Prediction of Steering and Manoeuvring of Ships 



Fig. 9 - Variable amplitude scotch yoke 



The translatory motion at each scotch yoke is transmitted to the model by a 

 rectangular crossframe and two links. The upper arm of each crossframe is 

 supported by four axial bearings which also permit the frames to swing forward 

 and aft. Constant drift angles can be introduced by sliding the short link on each 

 lower arm and clamping it (see Fig. 8), when it effectively becomes part of the 

 crossframe. The links are connected to axial and rotary bearings at each end 

 of a longitudinal beam on the model centre line. The beam is restrained at its 

 centre point to prevent cyclic longitudinal movements of the model, and it main- 

 tains the links at a constant distance from the centre of the mechanism during 

 all types of oscillatory motions. The beam together with the force gauges is 

 mounted in the model in the workshop prior to being transported to the carriage 

 and connected with the rest of the mechanism. This facilitates accurate posi- 

 tioning of the beam symmetrically around the origin of the model's body axes 

 as well as permitting model and mechanism to be transported and attached to 

 the c arriage in a simple and convenient manner. 



A Stromag electromagnetic tooth coupling in the main shaft permits the 

 phase angle between forward and aft scotch yokes to be precisely set in discrete 

 intervals of 1.5 degrees (Fig. 10). Also mounted on the coupling is the synchro- 

 nous switch that controls the integrator circuits used in conjunction with the 

 force -measurement system. The switch consists of a perspex ring having four 

 black lines spaced at 90 degree intervals. As the shaft rotates, the black lines 

 interrupt light beams shining on two photocells, giving rise to two trains of 

 electrical impulses. The perspex ring can be rotated relative to the main shaft 

 and locked so as to give impulses at positions of zero and maximum yaw or sway 

 displacement. This is accomplished by rotating the ring through half of the 

 phase angle between the scotch yokes. 



A mechanical microswitch, giving one impulse per revolution of the main 

 shaft, is used for identification purposes. 



331 



