Motion of a rigid body in an unsteady non-uniform heavy fluid :

Difficulties with an earlier set of equations are explained by deriving the equations of motion of a rigid body moving in a perfect fluid that is itself accelerating and contains velocity gradients. The equations are derived on the assumption that changes in the fluid velocity over the vehicles length are small compared to the velocity of the stream in its neighbourhood. It is shown how the resultant perfect fluid equations can be augmented to include viscous forces and moments derived from other theoretical or experimental sources.