SHIP MOTIONS 



217 



grated result and to eliminate the phase shift. This con- 

 ceivably can be accomplished by application of the 

 modern servomechanism iheory. 



The pressure gage and accelerometer unit have habitu- 

 ally been installed amidships. In this location the 

 accelerations are least, but hydrodynamic effects are at 

 their maximum. On cargo type and jjassenger ships, 

 the rectangular ship section with sharp turn of the bilge 

 and the presence of bilge keels make the evaluation of 

 hydrodynamic effects on the pressiu'e-gage readings 

 highly uncertain. The effect of the ship-made wave at 

 forward ship speed is also large. It appears to the 

 author that locating the sensing unit a short distance 

 ahead of the front edge of bilge keels is more favor- 

 able. A ship's cross section in this locality, more roundeil 

 and unbroken by bilge keels, is more amenable to the 

 evaluation of hydrodynamic eifects. In this location 

 the accelerations will be greater and more reliance has to 

 be placed on the methods of computing \'ertical dis- 

 placements from accelerations. A project is recom- 

 mended for comprehensive investigation of the problems 

 just outlined. 



32 Development of a Portable Wave Recorder is 

 desirable for use in connectiou with the portable record- 

 ing instrumentation listed as project 29. An instrument 

 which would be a coml)ination of acceleration integrater 

 and echo-ranging altimeter is visualized. Such an in- 

 strument can be clamped to the railing at the end of the 

 flying bridge or carried on a short outrigger. 



33 Development of a Portable Analyzer is recom- 

 mended. This is intendccl for use with either the port- 

 able motion recorder (project 29) or the complete record- 

 ing system (project 28). The analyzer must be capable 

 of spectral and cross-sijectral analyses. It is suggested 

 that the recording and analyzing equipment be stand- 

 ardized for a 1-hr duration of recording. 



A portable anal.vzer, based on the filtering principle, 

 can conceivably be developed as a self-contained com- 

 pact unit (a "suitcase") by the proper use of transis- 

 torized circuits. 



The author has repeatedly emphasized his belief that 

 the most rapid progress in understanding ship motions 

 will result from a combination of the personal impres- 

 sions of ship observers and on-the-spot analyses of re- 

 corded data. With a portalile analyzer the data taken 

 on the voyage woukl furtherm(jre, be immediately useful 

 upon return to the home port. Experience with elabo- 

 rately instrumented ships has been unsatisfactory be- 

 cause many months have been needed for the analyses of 

 data and in the end only a small part of the gathered 

 material has been utilized. 



34 Portable Strain Gage can well be used in con- 

 nection with recording ship motions on ships not ex- 

 tensively instrumented for strain measurements. Such 

 gages were used in the past for auxilliary measurements 

 in connection with extensive permanent instnmientation. 

 The gage suggested here is for use with the portable con- 

 tirmous recording eciuipment of project 29. It will give 

 indication of the stress variations connected with wave 



and ship motions and it will indicate stresses and vibra- 

 tions caused by slamming. Since a portable gage will be 

 applied to one side of the deck plate, it is suggested that 

 it be made with a long base (say 5 ft) to minimize the 

 errors caused by local plate deflections. Such a gage 

 can be installed in a protected location along the passage- 

 way on the strength de(;k. 



35 Measurements of Directional Sea Spectra by 

 fi'ee-floating buoys can be recommended during special 

 ship tests conducted in the vicinity of such buoys. 

 Dorrestein (1957) described buoys used for scalar sjjec- 

 trum measurements. \'osnessensky and Firsoff (li)57) 

 based their spectrum derivations on the results of data 

 obtained from buoys. The author understands that a 

 disposable i)U(jy for measurement of scalar wave spectra 

 was developed by DTMB. A buoy has apparently been 

 developed in England which will permit evaluation of 

 the directional spectra from wave slopes. 



A summary giving all available information on the 

 subject would he helpful in stimulating research. 



36 Methods of Measuring Directional Sea Spectra 

 from a ship at sea should be developed. As a starting 

 point, the author can suggest installation of four Tucker- 

 type ship-borne wave recorders (improved in project 31) 

 along the length of a ship. Analysis of the recorded data 

 can follow the pattern ba.sed on Barlier's work which was 

 described in project 38 of Chapter 1. The use of a ship 

 of low displacement/length ratio and low prismatic co- 

 efficient is suggested in this connection. If a special 

 weather ship were to be so equipped, the use of roll- 

 stabilizing tanks and omission of bilge keels is recom- 

 mendetl. With a ship hove-to in head seas, a reasonable 

 estimate of the directional distribution of waves can be 

 expected, at least for waves ^2 to 2 times as long as the 

 ship. Other, possibly simpler, approaches to this prob- 

 lem must be sought. 



37 Further Development of Pitch-Stabilizing Fins 

 is needed. This must follow from theoretical investiga- 

 tion, towing-tank model testing and observations on 

 ships at .sea. The efficacy of bow fins has been estab- 

 lished. Further activity is needed in the development of 

 fin shapes to minimize stalling effects, in the evaluation 

 and, if po.ssible, elimination of bow vibration and in the 

 cle\'elopment of structiu'al design criteria. 



The available data on stern fins are less complete and 

 further investigations are needed. In this ca.se particular 

 attention should be directed to the development of 

 suitable control functions. 



Nomenclature 



Symbols listed on pages 150 and 151 in connection with 

 Chapter 2 apply to Chapter 3 as well. NACA designa- 

 tions (page 151) are particularly important in connection 

 with Section 2.3 on six-component ship motions in waves. 

 Additional symbols are as follows: 



h = righting arm 



H = wave height (double iimplitiulc) 

 7'o = natural jjeriod of a ship's oscillation 

 7'e = period of wave encounter 



5 = rudder angle 



