Athletes rely on equipment and technique when swimming in open water. The movement of swimming has two forces, a resisting motion of drag and an assisting motion of propulsion. The forward motion and speed of a swimmer is determined by forces resisting motion (drag) and forces causing motion, propulsion. There are three main types of drag: form, surface and wave (Blazevich, 2012). To achieve an optimal swimming velocity, the goal of the swimmer is to create optimal propulsion and minimise drag.
HYDRODYNAMICS-DRAG
How can we improve a swimmers efficiency in open water?
Drag occurs when molecules of a
fluid (air or water) collide with an object and take energy away from it
(Blazevich, 2012). The bigger the surface area of the object or the faster the
molecules are moving the bigger the drag. The loss of energy from the object (swimmers
body) to the fluid (water) can be viewed as laminar flow: water moving towards
the swimmer in ordered layers ( refer to picture below). As the water approaches the swimmer, the layers
separate. As the water passes the swimmer, the water changes direction and
therefore, gains energy. The energy gained by the fluid is always equal to the
energy lost from the object and is called non-laminar flow or turbulent flow (Blazevich,
2012).
At some point of contact with the
swimmer the water flow may become turbulent as the fluid towards area of
low pressure and takes away energy from the swimmer.
FORM DRAG- relates to the surface area and shape of the
swimmer.
The only way to limit the affect
of drag on performance is to reduce the amount of surface area coming into
contact with the molecules (water). The swimmers surface area is affected by
their position in the water. Form drag or frontal resistance occurs at its
greatest when the swimmers body is too low in the water and therefore,
producing a larger surface area ( refer to picture below). The water pushes against the body in the
opposite direction, causing the swimmer to lose velocity (Blazevich, 2012). If
the swimmer continues to swim in a low body position, they then need to use
more energy in order to maintain velocity in comparison to a higher body
position.
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SURFACE DRAG
Surface drag is affected by the
roughness of the surface of the object. For a swimmer this relates to their
skin and swimming suit/bathers. As the water makes contact with the swimmers
body, small pockets of ridges in the skin or clothing catch the fluid, thus
allowing a force to be applied and energy transferred. Therefore, wearing
non-porous materials, allows the water to travel over the surface easily.
Wetsuits
To optimise body position, the entire body needs to be flat
in the water and to ensure the forces lifting the body are equal to the forces
pulling the body down. Therefore:
·
Body mass- gravity pulls the swimmer down
·
Buoyancy- is the force lifting the swimmer up in
the water
This means the buoyancy to body
weight ration must be equal across the entire body. As humans lungs have a
large volume with a low density and the legs are dense, there is a natural
tendency for the legs to drop or sink while swimming. The most effective way
to minimise the sinking of the legs is to use a wetsuit.
The wetsuit does the following to aid in buoyancy:
·
The wetsuit covers both the legs and is relative
to the volume of the legs, however it is very light so the buoyant forces
increase but does not increase the weight forces.
·
On the upper body the wetsuit adds some volume
but not as much in comparison to the effectiveness on the lower body.
Wetsuits assist with enhancing buoyancy by lifting the
swimmer higher in the water, therefore, reducing the amount of form drag and
aiding in overall performance (Cordain & Kopriva, 1991). Also by increasing
buoyancy it allows the swimmer to expend more effort in the propulsive
movement. Leaner swimmers, tend to benefit more from wetsuits than swimmers
with a larger mass due to their natural buoyancy (Cordain & Kopriva, 1991).
Swimming equipment of goggles reduce drag
reduction by 2.2% and swim caps by 3.4% as they help the swimmer become
streamline (UCAerospace2012).
The extra buoyancy to help the swimmer remain streamline
assists with the overall swimming time and allows the swimmer to concentrate on
creating horizontal propulsion through the water instead of focusing on
vertical propulsion to lift their body.
Speed Suits: Speed suits are made from a thin layer of material that mimics the skin of a shark. They compress the body circumference by up to 5%, which aids in decreasing the overall surface area of the swimmer (Phill Ligett UCAerospace2012).
