2 Spiral manoeuvres

  2.1.1 The direct spiral manoeuvre is an orderly sequence of turning circle tests to obtain a steady turning rate versus rudder angle relation (seefigure A4-2).

  2.1.2 Should there be reasons to expect the ship to be dynamically unstable, or only marginally stable, a direct spiral test will give additional information. This is a time-consuming test to perform especially for large and slow ships. A significant amount of time is needed for the ship to obtain a steady rate of change of heading after each rudder angle change. Also, the test is very sensitive to weather conditions.

  2.1.3 In the case where dynamic instability is detected with other trials or is expected, a direct spiral test can provide more detailed information about the degree of instability that exists. While this test can be time consuming and sensitive to weather conditions, it yields information about the yaw rate/rudder angle relation that cannot be measured by any other test.

  2.1.4 The direct spiral is a turning circle manoeuvre in which various steady state yaw rate/rudder angle values are measured by making incremental rudder changes throughout a circling manoeuvre. Adequate time must be allowed for the ship to reach a steady yaw rate so that false indications of instability are avoided.

  2.1.5 In cases where the ship is dynamically unstable it will appear that it is still turning steadily in the original direction although the rudder is now slightly deflected to the opposite side. At a certain stage the yaw rate will abruptly change to the other side and the yaw rate versus rudder angle relation will now be defined by a separate curve. Upon completion of the test the results will display the characteristic spiral loop as presented in figure A4-3.

  2.1.6 A direct spiral manoeuvre can be conducted using the following general procedure:

• .1 the ship is brought to a steady course and speed according to the specific initial condition;

• .2 the recording of data starts;

• .3 the rudder is turned about 15 degrees and held until the yaw rate remains constant for approximately one minute;

• .4 the rudder angle is then decreased in approximately 5 degree increments. At each increment the rudder is held fixed until a steady yaw rate is obtained, measured and then decreased again;

• .5 this is repeated for different rudder angles starting from large angles to both port and starboard; and

• .6 when a sufficient number of points is defined, data recording stops.

  2.2.1 The reverse spiral test may provide a more rapid procedure than the direct spiral test to define the instability loop as well as the unstable branch of the yaw rate versus rudder angle relationship indicated by the dotted curve as shown in figure A4-2. In the reverse spiral test the ship is steered to obtain a constant yaw rate, the mean rudder angle required to produce this yaw rate is measured. and the yaw rate versus rudder angle plot is created. Points on the curve of yaw rate versus rudder angle may be taken in any order.

  2.2.2 This trial requires a properly calibrated rate of turn indicator and an accurate rudder angle indicator. Accuracy can be improved if continuous recording of rate of turn and rudder angle is available for the analysis. Alternatively the test may be performed using a conventional autopilot. If manual steering is used, the instantaneous rate of turn should be visually displayed to the helmsman.

  2.3.1 The simplified spiral reduces the complexity of the spiral manoeuvre. The simplified spiral consists of three points which can be easily measured at the end of the turning circle test. The first point is a measurement of the steady state yaw rate at the maximum rudder angle. To measure the second point, the rudder is returned to the neutral position and the steady state yaw rate is measured. If the ship returns to zero yaw rate the ship is stable and the manoeuvre may be terminated. Alternatively, the third point is reached by placing the rudder in the direction opposite of the original rudder angle to an angle equal to half the allowable loop width. The allowable loop width may be defined as:

 When the rudder is placed at half the allowable loop width and the ship continues to turn in the direction opposite to that of the rudder angle, then the ship is unstable beyond the acceptable limit.