data and certain additional data on slam- 

 ming shed considerable light on the nature of the slam- 

 ming process. This will be discussed in connection with 

 stresses in Chapter 5. However, it can be mentioned 

 here that the added stresses caused by slamming oc- 

 casionally doubled the normal bending stress. 



The ships were not equipped with shipborne wave 

 recorders, so that correlation of ship motions with waves 

 was not possible. 



Figs. 50 and 51 give the distribution of roll and pitch 

 angles with the relative direction of the waves. E.x- 

 amination of Fig. 50 confirms the well-known fact that 

 fast ships roll the most in quartering seas. Fig. 51 is 

 interesting since it shows that, at 17 knots, essentially 

 the same amount of pitching was experienced in a range 

 of wind directions contained within a 90-deg sector from 

 head to beam seas. The reason for this evidently lies 

 in the short-crest edness of the natural irregular waves. 



5.2 Statistical Observations ot Sea. A large amount 

 of statistical data on sea waves and on ship motions has 

 been collected by the "Sammelstelle fiir Fahrtergebnisse 

 der Hamburgischen Schiffbau "\>rsuchsanstalt." Some 

 results were published by Hinterthan (1937), Hebecker 