Surface drag is the friction created by the fluid on the
swimmer. Recent research and design of swim suits have reported to have lower
drag coefficients with a small buoyancy effect (Blazevich, 2012). The idea of
these suits is it increases surface drag, allowing the water to remain attached
to the swimmer as a boundary layer, like a golf ball. The swimsuits have been made so that the surface is rough not smooth (refer to picture below). The
rough surface creates a small amount of turbulence in the thin layer of water
flowing close to the surface of the material (Blazevich, 2012). This is the same principle behind
the dimples in golf balls.
Wave Drag
Wave drag is created through the
interference of water and the air as the swimmer pushes through the water (Blazevich,
2012). The water in front of the swimmer acts as a resistance and pushes back
against them and in turn slows down their speed. Other water movement that also
forms around the body is due to the pressure difference and can take away
‘energy’ from the swimmer. As the swimmer increases their speed, so does the
length and height of the wave drag, as the waves travel at the same speed as
the swimmer (Blazevich, 2012). Research on measurements of drag have indicated
that total drag will continue to increase with velocity and will be smaller or
equal to the drag arising from the body travelling through the water (Blazevich,
2012). Furthermore, it can be thought
there is no particular speed to swim at, that will minimise wave drag, however,
swimming technique of the arm action and body roll might help reduce the wave
build up and in turn minimise drag.
Skilled swimmers have demonstrated
they can create smaller waves in comparison to less-skilled swimmers by (Blazevich,
2012):
·
Increasing body length- stretching the arm in
front of the body at the end of the recovery phase.
·
The arm entering the water might cause earlier
separation on the oncoming flow of water. Therefore, reducing the pressure at
the front of the head and reduce wave build-up.
·
Swimming with their head down, chin on their
chest, reduces the up and down movement of the body. It is thought, lower head
position reduces the pressure at the front of the head.
·
Body rotation reduces the effective surface
area.
·
Small leg kick, reduces the pressure difference
around the leg area and therefore, reduces
wave formation.
Overall to minimise wave drag the
best option is to swim as much as possible underwater, only lifting the head
occasionally to sight the direction is which the swimmer is aiming for i.e the
buoy.
Further ways to improve open water swimming
Technique is even more important,
as the body rotates; athletes use as twice as much kinetic energy on their
front as opposed to on their side. Rolling from one side of their body to the
other transfers power from the rotation to the propulsive arm, allowing for
more efficient stroke (Phill Ligett UCAerospace2012).
The biggest advantage in open
water swimming is ‘drafting’, swimming behind the person in front of you,
provides energy savings of 18-25% as the water is already moving forward (Phill Ligett UCAerospace2012).
SUMMARY:
In comparing the two types of
drag, the effects of surface drag are not as significant as those of form drag.
However, reductions in surface drag can have measurable effects on performance.
Hydrodynamics-propulsion
Newtons
Third Law, for every action there is a reaction. For the swimmer to move
forward in the water, they need to apply a backward force (Blazevich,
2012). As the aim of the swimmer is to move as quickly
as possible through the water, the focus becomes on power, action power v
reaction power. However, the reaction power is not equal to the action power as
water is not solid and moves when the force is applied. This means some of the
power is used to induce movement rather than propel. The idea is to increase
the amount of reaction power from the given action power (Blazevich,
2012).
The hands should have a relaxed
grip with the fingers slightly spread apart, (the thought of running them
through your hair) and should be the first thing to touch the water (refer to picture below). As the
water flows through it sticks to the fingers and therefore creates a larger
surface area. The increased surface area creates drag and in turn increases
propulsion. Furthermore, it can be thought taller swimmers with longer limbs
could be able to create greater drag forces which could be of benefit to them (Blazevich,
2012).
For a swimmer to maximise the
surface area to propel themselves forward, they begin with the arm
outstretched, having a high elbow, using the hand and forearm to pull through
the water. The elbow and wrist is flexed so the palm is facing backwards. The
stroke ends with the hand leaving the water close to the hip ( refer to picture below). The high elbow is especially important when the water is choppy, to clear the water and not make contact, if contact is made, the wave will force the swimmer backwards.
