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Propulsion:

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 .

Relevant parameters describing the hand as
hydrofoil (top). A hydrofoil subjected to flow (lower left) experiences a lift
and drag force. The same is true for the human hand (lower right). The magnitude
of the lift and drag forces depends on the angle of attack a and on the sweep
back angle y.

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 uh2 Cl S   and   D = r uh2 Cd
S

where L = lift force, D = drag
force, r = density of water,
uh = 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