3.4 Corrections from non-standard trial conditions
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Statutory Documents - IMO Publications and Documents - Circulars - Maritime Safety Committee - MSC/Circular.1053 – Explanatory Notes to the Standards for Ship Manoeuvrability – (Adopted on 16 December 2002) - Annex - Explanatory Notes to the Standards for Ship Manoeuvrability - Chapter 3 - Prediction Guidance - 3.4 Corrections from non-standard trial conditions

3.4 Corrections from non-standard trial conditions

Parent topic: Chapter 3 - Prediction Guidance

3.4.1 Loading condition

  3.4.1.1 In the case for predicting manoeuvrability of a ship in full load condition using the mathematical model through the sea trial results in ballast or heavy, ballast condition, the following two methods are used in current practice.

  Option 1:

  3.4.1.2 The manoeuvring performance in full load condition can be obtained from the criteria of measured performance during the sea trial in ballast condition (T) and the interaction factor between the criteria of manoeuvrability in full load condition and in a trial condition (F/B), that is as given below;

where,

  • B : the estimated performance in the condition of sea trial based on the numerical simulation using the mathematical model or on the model test;
  • F : the estimated performance in full load condition based on the numerical simulation using the mathematical model or on the model test;
  • T : the measured performance during the sea trial; and
  • R : the performance of the ship in full load condition.

  3.4.1.3 It should be noted that the method used to derive B and F should be the same.

  Option 2:

  3.4.1.4 The manoeuvring performance in the condition of sea trial such as ballast or heavy ballast are predicted by the method shown in appendix 2, and the predicted results must be checked with the results of the sea trial.

  3.4.1.5 Afterwards it should be confirmed that both results agree well with each other. The performance in full load condition may be obtained by means of the same method using the mathematical model.

3.4.2 Environmental conditions

  3.4.2.1 Ship manoeuvrability can be significantly affected by the immediate environment such as wind, waves, and current. Environmental forces can cause reduced course-keeping stability or complete loss of the ability to maintain a desired course. They can also cause increased resistance to a ship’s forward motion, with consequent demand for additional power to achieve a given speed or reduces the stopping distance.

  3.4.2.2 When the ratio of wind velocity to ship speed is large, wind has an appreciable effect on ship control. The ship may be unstable in wind from some directions. Waves can also have significant effect on course-keeping and manoeuvring. It has been shown that for large wave heights a ship may behave quite erratically and, in certain situations, can lose course stability.

  3.4.2.3 Ocean current affects manoeuvrability in a manner somewhat different from that of wind. The effect of current is usually treated by using the relative velocity between the ship and the water. Local surface current velocities in the open ocean are generally modest and close to constant in the horizontal plane.

  3.4.2.4 Therefore, trials shall be performed in the calmest weather conditions possible. In the case that the minimum weather conditions for the criteria requirements are not applied, the trial results should be corrected.

  3.4.2.5 Generally, it is easy to account for the effect of constant current. The turning circle test results may be used to measure the magnitude and direction of current. The ship’s track, heading and the elapsed time should be recorded until at least a 720° change of heading has been completed. The data obtained after ship’s heading change 180° are used to estimate magnitude and direction of the current. Position (xli′ yli′ tli) and (x2i′ y2i′ t2i) in figure 5 are the positions of the ship measured after a heading rotation of 360°. By defining the local current velocity for any two corresponding positions as the estimated current velocity can be obtained from the following equation:

 the estimated current velocity can be obtained from the following equation:

  3.4.2.6 If the constant time interval, δt = (t2i - t1i), is used this equation can be simplified and written:

 The above vector, , obtained from a 720° turning test will also include the effect of wind and waves.

  3.4.2.7 The magnitude of the current velocity and the root mean square of the current velocities can be obtained from the equations:

 vc(RMS) represents the non-uniformity of vi which may be induced from wing, waves, and non-uniform current.

  3.4.2.8 All trajectories obtained from the sea trials should be corrected as follows:

 where is the measured position vector and is the corrected one of the ship and x’(t) = x(t) at t = 0.

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