Fig. 9. The propulsive forces (drag, lift and inertia combined) generated by the feet of the great cormorant during the stroke phase (fields 1–6). Each blue line is the mean force vector from all the birds for a specific field. Fields are separated by 0.02 s time intervals and sequenced by adjacent numbers. Forces' magnitude and direction are estimated according to the model (see text) using low aspect ratio wing calculations. Values include the forces of both feet. The overall propulsive force (the resultant vector of fields 1–6; in red) is directed at 44° below the actual swimming direction of the birds, thus contributing a large vertical component to aid in resisting buoyancy. The model accounts for motion in the XY plane and disregards the contribution of lateral motion. The red arrow is the swimming direction of the birds. Variation (S.E.M.) of the vectors' direction and magnitude is mentioned in the table.