Tham khảo tài liệu 'practical ship hydrodynamics episode 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Ship manoeuvring 171 it is important to consider that the ship is faster at the beginning of the turning circle and slower at sustained turning. The heeling angle exceeds dynamically the statical heel angle due to forces listed above. The turning circle test is used to evaluate the turning ability of the ship. 2. Spiral manoeuvres We distinguish between - Direct spiral manoeuvre Dieudonne With the ship on an initial straight course the rudder is put hard to one side until the ship has reached a constant rate of change of heading. The rudder angle is then decreased in steps typically 5 but preferably less near zero rudder angle and again held until a steady condition is reached. This process is repeated until the rudder has covered the whole range to the maximum rudder angle on the other side. The rate of turn is noted for each rudder angle. The test should be performed at least for yaw unstable ships going both from port to starboard and from starboard to port. - Indirect reverse spiral manoeuvre Bech The ship is steered at a constant rate of turn and the mean rudder angle required to produce this yaw rate is measured. This way points on the curve rate of turn vs. rudder angle may be taken in any order. The spiral test results in a curve as shown in Fig. . The spiral test is used to evaluate the turning ability and the yaw stability of the ship. For yaw unstable ships there may be three possible rates of turn for one given rudder angle as shown in Fig. . The one in the middle dotted line represents an instable state which can only be found by the indirect method. In the direct method the rate of turn switches at the vertical sections of the curve suddenly to the other part of the curve if the rudder angle is changed. This is indicated by the dotted arrows in Fig. . Rate of turn Curve obtained--from reversed spiral Figure Results of spiral tests for yaw stable and yaw unstable ship Rate of turn Left Rudder angle d Right stb Curve obtained from direct or
