Prediction of Steering and Manoeuvring of Ships 



channels, and freely decaying oscillators. All of these systems have major dis- 

 advantages, being either inaccurate or unwieldy or both. The use of a rotating 

 arm in a circular tank is at present by far the most widely used approach and is 

 furthermore a most satisfactory means of obtaining crosscoupling terms in an- 

 gular velocity and drift angle, and angular velocity and rudder angle. The dis- 

 advantages of a rotating arm, apart from its high capital costs, are that it is not 

 practicably possible to measure acceleration derivatives and that it is not well 

 suited to the generation of small angular velocities. 



The planar-motion mechanism system conceived and developed by Gertler 

 (6) and Goodman (7) provides a means of conducting captive model tests in which 

 angular and straight-line motion can be imposed on a model in a conventional 

 towing tank. Developed as a technique for submerged body research, the orig- 

 inal mechanism generated motions of the body in the vertical plane. For appli- 

 cation to surface ships, a planar-motion mechanism must operate in the hori- 

 zontal plane, and the following description of the HyA planar -motion mechanism 

 system is consequently given in terms of sway and yaw motions. 



A planar-motion mechanism can be used in two different modes of opera- 

 tion, designated static and dynamic. In the static mode the model is constrained 

 to travel along a straight path at constant velocity, and the mechanism is used 

 to set discrete drift angles. Figure 2 schematically represents the three main 

 types of tests made in the static mode. Forces and moments resulting from 

 drift angle, rudder angle, and combinations of drift and rudder angles are 

 measured in these tests. 



SUtk DriH Angl« Test 



Fig. 2 - Various examples of suiic Rudder Angle Ttei 

 tests executed in the static 

 mode of planar-motion mech- 

 anism operation 



-^^^^ 



sialic Drift and Rudder Angle lest 



The unique feature of the planar-motion mechanism is its ability to gener- 

 ate oscillatory motions which are produced in the dynamic mode of its opera- 

 tion. Sinusoidal motions are imposed on the model with sway and yaw phased 

 in such a way as to produce conditions of "pure sway" and "pure yaw." In the 

 "pure-sway test," the bow and stern are oscillated in phase, and pure side- 

 velocity and acceleration result, as shown schematically in Fig. 3. 



In the "pure-yaw test" bow and stern are oscillated with phase-angle chosen 

 such that pure angular velocity and acceleration result. Various examples of 



323 



