MODELING OF SHIP DYNAMIC FOR SIMULATION

The basic capability that makes simulation possible is that of modeling (predicting and imitating) the trajectory of the ship under various conditions of the environment and controlling actions and forces. This section discusses the present state of ship trajectory modeling. It also addresses the modeling process for ship dynamics and examines issues of accuracy of trajectory prediction and the difficulty in the use of these models for training.

Accurate modeling of ship maneuvering behavior requires an understanding of and a predictive capability for the significant physical forces involved. This understanding may be based on empirical knowledge gained from full-scale and model experiments or on theoretical descriptions of the pertinent physical processes. In practice, both approaches are useful and are often employed in complementary roles.

Physical Modeling
Technical Factors

The principal mechanisms affecting the fluid pressure acting on a ship, and thus its dynamics, are wave effects and fluid viscosity (friction). To correctly represent wave effects, model tests are performed with Froude scaling (model velocity reduced in relation to the actual ship by the square root of the ratio of the model and the ship's length). It is impossible to correctly scale viscous forces, which are exaggerated at the model scale.

Scale models of ships are used in towing tanks to make engineering estimates of various hydrodynamic parameters, including the resistance (drag force) in calm water and motions in waves. Free-running models with self-propulsion and radio control are used to assess standard maneuvering characteristics, such as turns and zig-zags. Captive models are more useful in developing models because different components of the forces and moments acting on the ship can be measured separately, during prescribed maneuvers, and used in more general contexts to reproduce arbitrary trajectories, as is required in simulation.

Application of Physical Models to Training

Sufficiently large-scale models with self-propulsion and steering can be maneuvered by an onboard crew to simulate actual vessel maneuvers in suitable lakes or other sheltered waters. Manned models offer the advantages of relatively accurate hydrodynamic representation and realistic scenarios, particularly for low-speed operations, berthing, and ship interaction forces.

The hydrodynamic representation of vessel maneuvering behavior is reasonable because actual hull forms are used at the appropriate scale with appropriate



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