Why Phelps’ Dolphin Kick Works

· fluid mechanics

The Dolphin Flipper Acts Like a Wing

Based on the New Theory of Flight explaining Why a Propeller Gives ThrustWhy 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:

This movie shows that the feet in both backstroke and freestyle act like a dolphin flipper.

Freestyle Armstroke

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):


Both the arms, in particular the hands, and the legs seem to act like a bit like wings in the breaststroke: The arms are first spread out and are then during backstroke bent down and in a circular motion folded towards the body. The legs are first bent and spread apart and are then during back-kick straightened and folded together, as shown in the animation , movie1 , movie2 and the following drawing from Gator Swim Team :

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