Stability versus maneuverability in aquatic locomotion

The dictionary definition of stability as "Firmly established,not easily to be changed" immediately indicates the conflictbetween stability and maneuverability in aquatic locomotion.The present paper addresses several issues resulting from theseopposing requirements. Classical stability theory for bodiesmoving in fluids is based on developments in submarine and airshipmotions. These have lateral symmetry, in common with most animals.This enables the separation of the equations of motion intotwo sets of 3 each. The vertical (longitudinal) set, which includesmotions in the axial (surge), normal (heave) and pitching directions,can thus be separated from the lateral-horizontal plane whichincludes yaw, roll and sideslip motions. This has been founduseful in the past for longitudinal stability studies basedon coasting configurations but is not applicable to the analysisof turning, fast starts and vigorous swimming, where the lateralsymmetry of the fish body is broken by bending motions. Thepresent paper will also examine some of the aspects of the stabilityvs. maneuverability tradeoff for these asymmetric motions. Ananalysis of the conditions under which the separation of equationsof motions into vertical and horizontal planes is justified,and a definition of the equations to be used in cases wherethis separation is not accurate enough is presented.