Ship Maneuvering in Deep and Confined Waters 



I. INTRODUCTION 



On Course-Keeping in Deep vVater 



The average depth of the oceans is some 3800 m. Small 

 native crafts still steer their ways between nearby islands in these 

 oceans. New ships are built to transport ever larger quantities of 

 containers or bulk cargoes at a minimum of financial expense 

 between the continents. 



It Is not necessarily obvious that the helmsman shall be able 

 to control a mammoth tanker on a straight course. A few years ago 

 ship operators were stirred by the published results of an analytical 

 study, Interesting In Itself, which In fact did Indicate, that manual 

 control of ships would be Impossible beyond a certain size. Upon 

 request by the shipbuilders a series of real-time simulator studies 

 were Initiated at SSPA In autumn 1967 to Investigate manual as well 

 as automatic control of large tankers then building, [ l]. 



At an early stage of these tests the helmsman was found to 

 constitute a remarkably adaptive control, which could not be simu- 

 lated by a simple transfer function. As could be expected a rate dis- 

 play proved to make course keeping more easy; the rate signal was 

 even more essential to the auto pilot. 



The simulator findings were confirmed In subsequent proto- 

 type trials. The diagramis of Fig. 1 compare simulator and proto- 

 type rates of change of heading and yaw accelerations for a large 

 tanker as steered by the author In a Force 6 following sea. (In the 

 simulator case the sea disturbance was represented by a cut-off 

 pseudo-randonn white noise of predetermined root mean square 

 strength, that was fed Into the yaw loop.) This particular tanker Is 

 dynamically unstable on a straight course, and the steady-state 

 4j(6)-dlagram from a deep-water spiral test exhibits a hysteresis 

 loop with a total height of 0. 5 °/s and a total width of a little more 

 than 3° of helm. If yaw rate Is maintained within some 40 per 

 cent of the loop height value It has been found possible to control the 

 straight heading by use of small helm only. 



The use of the computer-type simulator for the prediction of 

 ship behaviour Implies the adoption of a suitable mathematlceil model 

 and the knowledge of a numiber of coefficients In this model. An 

 elite rnatlve technique that simulates full-scale steering by controlled 

 free-sailing ship models Is still In use. Mostly the steering has been 

 exercised by manual operation of the controls, and It has been claimed 

 that at least comparative results should then be valid. It Is likely 

 that the truth of this statement depends on the actual speed and size 

 (euid time constants) of the prototype ship as well as of the model 

 scale ratio used. 



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