BIOMECHANICSBIOMECHANICS

The Fluid Mechanics of The Fluid Mechanics of SwimmingSwimming

HydrostaticsHydrostaticsBuoyancy and FloatationBuoyancy and Floatation

Why does a plank always adjust it’s Why does a plank always adjust it’s position in water to float position in water to float horizontally, not vertically?horizontally, not vertically?

Why, if you had an aluminium dinghy Why, if you had an aluminium dinghy and a block of aluminium the same and a block of aluminium the same weight, the dinghy will float and the weight, the dinghy will float and the block sink?block sink?

When they try to float on the surface When they try to float on the surface without moving some people manage without moving some people manage it easily while others always sink. it easily while others always sink. Why?Why?

Why is it easier to pick up and hold Why is it easier to pick up and hold an object in water than doing it on an object in water than doing it on land?land?

An individual who floats is said to An individual who floats is said to display positive buoyancydisplay positive buoyancy

Others tend to sink in the water and Others tend to sink in the water and display negative buoyancy display negative buoyancy (OHT #1)(OHT #1)

A person’s ability to float may A person’s ability to float may influence performance at both the influence performance at both the beginner and championship levelsbeginner and championship levels

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

Archimedes PrincipleArchimedes Principle

The reason why one object floats better The reason why one object floats better in the water than another is determined in the water than another is determined by Archimedes Principle of Forces which by Archimedes Principle of Forces which states that:states that: ‘‘A body submerged in a liquid is buoyed up A body submerged in a liquid is buoyed up

by a force equal to the weight of the fluid by a force equal to the weight of the fluid displaced’displaced’

This means that an object will float in a This means that an object will float in a fluid if its density is fluid if its density is less thanless than the density the density of that fluidof that fluid

The density of fresh water is 1.00, and if The density of fresh water is 1.00, and if an object has a density of less than this an object has a density of less than this it will floatit will float

If the object’s density is greater than If the object’s density is greater than 1.00 it will sink1.00 it will sink

Also known as the ‘specific gravity’ of Also known as the ‘specific gravity’ of an objectan object

The effective specific gravity of the The effective specific gravity of the human body is 0.95human body is 0.95

This means that in general the human This means that in general the human body has the ability to floatbody has the ability to float

A lump of steel will sink because it is A lump of steel will sink because it is unable to displace water that equals its unable to displace water that equals its weight weight (OHT #2)(OHT #2)

But steel of the same weight but shaped But steel of the same weight but shaped as a bowl, will floatas a bowl, will float

This is because the weight gets This is because the weight gets distributed over a larger area and the distributed over a larger area and the steel displaces water equal to its weightsteel displaces water equal to its weight

A heavily laden ship floats because A heavily laden ship floats because its total weight is exactly equal to its total weight is exactly equal to the weight of the water it displacesthe weight of the water it displaces

It is this weight that exerts the It is this weight that exerts the buoyant force supporting the shipbuoyant force supporting the ship

DVD 02:10 – 04:21DVD 02:10 – 04:21

Factors Affecting the Ability to FloatFactors Affecting the Ability to Float

Lung CapacityLung Capacity The volume of air in the lungs has a The volume of air in the lungs has a

pronounced effect on an individuals ability pronounced effect on an individuals ability to floatto float

Deep inhalation = considerable volume of Deep inhalation = considerable volume of air added to lungs = specific gravity reduced air added to lungs = specific gravity reduced substantiallysubstantially

The majority of males and females will float The majority of males and females will float if they have taken a full inhalation of airif they have taken a full inhalation of air

But the majority of males will sink unless But the majority of males will sink unless they have more than residual air in the lungsthey have more than residual air in the lungs

Body BuildBody Build Individuals with a high fat content will tend Individuals with a high fat content will tend

to be good floaters when compared with to be good floaters when compared with those who are heavily boned and well those who are heavily boned and well muscledmuscled

This is because of the specific gravity of body This is because of the specific gravity of body tissue:tissue: Bone = 1.5 – 2.0Bone = 1.5 – 2.0 Muscle = 1.0Muscle = 1.0 Fat = 0.8Fat = 0.8

Endomorphs – overweight = floaterEndomorphs – overweight = floater Mesomorphs – little fat, high proportion muscle, Mesomorphs – little fat, high proportion muscle,

bone = sinkerbone = sinker Ectomorphs – slim, slight = floaterEctomorphs – slim, slight = floater

