The Dolphin Flipper Acts Like a Wing
Based on the New Theory of Flight explaining Why a Propeller Gives Thrust, Why Birds Can Fly and more generally Why It Is Possible to Fly , we can understand why the dolphin kick in swimming can give such a strong forward thrust. The dolphin kick helped Michael Phelps to win seven Olympic Gold Medals 2008, because:
The dolphin kick can be performed attaching a mono fin to your feet, which allows you to mimic the kick of a real dolphin tail:
In downstroke a dolphin flipper acts like a wing generating lift L, just like a birds wing in downstroke according to Why Birds Can Fly, and thus gives forward thrust because of the angle of the tail to the horisontal, at the price of drag D. The lift/drag ratioL/D for a wing can be between 10 and 20, and thus a strong forward thrust can be generated in a quick downstroke with moderate force. Evidently, it is important that a monofin is flexible in order to make a good angleto the horisontal and thus give lift a forward component with thrust. A stiff monofin makes smaller angle and less thrust.
You can study the dolphin kick with bare feet in the following movie showing the mechanics of the kick. Visibly the kick consists of (assuming horisonal position with belly down): a kick-up by bending the knees with the feet together a kick-down by straightening the knees withthe legs somewhat spread andthe feet turned inward with toes together.
We see that in the kick-down the feet form sort of a dolphin tail or wing,the function of which we can explain and understand. The kick-up with the feet together is a bit similar but the wing area and thus lift/thrust is (probably) smaller. The large foot blades of Phelps gives large lift area and strong forward thrust. Compare Fluid Dynamics of Swimming:
In thefreestyle armstrokethe arm also acts like wing as shown in this movie. You see in particular that the arm moves in a vertical plane backwards with the hands twisted with thumb back mimicing a verticalwing with leeward side facing forward giving lift and thus forward thrust (Scwimmat Limmat Zurich):