Propulsion mechanics swimming the front crawl

The mechanics involved in the generation of propulsive forces received scarce attention until the late 1960s when Counsilman published his famous kinematic analysis of the swimming strokes and began to speculate on the fluid dynamic mechanism of propulsion. Previously, propulsive forces on the surface of the hand were thought to be created in a similar fashion to those on the surface of an oar. It was reasoned that the drag forces generated by moving the hand backward would propel swimmers forward as a direct application of Newton's Third Law (action = - reaction).

The hand is pulled straight back, creating a high pressure zone on the palm, and a low pressure zone on the back of the hand. The resulting propulsive drag force would propel swimmers forward corresponding to the caterpillar paddle-wheel (right). However, in reality this form of propulsion is rather ineffective, because turbulence from one blade affects the ability of the following blade to create drag. Also, a relative small mass is given a rather large velocity change, leading to large losses of kinetic energy to the water.

Counsilman modified this view and drew attention to the importance of lift forces, which act perpendicular to the direction of hand movement and stated that both lift and drag forces are important for propulsion. This modified theory could explain the sculling movements during the arm pull observed with underwater cinematography .

The suggested hydrofoil behaviour of the hand was investigated in depth by Schleihauf. According to hydrodynamic theory the drag and lift force can be derived using the following equations:

L = r u_h² Cl S   and   D = r u_h² Cd S

where L = lift force, D = drag force, r = density of water, u_h = hand velocity, Cl = lift coefficient, Cd = drag coefficient, and S = propelling surface of the hand. Values of Cl and Cd for the human hand as a function of angle of attack and sweep back angle were determined in a fluid lab using hand models The propulsive forces generated by the hand during a pull may now be estimated given the values of Cl and Cd for the hand and the velocity and orientation of the hand during the stroke. It was concluded that lift forces play an important role in propulsion.

However, in recent studies comparison was made between the propulsive forces calculated using Schleihauf's approach and those measured on the M.A.D.-system. In general, a reasonable degree of agreement between the two methods was observed (Fmad = 34 N, FSchlei = 30 N, r = 0.76). However, the calculated propulsive forces were on average 10% lower than the measured values. In a replication of Schleihauf's work Berger et al. (1996) found similar values of Cl and Cd of a model hand. Again, the calculated quasi-steady forces were considerably lower (17%) than the measured propulsive forces (M.A.D.-system) (Berger et al., 1997), suggesting that significant unsteady mechanisms must be present.

An alternative propulsion mechanism is proposed. The rotation of the arm leads to a proximo-distal pressure gradient, which induces an axial flow towards the hand. This axial flow along the trailing side of the arm enhances the pressure difference over the hand, thus assisting propulsion by paddling. It is suggested that axial flow could play an important role wherever a propelling element is rotated (e.g. fins, paddles, wings, legs).

Recently a paper was published on this theory: Toussaint et al, PUMPED-UP PROPULSION DURING FRONT CRAWL SWIMMING, Medicine and Science in Sports and Exercises, volume 34: pages 314-319, 2002, click for download of pdf

 The following are titles of publications related to this topic, of the highlighted titles abstracts are available by clicking on the appropriate title:

Hydrodynamic drag and lift forces on human hand/arm models.

Technique and energy losses in front crawl swimming

Propulsive force in front crawl swimming.

Pumped-up propulsion during front crawl swimming.

Ongoing research in the fluid dynamic background of propulsion

Pictures of tests on arm model in Ship Hydromechanics Lab, Delft, University of Technology

More pictures Delft       Quicktime Video Delft        Results of Experiments in Delft

pictures of Experiments in Sloterparkbad Amsterdam March 2002 (3D analysis)

pictures of Experiments in Sloterparkbad Amsterdam January 2003 (3D analysis + pressure measurements)

©2002 Huub Toussaint

version: January 19, 2003