216 



THEORY OF SEAKEEPING 



ship's natural period persist and are superposed on 

 rolling resulting from other causes. 



24 Rudder-Control Functions 5 = i{f\p dt, i/-, i/-, xf) 

 have been examined in the past solely with regard to 

 minimizing yawing exciu-sion. The yaw-roll coupling 

 and the resultant effect on rolling have not been con- 

 sidered. It is suggested that the rudder-control function 

 be examined with regard to the side-swaying and rolling, 

 as weW as yawing characteristics of ships. A more sea- 

 kindly ship can conceivably be obtained by permitting 

 greater yawing excursions, while minimizing lateral 

 accelerations caused by side-swaying and rolling. 

 Greater safety (in regards to the shifting of cargo) maj' 

 thereby be secured, in addition to greater comfort of 

 passengers and crew. 



25 Mathematical Representation of Manual Rudder- 

 Control Functions is needed for proper representation in 

 model tests in oblicjue seas. It also maj' be u.seful as a 

 target in the design of automatic steering systems. It 

 is believed possible to derive such a mathematical repre- 

 sentation by a statistical study of the simultaneous 

 records of a ship's yawing and rudder motions. 



The control functions usually discussed in^'oh-e one or 

 more functions of \j/ (see under project 24). It appears to 

 the author that an important characteristic of manual 

 control ma\' be almost continual shifting of emphasis from 

 one function of i/- to another. This shifting may well 

 be connected with changes of sign of dN^/bip caused by 

 the pitching of the ship and its phase relationship to the 

 waves. 



26 A Theoretical Investigation of Steering Charac- 

 teristics under the Combined Action of Waves and Wind 

 would work toward improving the steering characteristics 

 of ships and wouki help in the interpretation of model 

 tests. The steering characteristics of a ship depend on 

 wind-caused forces and moments as well as on the waves. 

 Erroneous conclusions may be deri\-ed from towing- 

 tank tests, if the wind effects (not present in these tests) 

 are not kept in mind. In most ca.ses these effects will 

 have to be allowed for intuitively. This process will be 

 made more reliable if a theoretically derived example is 

 available. 



27 For Experiments Taking Wind Effects Into Ac- 

 count, the wind-caused force and the center of pressure 

 can be measured in a wind tunnel. These will depend 

 on the relative mean wind direction but will not vary 

 appreciably' \\ith the normal yawing of a ship in waves. 

 The wind effect can be included, therefore, in towing- 

 tank tests in oblique waves as a constant force applied 

 in a specified direction at a specified point on the model. 

 It also can be taken as a function of the pitch angle. 

 The wind gustiness is neglected in the method proposed 

 here. 



28 Uniform Instrumentation sh(juld lie developed for 

 fully instrumented ships. The output of all sensing 

 elements should t:>e such as to permit the use of identical 

 amplifiers and recorders. Duplication of all sensing 

 elements and provision for instant switching to a spare 

 amplifier or recorder are desirable. Records should 



include the wave height, all six modes of ship motions, 

 rudder motions and possibly wind strength and direction. 

 All records must be connected by a timing code. The de- 

 velopment of transistorized ecjuipment will simplify in- 

 stallation by making it independent of a ship's electrical 

 system, and also can be expected to shorten the time 

 needed to put equipment in operation for each test run. 

 Magnetic-tape recorders should be supplemented by 

 visual recorders and/or tabulating devices. The visual 

 records in this case are not intended for precise analysis 

 and should present information in compact form for 

 scanning and rough analysis by observers. 



29 Portable Instrumentation for recording ship 

 motions should be developed. Rugged and compact 

 instrvmients have been used in missiles and research 

 rockets (for instance Kenimer, 19.55). The rapidly pro- 

 gressing transistor techniciue will also permit the develop- 

 ment of compact multichannel equipment. Six ship- 

 motion components can be sensed by entirely self-con- 

 tained equipment using accelerometers and possibly 

 small gyros. Additional recording channels and external 

 jacks should be provided for connection to ship's in- 

 struments, particularly to wave-height gage, when these 

 are available. An hour capacity of continuous record- 

 ing is suggested. 



The need for this equipment is indicated by the diffi- 

 cult.v in finding suitable weather conditions on trips 

 made by a few instrumented ships. For instance, only 

 mild weather was encountered by the SS Ocean Vulcan 

 in a time interval of many months. Reaching a satis- 

 factory understanding of ship conditions at sea requires 

 the collection of a much greater amount of data than is 

 feasible by using a few fully instrumented ships. The 

 use of .self-contained portable equipment will permit 

 more observations on shi]is (jf all kinds. 



30 Ship-Borne Wave Recorder, such as is now a\-ail- 

 able, cannot be considered portable eciuipment. It is, 

 however, a relatively simple instrument and its instal- 

 lation is recommended on as many ships as possible as a 

 part of the general investigation of ocean waves. In par- 

 ticular its installation is recommended on all weather 

 ships. It is suggested that external jacks be incor- 

 porated in its construction to permit ready connection to 

 other instruments, j^articularly to the portable equip- 

 ment listed under jiroject 2'J. 



31 Tucker's Shipbome Wave Recorder is the most 

 practical instrument available to date. However, in its 

 original design form it is intended for weather ships in 

 ho^'e-to condition, and its performance can probably be 

 improved (for use on larger ships) with additional 

 development. A hydrodynamic problem recommended 

 as project 23 of Chapter 1, was directed at evaluation of 

 ship-wave interference efTects on the pressure recorded 

 by the gage. Two additional projects can be recom- 

 mended here; namely, improvement of the electronic 

 integration of accelerations and investigation of the most 

 favorable location for the pressure and acceleration- 

 sensing assembly. The objecti^'e of the first is to 

 eliminate the influence of wave frequency on the inte- 



