Lift in Swimming

     In 1969 James Counsilman made the groundbreaking announcement that lift was the most important force in swimming.  His study, the first to use underwater cameras, showed that a swimmer's arm crossed over the body's axis of rotation in an S-shaped pattern, producing lift like a propeller on an airplane.⁴ (see diagram 1)   Lift always acts in the direction perpendicular to motion.  This effect can also be seen in the sculling patterns of the arms in the breast and butterfly strokes.  (see video) This completely contradicted what coaches had previously believed.  They had taught their swimmers to pull the arm straight back, any curve was seen to be a detriment to the stroke.  Counsilman showed how the Bernoulli principle affects swimming.  The Bernoulli principle is a law of fluid dynamics which states that if the speed of a fluid particle increases as it travels in a horizontal streamline, then the pressure decreases and vise versa.¹   This explains how a foil acts in a fluid, like an airplane wing in the air or a foil-shaped hand in water. (see diagram 2)  A good example this principal is dangling a spoon over a fast stream of water flowing over the convex side.² The water will draw the spoon in, rather than pushing it out as expected. The water has to move faster over the convex side to keep up with the other side.  This creates a low pressure cell on the convex side, and a high pressure cell on the concave side.  The S-shaped pattern of the swimmer's hand produces forward lift in much the same way.  The resistance from the resulting lift is used to apply thrust.²
       Another way a swimmer produces lift is explained by the Magnus effect.²  The Magnus effect is how a fluid flows around a rotating cylinder, the  forearm for example.  The fluid flowing past the forearm appears as if it is rotating around the arm, this is called a bound vortex.  Moving the arm with the vortex through moving water decreases the pressure above the arm and increases the pressure below the arm, Bernoulli's principle.  In this way the arm produces lift in a similar way as the hand.  However, if the swimmer loses the vortex for some reason, no lift will take place.  This also explains the curved path of a golf ball.²

Diagram 1     http://wings.ucdavis.edu Diagram 2