SHIP MOTIONS 



183 



(!. Kempf and Hans Hopix' (l!t2(i) of Hamburgische 

 Schiffbau Versuchsanstalt, Ilainhuri;, (Icrmany, took 

 observations on a passenger liner, l-'urthermore, a 

 special division was established at the latter laboratory 

 for the specific purpose of collecting data on ships at sea 

 ("Sammelstelle fiir Fahrtergehnisse"). 



Observations aboard ships at sea are made for the pui'- 

 pose of establishing environmental conditions and 

 evaluating a ship's response to these conditions. The 

 first kind consists of ([uantitative descriptions of wind 

 and waves. This subject was treated in Chapter 1 of 

 the present monograph. Historically, however, suf- 

 ficient oceanographic data had not been amassed in the 

 time of Kent and Kempf. The first substantial organ- 

 ization of such data by Sverdrup and Munk (19-1:6, 1947) 

 took place some 20 years later, and was directed pri- 

 marily at the evaluation of swells. Furthermore, the 

 changeable nature of the sea recjuires that there be close 

 co-ordination between investigations of wind and waves 

 and of ship responses. Therefore, environmental con- 

 ditions have ftirmed an important part of ail ship ob- 

 servations at sea. 



The response of a ship to wind and waves can he con- 

 sidered under three headings: 



1 Ship resistance and projiulsion characteristics. 



2 Ship motions. 



3 Ship .stresses, deflections and vibrations. 



The first will be considered in Chapter 4, the second 

 in the present chapter and the third partially in the pres- 

 ent chapter, but more completely in Chapter 5. How- 

 ever, a complete separation in the discussion of all these 

 aspects is not possible. In most tests at sea all are con- 

 sidered and usually reported together. P'urthermore, 

 taken together, all three aspects define the seagoing 

 ([uality of a ship. 



Treatment of sea oliservat ions isdix'ided into: 



(a) A detailed investigation of wind, wave, ship 

 motion, and stress phenomena. 



(b) A statistical talnilation of the data. 



The establishment of a quantitative cau.se-effect re- 

 lationship is sought in the first part. Ship motion is 

 evaluated in terms of the wind and waves which cause 

 it. Structural loading is evaluated in terms of water 

 pressures and ship accelerations. A complete instant- 

 by-instant history of ship l)chavior under certain given 

 conditions is the aim of such detailed investigations. 



In the second part, the aim is to establish, on a statis- 

 tical basis, (luantities which occur in Nature. A large 

 number of simple quantitative obser\-ations of a single 

 characteristic is collected and presented in the form of 

 tables or plots. Data on one characteristic, for example 

 pitch angles or stresses in a certain part of a ship, are 

 collected independently of another. The only connec- 

 tion between recorded (juantities of two characteristics 

 is that they were recorded simultaneously. Direct 

 causal relationships between any two characteristics 

 cannot be obtained. Only a general correspondence of 

 averages over a given time interval is established. For 

 instance, the information may be gi\'en in a table show- 



ing the number of times a certain high bending stress 

 was reached in a given period of time during which the 

 wind had a certain average or ma.ximum \elocity. 



Statistical information is needed in order to establish 

 the levels of severity of various sea and ship-behavior 

 characteristics with which ship designers and mariners 

 must cope. More detailed information is needed for 

 design purposes, particularly for establishing the effect 

 of a ship's form on its seakindliness. All characteristics 

 of ship behavior at sea are so complex that useful deduc- 

 tions can be made only by freciuent reference to theore- 

 tical w(jrk and towing-tank data. Such references are 

 found, therefore, in most papers reporting sea observa- 

 tions. 



The treatment of sea observations outlined in the fore- 

 going is typical of past practice and applies to the ab- 

 stracted data presented in the following text. The last 

 two paragraphs abo\'e, in particular, refer to the "histo- 

 grammatic^" approach. Of the recently developed 

 statistical approach, ba.sed on the properties of time 

 series, which was described in Section 3, the only |)ub- 

 lished applications to a ship at sea are in Cartwright and 

 Rydill (1957) and Cartwright (1957, 1958). This ap- 

 l^roach occupies an intermediate position between the 

 two described in this section. While it provides in- 

 formation on causal relationships among waves, ship 

 form and ship motions, the results are given in the form 

 of statistical averages rather than as a detailed history. 



5.1 Observations Considering Sea Action on a Ship: 

 5.11 Observations of Kent, Kempf and Hoppe. ,J. L. 

 Kent (1924, 1927a) made six ocean voyages on three 

 passenger liners, one express cargo ship and two tankers. 

 In 1924 he reported on the methods of measurement and 

 the results for four ships under the following headings: 



1 A general description of the observations taken 

 and the manner in which they were obtained. 



2 Description of the wind and waves encountered, 

 and the effect of the former on the latter. 



3 Analysis of the part played by the weather in 

 reducing ship speed under various weather conditions. 



4 Elffect of the wind and ship form on steering of 

 ships. 



5 Summary of the principal factors causing loss of 

 ship speed at sea. 



The data on two passenger liners w^ere reported in 1927 

 under the following headings: 



1 Wind speed and wave dimensions. 



2 Speed and power dining \-oyages. 



3 Fine weather propulsion data. 



4 Rough weather propulsion data. 



5 Propeller thrusts. 



6 Weather producing propeller thrusts. 



7 Fluctuation of torque on propeller shafts. 



8 Pitching experienced on both ships. 



9 Rolling experienced on both ships. 



10 Comparison of rolling of the two ships. 



1 1 Notes on pounding data. 



1 2 Notes on steering data. 



All of the observations were made by J. L. Kent alone. 



