On Unusual Phenomenon in Manoeuvrability and its Successful Countermeasure of a Fishery Research Vessel

Abstract

These days we sometimes have experienced or heard of full-bodied ships with so-called “unusual phenomena” in manoeuvrability, and the number of those ships seems to increase rapidly owing to the growing fullness of ships.Herewith in this paper an example of such a phenomenon appeared in a fishery research vessel is dealt. Turning tests and zig-zag tests were held, using both the ship and the 1/10 scale model. Oblique towing tests, β-varying tests (changing drift angle gradually) and observation of the stern flow, furthermore, were held in a towing tank using the model.Those experiments certify the existence of two separate flow fields around the hull and the occurrence of switching the two states. The large clearance above the propeller and the rudder as well as the full stern form permits the flow passing from starboard to port or vice versa through the clearance. By this flow, two types of unsymmetrical flow separation may occur. The alteration of the flow direction through the clearance accompanies the changes in the separation, the flow field and the force acting on the hull.Basic characteristics of the “unusual phenomena” observed both in the ship and the model can be explained by the unsymmetrical separation and its alteration. The apparent difference between the ship and the model can be also treated by the conception of “switching line”. The “switching line” is defined as the critical state of motion at which the alternation of the flow occurs.In consideration of those experiments, a center fin shutting the clearance was adapted to both the ship and the model. The results are fully satisfactory. In model tests the unsymmetrical separation is no longer observed. Therefore, the course stability recovers almost completely and the ship can keep the course without any difficulty.