SexSex Females tend to have greater proportions of fat Females tend to have greater proportions of fat

which aids their floatation because it lowers which aids their floatation because it lowers their specific gravitiestheir specific gravities

Muscular TensionMuscular Tension Can disallow the lungs to fill with air adequately, Can disallow the lungs to fill with air adequately,

having a negative effect on floatationhaving a negative effect on floatation

Reduction in Buoyancy ForceReduction in Buoyancy Force The upward buoyancy force is reduced whenever The upward buoyancy force is reduced whenever

a part of the body comes out of the water, a part of the body comes out of the water, therefore swimmers should keep as many parts therefore swimmers should keep as many parts in the water as possiblein the water as possible

Centre of Buoyancy / Centre of Centre of Buoyancy / Centre of GravityGravity

Many individuals have the ability to Many individuals have the ability to float but cannot assume a horizontal float but cannot assume a horizontal body position as the legs tend to sinkbody position as the legs tend to sink

This is because in air, the body This is because in air, the body rotates around the COG, while in rotates around the COG, while in water the axis of movement is the water the axis of movement is the COBCOB

The actual floating position for a human The actual floating position for a human body is when the COG is vertically body is when the COG is vertically aligned with the COBaligned with the COB

The legs sink until the COG (hip region) The legs sink until the COG (hip region) and the COB (lung region) come into and the COB (lung region) come into vertical alignment vertical alignment (OHT #3)(OHT #3)

This is why a person with a COG higher This is why a person with a COG higher up in their body has dragging legs, up in their body has dragging legs, creating a lot of dragcreating a lot of drag

DVD 00:35 – 02.10DVD 00:35 – 02.10

HydrodynamicsHydrodynamicsThe Study of Propulsion The Study of Propulsion and Resistance in Waterand Resistance in Water

Propulsion is the force which drives Propulsion is the force which drives an object through the wateran object through the water

Human propulsion in water is Human propulsion in water is generated by the use of…..generated by the use of….. Arms, hands, legs and feetArms, hands, legs and feet

The structure of the shoulder, elbow The structure of the shoulder, elbow and wrist joints allow a wide range and wrist joints allow a wide range of movementof movement

These joints can be rotated to These joints can be rotated to produce the required forces more produce the required forces more functionally than the hips, knees and functionally than the hips, knees and anklesankles

Basic Forms of PropulsionBasic Forms of Propulsion

PaddlingPaddling Pulling and pushing action of hands and Pulling and pushing action of hands and

arms in waterarms in water ScullingSculling

Movement of hands through the water at Movement of hands through the water at approximately right angles to the approximately right angles to the direction of intended traveldirection of intended travel

FinningFinning Leg kicking action of freestyle, Leg kicking action of freestyle,

backstroke and butterflybackstroke and butterfly

Newton’s 3rd LawNewton’s 3rd Law For every action there is an equal and For every action there is an equal and

opposite reactionopposite reaction

Swimmers must try to push the water Swimmers must try to push the water backwards rather than downwards and backwards rather than downwards and upwardsupwards

When a swimmer pushes the water When a swimmer pushes the water downwards this results in the body being downwards this results in the body being forced upwardsforced upwards

As the arm pushes backwards the body is As the arm pushes backwards the body is propelled forwards propelled forwards (OHT #4)(OHT #4)

While the hand presses upward the While the hand presses upward the body is forced downbody is forced down

The consequence of these actions is The consequence of these actions is that the body bobs up and down as that the body bobs up and down as the swimmer moves through the the swimmer moves through the waterwater

At the advanced swimmer stage At the advanced swimmer stage resistance must be kept to a resistance must be kept to a minimumminimum

Bernoulli’s PrincipleBernoulli’s Principle

Greater propulsion in water is obtained by Greater propulsion in water is obtained by moving a large amount of water a short distance moving a large amount of water a short distance than by moving a small amount a great distancethan by moving a small amount a great distance

If a swimmer pulls their hand in a straight If a swimmer pulls their hand in a straight line it is pushing a small volume of water a line it is pushing a small volume of water a long waylong way

Once the water has started moving Once the water has started moving backwards the swimmer cannot apply as backwards the swimmer cannot apply as much force on the water as his hand meets much force on the water as his hand meets less resistanceless resistance

To obtain maximum propulsion the To obtain maximum propulsion the hand must move faster than the hand must move faster than the waterwater

