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Abstract The elongated-body theory of the reactive forces on a fish moving in water (that is, forces resulting from the inertia of associated water movements) is extended so that a prediction of instantaneous reactive force between fish and water is obtained for fish motions of arbitrary amplitude, regular or irregular (§2). A preliminary application of the theory to the balance of reactive thrust and resistive drag in regular carangiform swimming of fishes with slender caudal fins is made (§3). Comparison with data (Bainbridge 1963) on the dace Leuciscus suggests that an important feature of this balance may be a substantial enhancement of drag for such fishes when swimming movements commence, an enhancement here interpreted in terms of a boundary-layer-thinning mechanism first suggested by Dr Quentin Bone.
M. J. Lighthill (1971) studied this question.