Fluid Mechanics 2 –
Hydrodynamics: Propulsion

Hydrodynamics
The study of propulsion and resistance in water
Propulsion the force which drives an object through the water
Human propulsion in water is generated by the use of…
Arms, hands, legs and feet
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

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
Prior Learning:
Newton’s 3rd Law
“For every action there is an equal and opposite reaction”

rd
The consequence is that the body bobs up and down as the swimmer moves through the water.

In addition, if the swimmer swings their arms to the side, the reaction is for the legs to swing to the opposite side with an equal amount of force. = “Snaking” pathway
At the advanced swimmer stage resistance must be kept to a minimum in order to improve propulsion.
Summary
The propulsive forces generated by the hands, arms, legs and feet should be directly opposite to direction of preferred movement.

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
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

Note – S Shape
Video - Freestyle Arm Pull
Note – Hand position

Bernoulli also found that:
“In a region of high flow velocity a low pressure zone is created and in a region of low velocity, a high pressure zone results”.

Bernoulli Effect
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.
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

Application in Swimming?
When we move our cupped hand through the water, water travels faster over the knuckles than over the palm.
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

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
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

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

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
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’

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
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

Other factors…
• The size of the hand
• The shape of the hand
• Slippage
• Feathering
(diagrams in handout)

Explain, in detail, how Bernoulli’s principles of propulsion affected your first swimming performance.