The best method is to seek The best method is to seek stationary or ‘still’ water by using a stationary or ‘still’ water by using a curved pathway for the hands curved pathway for the hands (OHT (OHT #5)#5)

Bernoulli EffectBernoulli Effect (OHT #6)(OHT #6)

When fluid particles travel over an When fluid particles travel over an object shaped like a wing where object shaped like a wing where there is a convex shape on one of the there is a convex shape on one of the sides, the particles that travel over sides, the particles that travel over the larger area meet up at the back the larger area meet up at the back of the wing at the same time as the of the wing at the same time as the particles that travelled over the particles that travelled over the flatter surface on the other sideflatter surface on the other side

Therefore, the particles that went over Therefore, the particles that went over the convex side must have moved the convex side must have moved fasterfaster

Bernoulli found that when particles Bernoulli found that when particles move fast they create low pressure, move fast they create low pressure, and a higher pressure on the other and a higher pressure on the other side – and side – and liftlift can occur can occur

So when we move our cupped hand So when we move our cupped hand through the water, water travels faster through the water, water travels faster over the knuckles than over the palmover the knuckles than over the palm

This creates a low pressure by our This creates a low pressure by our knuckles and a higher pressure knuckles and a higher pressure under our palmsunder our palms

We can use this lift force that occurs We can use this lift force that occurs to propel us through the waterto propel us through the water

In breaststroke, bringing cupped In breaststroke, bringing cupped hands in towards the chest creates hands in towards the chest creates lift that enables the breathing lift that enables the breathing subroutine of the strokesubroutine of the stroke

Propulsive Lift Force on the HandPropulsive Lift Force on the Hand

Lift always acts in a direction Lift always acts in a direction perpendicular to the flowperpendicular to the flow

Lift force is felt as pressure on the palms Lift force is felt as pressure on the palms of the hands when the slightly pitched or of the hands when the slightly pitched or tilted hand moves through the watertilted hand moves through the water

E.g. sculling action; As the hand moves E.g. sculling action; As the hand moves through the water at a slight angle, a through the water at a slight angle, a pressure differential is created on pressure differential is created on alternate sides of the hand alternate sides of the hand (OHT #7)(OHT #7)

Since motion occurs from high Since motion occurs from high pressure to low pressure, the pressure to low pressure, the propulsive lift is perpendicular to the propulsive lift is perpendicular to the direction of the path of the handdirection of the path of the hand

E.g. treading water, sculling with E.g. treading water, sculling with horizontal hand movements. Lift horizontal hand movements. Lift force is produced on the hands and force is produced on the hands and maintains the head above the maintains the head above the surfacesurface

Propulsive Drag Force on the HandPropulsive Drag Force on the Hand

Propulsive drag is created by the Propulsive drag is created by the backward movement of the hand through backward movement of the hand through the waterthe water

As the hand is pulled or pushed against As the hand is pulled or pushed against the water, a high pressure zone is the water, a high pressure zone is created on the palm of the hand and a created on the palm of the hand and a low pressure zone on the back low pressure zone on the back (OHT #8)(OHT #8)

The difference in pressure creates a The difference in pressure creates a force on the swimmer’s hand which force on the swimmer’s hand which moves the swimmer forwardmoves the swimmer forward

Drag can definitely hinder the Drag can definitely hinder the progress of a swimmer – as you will progress of a swimmer – as you will see when we talk about resistancesee when we talk about resistance

But without it, a swimmer will not be But without it, a swimmer will not be able to move in waterable to move in water

Think of a sprinter on land – to gain Think of a sprinter on land – to gain greater speed they wear spikes to greater speed they wear spikes to get more friction on the track to aid get more friction on the track to aid their propulsiontheir propulsion

Swimmers must do the same, but Swimmers must do the same, but must be careful about not creating must be careful about not creating too much drag to slow them downtoo much drag to slow them down

Drag can aid propulsion by the hand Drag can aid propulsion by the hand ‘grabbing’ the water‘grabbing’ the water

Newton’s 3rd law – action of Newton’s 3rd law – action of grabbing the water, reaction of the grabbing the water, reaction of the body going forwardbody going forward

The hand pulling backwards The hand pulling backwards produces a high pressure in the produces a high pressure in the palm and a low pressure at the back palm and a low pressure at the back of the handof the hand

