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3.3 Performance Appraisal
Fluid Mechanics 3:
Hydrodynamics - Resistance
Resistance
“the forces acting against the swimmer in their efforts to propel
themselves through the water.”
Water offers a far higher resistance to objects moving through it than does
air.
Because of this, it is very important for a swimmer to obtain and maintain a
streamlined position when performing a stroke.
Resistance or the slowing down effect of the water is also known as ‘drag’.
There are three major types of drag:
• Surface drag
• Form drag
• Wave drag
Surface Drag (or skin friction resistance)
When swimming, the water must move around your body and limbs
A thin layer of water next to the body actually sticks to it, and moves
with it causing up to 30% resistance.
The overall effect of this is a considerable drag on the forward progress
of the swimmer.
How to overcome?
Form Drag (or Tail Suction Resistance, or Eddy Drag)
Depends on the size, shape and speed of the swimmer
When the irregular shaped human body is propelled through the
water, the flow lines don’t remain smooth.
Instead they are deflected and break up into a number of whirls
creating a great deal of turbulence
Form Drag (or Tail Suction Resistance, or Eddy Drag)
Depends on the size, shape and speed of the swimmer
When the irregular shaped human body is propelled through the
water, the flow lines don’t remain smooth.
Instead they are deflected and break up into a number of whirls
creating a great deal of turbulence
This type of resistance is very costly in terms of energy output
The greater the frontal area hitting the water, the greater the
eddy resistance
Where will form drag feature most in your swimming performance?
How can you minimise its effects?
Wave Drag (or Frontal Resistance)
Caused by waves developing on the water’s surface in the form of
a bow wave. Determined by the amount of surface area exposed
to the direction of forward movement.
Swimmers must maintain a swimming position that is as
streamlined as possible – i.e. present as small a surface area as
possible to the water
Position of head is important. If too high, wave drag increases
Applying Biomechanical Principles in Swimming
Newton’s first law (The law of inertia)
“An object at rest will remain at rest unless acted on by an
unbalanced force. An object in motion continues in motion with
the same speed and in the same direction unless acted upon by an
unbalanced force”
A constant application of force must be applied to swim at a uniform
speed.
With swimming, the resistance to movement is so great that the body
almost immediately stops when propulsion stops.
Breaststroke, which involves a glide, uses momentum but if the glide is
held too long, resistance overcomes the moving body and it comes to rest
It then requires excess energy to regain the momentum
Applying Biomechanical Principles in Swimming
Newton’s second law
“Acceleration is produced when a force acts on a mass. The
greater the mass (of the object being accelerated) the greater the
amount of force needed (to accelerate the object)”
A swimmer accelerates forward by increasing their stroke rate
An even application of propulsion is more efficient in propelling a
body forward than a fluctuating force
If a swimmer accelerates and decelerates in a stop-and-go
manner, much of the force that could be used to overcome water
resistance will be lost in overcoming inertia
Student Task
Explain, in detail, the
effects resistance had on
your initial swimming
performance.