3.2 Model tests
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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 - 3.2 Model tests

3.2 Model tests

 There are two commonly used model test methods available for prediction of manoeuvring characteristics. One method employs a free-running model moving in response to specified control input (i.e. helm and propeller); the tests duplicate the full-scale trial manoeuvres and so provide direct results for the manoeuvring characteristics. The other method makes use of force measurements on a "captive" model, forced to move in a particular manner with controls fixed; the analysis of the measurements provides the coefficients of a mathematical model, which may be used for the prediction of the ship response to any control input.

Parent topic: Chapter 3 - Prediction Guidance

3.2.1 Manoeuvring test with free-running model

  3.2.1.1 The most direct method of predicting the manoeuvring behaviour of a ship is to perform representative manoeuvres with a scale model. To reduce costs by avoiding the manufacture of a special model for manoeuvring tests, such tests may be carried out with the same model employed for resistance and self-propulsion tests. Generally it means that a relatively large model will be used for the manoeuvring tests, which is also favourable with regard to reducing scale effects of the results.

  3.2.1.2 The large offshore, sea-keeping and manoeuvring basins are well suited for manoeuvring tests with free-running models provided they have the necessary acquisition and data processing equipment. In many cases, conventional towing tanks are wide enough to allow the performance of the 10°/10° zig-zag test. Alternatively, tests with a free-running model can be conducted on a lake. In this case measuring equipment must be installed and the tests will be dependent on weather conditions. Both laboratory and open-air tests with free-running models suffer from scale effects, even if these effects to a certain extent will be reduced by using a large model for the tests. Sometimes it has been attempted to compensate for scale effects by means of an air propeller on board the model. Another improvement is to make the drive motor of the ship model simulate the characteristics of the main engine of the ship with regard to propeller loading.

  3.2.1.3 Manoeuvres such as turning circle, zig-zag and spiral tests are carried out with the free-running model, and the results can be compared directly with the standard of manoeuvrability.

  3.2.1.4 More recently, efforts have been made at deriving the coefficients of mathematical models from tests with free-running models. The mathematical model is then used for predicting the manoeuvring characteristics of the ship. Parameter identification methods have been used and this procedure has been combined with oblique towing and propulsion tests to provide some of the coefficients.

3.2.2 Manoeuvring tests with captive model

  3.2.2.1 Captive model tests include oblique-towing tests in long narrow tanks as well as "circling" tests in rotating-arm facilities, but in particular such tests are performed by the use of a Planar Motion Mechanism (PMM) system capable of producing any kind of motion by combining static or oscillatory modes of drift and yaw. Generally, it may be said that captive model tests suffer from scale effects similar to those of the free-running tests, but corrections are more easily introduced in the analysis of the results.

  3.2.2.2 In using captive model tests due account of the effect of roll during manoeuvring should be taken.

  3.2.2.3 The PMM has its origin in devices operating in the vertical plane and used for submarine testing. The PMM makes it possible to conduct manoeuvring tests in a conventional long and narrow towing tank. The basic principle is to conduct various simpler parts of more complex complete manoeuvres. By analysis of the forces measured on the model the manoeuvring behaviour is broken down into its basic elements, the hydrodynamic coefficients. The hydrodynamic coefficients are entered into a computer based mathematical model and the results of the standard manoeuvres are predicted by means of this mathematical model.

  3.2.2.4 A rotating arm facility consists of a circular basin, spanned by an arm from the centre to the circumference. The model is mounted on this arm and moved in a circle, varying the diameter for each test. The hydrodynamic coefficients related to ship turning as well as to the combination of turning and drift will be determined by this method. Additional tests often have to be conducted in a towing tank in order to determine hydrodynamic coefficients related to ship drift. As in the case of the PMM the manoeuvring characteristics of the ship are then predicted by means of a mathematical model using the coefficients derived from the measurements as input.

3.2.3 Model test condition

 The Standards are applicable to the full load condition of the ship. The model tests should therefore be performed for this condition. For many ships the delivery trials will be made at a load condition different from full load. It will then be necessary to assess the full load manoeuvring characteristics of the ship on the basis of the results of manoeuvring trials performed at a condition different from full load. To make this assessment as reliable as possible the model tests should also be carried out for the trial condition, meaning that this condition must be specified at the time of performing the model tests. The assumption will be that when there is an acceptable agreement between model test results and ship trial results in the trial condition, the model test results for the loaded condition will then be a reliable basis for assessing the manoeuvring characteristics of the ship.

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