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Vessel data: Prescribed motion |
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Vessel data: Prescribed motion |
The prescribed motion data only apply if the vessel's primary motion is prescribed. It enables you to drive the vessel around the sea surface along a predetermined path, by specifying how the vessel's primary position and heading change during the simulation.
The vessel is driven by defining, for each stage of the simulation, the velocity (speed and direction) of the primary position and the rate of change of the heading.
| Warning: | Prescribed motion can cause discontinuities of velocity at stage boundaries, and these may cause transients in the system. OrcaFlex issues a warning at the start of the simulation if the data will result in a discontinuous vessel velocity. |
For each simulation stage. the speed of the motion can be given either directly as a constant speed or indirectly as speed change.
A constant speed applies throughout the stage.
Speed change is the amount by which speed is incremented (or decremented) during the stage. For example, a speed change of 1m/s over a stage of length 10 seconds causes a uniform acceleration of 0.1m/s2 to be applied throughout the stage, so that if the vessel starts at rest then at the end of the stage the vessel is travelling at 1m/s.
| Note: | Negative speeds may be given. |
For each stage the direction of the motion can be given, either relative to the global X direction or relative to vessel heading.
The direction value is interpreted as an azimuth. So a value of 90° relative to global $X$ direction, for instance, would result in the motion being in the global $Y$ direction. And if the vessel's heading was 130° and the direction of motion was 50° relative to the vessel heading, then the motion would be an azimuth of 130° + 50° = 180° relative to global, that is in the global $-X$ direction.
| Note: | The direction of motion is not restricted to be in the same direction as the vessel heading. |
In addition to varying the velocity of the primary position, you can define a rate of turn for each stage. This is the angle change per second to be applied to the vessel's heading throughout the stage.
If the direction of motion is specified relative to the vessel heading and the rate of turn is non-zero, then the vessel velocity direction will vary during the stage. In addition, if the direction is 0° relative to vessel heading, then the vessel velocity, whilst varying, will be always in the vessel heading direction.
If the direction is specified relative to the global $X$ direction, then the vessel velocity direction is not affected by the rate of turn.