1. The Fluid Mechanics of Swimming
BIOMECHANICS
The Fluid Mechanics of Swimming

2. Hydrostatics Buoyancy and Floatation
Why does a plank always adjust it’s position in water to float horizontally, not vertically?
Why, if you had an aluminium dinghy and a block of aluminium the same weight, the dinghy will float and the block sink?

3. When they try to float on the surface without moving some people manage it easily while others always sink. Why?
Why is it easier to pick up and hold an object in water than doing it on land?

4. An individual who floats is said to display positive buoyancy
Others tend to sink in the water and display negative buoyancy (OHT #1)
A person’s ability to float may influence performance at both the beginner and championship levels

5. Beginners who have positive buoyancy are likely to learn to swim faster than those who float with difficulty, or not at all

6. Archimedes Principle
The reason why one object floats better in the water than another is determined by Archimedes Principle of Forces which states that:
‘A body submerged in a liquid is buoyed up by a force equal to the weight of the fluid displaced’
This means that an object will float in a fluid if its density is less than the density of that fluid

7. The density of fresh water is 1
The density of fresh water is 1.00, and if an object has a density of less than this it will float
If the object’s density is greater than 1.00 it will sink
Also known as the ‘specific gravity’ of an object
The effective specific gravity of the human body is 0.95

8. This means that in general the human body has the ability to float
A lump of steel will sink because it is unable to displace water that equals its weight (OHT #2)
But steel of the same weight but shaped as a bowl, will float
This is because the weight gets distributed over a larger area and the steel displaces water equal to its weight

9. A heavily laden ship floats because its total weight is exactly equal to the weight of the water it displaces
It is this weight that exerts the buoyant force supporting the ship
DVD 02:10 – 04:21

10. Factors Affecting the Ability to Float Lung Capacity
The volume of air in the lungs has a pronounced effect on an individuals ability to float
Deep inhalation = considerable volume of air added to lungs = specific gravity reduced substantially
The majority of males and females will float if they have taken a full inhalation of air
But the majority of males will sink unless they have more than residual air in the lungs

11. Body Build
Individuals with a high fat content will tend to be good floaters when compared with those who are heavily boned and well muscled
This is because of the specific gravity of body tissue:
Bone = 1.5 – 2.0
Muscle = 1.0
Fat = 0.8
Endomorphs – overweight = floater
Mesomorphs – little fat, high proportion muscle, bone = sinker
Ectomorphs – slim, slight = floater

12. Reduction in Buoyancy Force
Sex
Females tend to have greater proportions of fat which aids their floatation because it lowers their specific gravities
Muscular Tension
Can disallow the lungs to fill with air adequately, having a negative effect on floatation
Reduction in Buoyancy Force
The upward buoyancy force is reduced whenever a part of the body comes out of the water, therefore swimmers should keep as many parts in the water as possible

13. Centre of Buoyancy / Centre of Gravity
Many individuals have the ability to float but cannot assume a horizontal body position as the legs tend to sink
This is because in air, the body rotates around the COG, while in water the axis of movement is the COB

14. The actual floating position for a human body is when the COG is vertically aligned with the COB
The legs sink until the COG (hip region) and the COB (lung region) come into vertical alignment (OHT #3)
This is why a person with a COG higher up in their body has dragging legs, creating a lot of drag
DVD 00:35 – 02.10

15. Hydrodynamics The Study of Propulsion and Resistance in Water
Propulsion is the force which drives an object through the water
Human propulsion in water is generated by the use of…..
Arms, hands, legs and feet

16. The structure of the shoulder, elbow and wrist joints allow a wide range of movement
These joints can be rotated to produce the required forces more functionally than the hips, knees and ankles

17. Basic Forms of Propulsion Paddling
Pulling and pushing action of hands and arms in water
Sculling
Movement of hands through the water at approximately right angles to the direction of intended travel
Finning
Leg kicking action of freestyle, backstroke and butterfly

