When a body is moving through the water, the
main forces acting in the opposite direction to the motion
of the body are hydrodynamic damping forces. These
damping forces are mainly due to drag and lifting forces,
as well as lineal skin friction .Damping forces have a
significant effect on the dynamics of an underwater
vehicle which leads to nonlinearity. Lineal skin friction
can be considered negligible when compared to drag
forces, and therefore, it is usually sufficient to only take
into account the latter when calculating damping forces.
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