In the 1960’s the ‘S’ pull was discovered as a
sculling movement (Blazevich, 2012). Essentially the hand moves laterally through the water and
is on a slight tilt towards the incoming water. This lateral movement of the
hand creates lift on the palm of the hand and effectively the hand can pull. Furthermore,
by using a slight ‘S’ pull, allows the swimmer to pull through still water.
From pushing against still water the swimmer can exert more force and hence
giving more propulsion. Furthermore, the efficiency of swimming can be improved
by applying the forces in line with the direction of the buoy.
There is debate to whether the ‘S’
pull has its advantages. Research using particle image called Velocimetry which
tracks the motion of water and quantifies how fast it is moving. This
technology has proven the change in direction produces a jet flow that itself
is propulsive. This new information suggests the greatest propulsion is when
the hand is moving slowly, i.e. during the change in direction of the hand
(kamata, et al 2006). These subtle changes in hand movement are very important
in optimising propulsion. At this point
the researchers are not suggesting for swimmers to change their stroke movement,
however, encourage them to ‘feel for the water’ and recognise that the
directional changes the hands make are in fact generating propulsion (kamata et
2006).
The breathing action in swimming inflicts
the swimmers streamline position and propulsion as the heads moves out of
normal position to take in air (Pedersen & Kjendlie, 2006). This means the
more often the swimmer breaths, the less speed they will have overall. Tests
have indicated if a swimmer breaths every 3, their speed should not decrease due
to the breathing action (Pedersen & Kjendlie, 2006). If the water is choppy, it is best to breath to the side the wave is not crashing, this avoids receiving a large amount of water in the mouth.
How else we can use this information:
These principals can be taken to other swimming strokes in regards of how to minimise drag and at the same time maximise propulsion.
Breaststroke swimmers
propel themselves under the water
and only surface to breathe, this motion does create wave drag but is minimised
due to the swimmer keeping their hands out in front like a point. In butterfly
the underwater arm movement is quite similar to freestyle and when turning use
the underwater dolphin kick to their benefit. Furthermore, as more research is
done on hydrodynamics and our understanding increases there will be more
advances in the speed that water-based sports can reach. This can include speed
boats, yachts and jet skis.
The Answer
For effective
open-water swimming, drag needs to be minimised and at the same time increase
propulsion and this is achieved through modifying the swimming technique:
·
Head position low- chin tucked onto the chest.
To minimise wave drag by keeping the body under the water and keeping the
streamline position.
·
Small leg kick maintained at all time- will assist
keeping the hips up and reduce wave drag.
·
Use of wetsuit or speed suit with swim cap and
goggles.
·
If possible swimming directly behind another
swimmer.
·
Streamline position, lifting head only
occasionally to sight the buoy, as lifting will increase frontal resistance and
form drag.
·
Maintain alignment of the direction of the buoy.
·
Angled
hand position when entering the water.
·
Transferring the power from the rotation to the
propulsive arm, this allows for a more efficient technique.
·
Fingers slightly spaced apart to increase the
effective surface area and in turn increase the drag and lift forces of
propulsion.
·
High elbow, wrist slightly flexed and ‘S’ pull
to grip the water and propel forward.
Combining these technique principles will give a swimmer the best opportunity to achieving optimal results in open water swimming.
References
Cislunar Aerospace, Inc (1998). Why does a gold ball have dimples? Retrieved
from http://www.fi.edu/wright/again/wings.avkids.com/wings.avkids.com/Book/Sports/instructor/golf-01.html
Cordain, L, & Kopriva, R. (1991). Wetsuits,
body density and swimming performance. Department of Exercise and Sport
Science, Colorado University.
Kamata, E., Miwa, T., Matsuuchi, K., Shintani, H., and Nomura, T.
(2006). Analysis of sculling propulsion mechanism using two components particle
image velocimetry. International Symposium on Biomechanics and Medicine in Swimming,
50-52.
Pedersen, T., Kjendlie, P. (2006). The effect of the breathing action
on velocity in front crawl sprinting. International Symposium on Biomechanics and Medicine in Swimming,
75-77.
UCAerospace2012.
(2012, October, 29th). 2012 Ironman World Championship Kona.
Retrieved from http://www.youtube.com/watch?v=gge7Ag__Pm8