18. As the arm pushes backwards the body is propelled forwards (OHT #4)
Newton’s 3rd Law
For every action there is an equal and opposite reaction
Swimmers must try to push the water backwards rather than downwards and upwards
When a swimmer pushes the water downwards this results in the body being forced upwards
As the arm pushes backwards the body is propelled forwards (OHT #4)

19. While the hand presses upward the body is forced down
The consequence of these actions is that the body bobs up and down as the swimmer moves through the water
At the advanced swimmer stage resistance must be kept to a minimum

20. Bernoulli’s Principle
Greater propulsion in water is obtained by moving a large amount of water a short distance than by moving a small amount a great distance
If a swimmer pulls their hand in a straight line it is pushing a small volume of water a long way
Once the water has started moving backwards the swimmer cannot apply as much force on the water as his hand meets less resistance

21. To obtain maximum propulsion the hand must move faster than the water
The best method is to seek stationary or ‘still’ water by using a curved pathway for the hands (OHT #5)

22. Bernoulli Effect (OHT #6)
When fluid particles travel over an object shaped like a wing where there is a convex shape on one of the sides, the particles that travel over the larger area meet up at the back of the wing at the same time as the particles that travelled over the flatter surface on the other side

23. Therefore, the particles that went over the convex side must have moved faster
Bernoulli found that when particles move fast they create low pressure, and a higher pressure on the other side – and lift can occur
So when we move our cupped hand through the water, water travels faster over the knuckles than over the palm

24. This creates a low pressure by our knuckles and a higher pressure under our palms
We can use this lift force that occurs to propel us through the water
In breaststroke, bringing cupped hands in towards the chest creates lift that enables the breathing subroutine of the stroke

25. Propulsive Lift Force on the Hand
Lift always acts in a direction perpendicular to the flow
Lift force is felt as pressure on the palms of the hands when the slightly pitched or tilted hand moves through the water
E.g. sculling action; As the hand moves through the water at a slight angle, a pressure differential is created on alternate sides of the hand (OHT #7)

26. Since motion occurs from high pressure to low pressure, the propulsive lift is perpendicular to the direction of the path of the hand
E.g. treading water, sculling with horizontal hand movements. Lift force is produced on the hands and maintains the head above the surface

27. Propulsive Drag Force on the Hand
Propulsive drag is created by the backward movement of the hand through the water
As the hand is pulled or pushed against the water, a high pressure zone is created on the palm of the hand and a low pressure zone on the back (OHT #8)
The difference in pressure creates a force on the swimmer’s hand which moves the swimmer forward

28. Drag can definitely hinder the progress of a swimmer – as you will see when we talk about resistance
But without it, a swimmer will not be able to move in water
Think of a sprinter on land – to gain greater speed they wear spikes to get more friction on the track to aid their propulsion

30. Swimmers must do the same, but must be careful about not creating too much drag to slow them down
Drag can aid propulsion by the hand ‘grabbing’ the water
Newton’s 3rd law – action of grabbing the water, reaction of the body going forward

31. The hand pulling backwards produces a high pressure in the palm and a low pressure at the back of the hand
Scientists have concluded that the swimmer gains propulsion by both drag and lift and by changing sequences of the hand during the stroke to get a ‘resultant force’

32. Angle of Attack
The swimmer needs to continually change the pitch of the hand as it travels it’s curved path so that it is using both drag and lift forces to maximum effect (OHT #9)
The angle should be about 45 degrees so that the resultant force is an equal contribution of both lift and drag – so that the body moves forward
DVD 06:42 - end

33. Resistance
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’

34. Resistance / drag is the force exerted by the fluid against the body which reduces its speed
Skin Friction Resistance (or Surface Drag) (OHT #10)
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

35. The overall effect of this is a considerable drag on the forward progress of the swimmer
‘Drag free’ swimming costumes and shaving down
Tail Suction Resistance (or Eddy Drag)
Depends on the size, shape and speed of the swimmer

36. This type of resistance is very costly in terms of energy output
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

37. Frontal Resistance
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
DVD 04:21 – 06:42

38. Newton’s 1st Law
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

39. 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

40. Newton’s 2nd Law
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