Swimming Performance and Hydrodynamic Characteristics of Harbor Seals Phoca vitulina

Hydrodynamic characteristics, including fineness ratios, coefficients of drag, and the influence of body size, shape, and position on drag were examined for harbor seals (Phoca vitulina). Drag for towing animals increased curvilinearly with velocity over the range 0.7 to 3.5 m·s⁻¹ and was described by the equations: Drag(N) = 6.49 velocity (m·s−1)0.79 for a submerged adult seal, and Drag(N) = 1.29 e1.9 velocity (m·s⁻¹) for the seal towing at the water surface. The importance of streamlining was demonstrated by comparing these values with those of a towing human subject. At 2.0 m·s⁻¹ drag for a submerged human was 113 N, representing a 5.0-fold and a 1.9-fold increase in drag over submerged and surface towing seals, respectively. Coefficients of drag (Cd) followed this trend and were three times greater for the human subject. Alterations in body configuration of the seals, as occurs during stroking movements, resulted in an increase in body drag. From our calculations, body drag during gliding phases may more than double during active swimming by seals and influences the swimming performance of these animals.