Scientists have concluded that the Scientists have concluded that the swimmer gains propulsion by both swimmer gains propulsion by both drag and lift and by changing drag and lift and by changing sequences of the hand during the sequences of the hand during the stroke to get a ‘resultant force’stroke to get a ‘resultant force’

Angle of AttackAngle of Attack

The swimmer needs to continually change The swimmer needs to continually change the pitch of the hand as it travels it’s the pitch of the hand as it travels it’s curved path so that it is using both drag curved path so that it is using both drag and lift forces to maximum effect and lift forces to maximum effect (OHT #9)(OHT #9)

The angle should be about 45 degrees so The angle should be about 45 degrees so that the resultant force is an equal that the resultant force is an equal contribution of both lift and drag – so that contribution of both lift and drag – so that the body moves forwardthe body moves forward

DVD 06:42 - endDVD 06:42 - end

ResistanceResistance

Water offers a far higher resistance to Water offers a far higher resistance to objects moving through it than does objects moving through it than does airair

Because of this, it is very important Because of this, it is very important for a swimmer to obtain and maintain for a swimmer to obtain and maintain a streamlined position when a streamlined position when performing a strokeperforming a stroke

Resistance or the slowing down effect Resistance or the slowing down effect of the water is also known as ‘drag’of the water is also known as ‘drag’

Resistance / drag is the force exerted Resistance / drag is the force exerted by the fluid against the body which by the fluid against the body which reduces its speedreduces its speed

1.1. Skin Friction Resistance (or Surface Skin Friction Resistance (or Surface Drag)Drag) (OHT #10)(OHT #10)

When swimming, the water must move When swimming, the water must move around your body and limbsaround your body and limbs

A thin layer of water next to the body A thin layer of water next to the body actually sticks to it, and moves with itactually sticks to it, and moves with it

The overall effect of this is a The overall effect of this is a considerable drag on the forward considerable drag on the forward progress of the swimmerprogress of the swimmer

‘‘Drag free’ swimming costumes Drag free’ swimming costumes and shaving downand shaving down

2.2. Tail Suction Resistance (or Eddy Tail Suction Resistance (or Eddy Drag)Drag)

Depends on the size, shape and Depends on the size, shape and speed of the swimmerspeed of the swimmer

When the irregular shaped human When the irregular shaped human body is propelled through the water, body is propelled through the water, the flow lines don’t remain smooththe flow lines don’t remain smooth

Instead they are deflected and break Instead they are deflected and break up into a number of whirls creating a up into a number of whirls creating a great deal of turbulencegreat deal of turbulence

This type of resistance is very costly in This type of resistance is very costly in terms of energy outputterms of energy output

The greater the frontal area hitting the The greater the frontal area hitting the water, the greater the eddy resistancewater, the greater the eddy resistance

3.3. Frontal ResistanceFrontal Resistance

Determined by the amount of surface area Determined by the amount of surface area exposed to the direction of forward movementexposed to the direction of forward movement

Swimmers must maintain a swimming position Swimmers must maintain a swimming position that is as streamlined as possible – i.e. present that is as streamlined as possible – i.e. present as small a surface area as possible to the wateras small a surface area as possible to the water

DVD 04:21 – 06:42DVD 04:21 – 06:42

Newton’s 1st LawNewton’s 1st Law

A constant application of force must A constant application of force must be applied to swim at a uniform be applied to swim at a uniform speedspeed

With swimming the resistance to With swimming the resistance to movement is so great that the body movement is so great that the body almost immediately stops when almost immediately stops when propulsion stopspropulsion stops

Breaststroke, which involves a glide, Breaststroke, which involves a glide, uses momentumuses momentum

But if the glide is held too long, But if the glide is held too long, resistance overcomes the moving resistance overcomes the moving body and it comes to restbody and it comes to rest

It then requires excess energy to It then requires excess energy to regain the momentumregain the momentum

Newton’s 2nd LawNewton’s 2nd Law

A swimmer accelerates forward by A swimmer accelerates forward by increasing their stroke rateincreasing their stroke rate

An even application of propulsion is An even application of propulsion is more efficient in propelling a body more efficient in propelling a body forward than a fluctuating forceforward than a fluctuating force

If a swimmer accelerates and If a swimmer accelerates and decelerates in a stop-and-go manner, decelerates in a stop-and-go manner, much of the force that could be used to much of the force that could be used to overcome water resistance will be lost in overcome water resistance will be lost in overcoming inertiaovercoming inertia