Notions of hydrodynamics
Moving underwater with a monofin efficiently in order to achieve performance requires an understanding of monofin swimming technique. To understand this technique, you need to have an idea of the laws of hydrodynamics. These laws provide an understanding of the environment in which you operate and help establish some of the technical principles that I will discuss in the next article.
As you move through the water you will encounter and create resistance. These resistances tend to slow down your movement : we talk about swimming resistances.
Some technical terms translated from French maybe do not correspond exactly to the terminology used in English.
I hope to find among you a reader who will help me to be more precise. Send me message to : firstname.lastname@example.org
To understand the different forms of resistance you must first identify two things : the concept of the « frontal area » and the « shape coefficient ».
– the concept of the « frontal area » : It is the frontal surface area of the body that is exposed to the resistance of the water while moving forwards.
– the « shape coefficent » : It’s the total body shape. The same frontal area can be presented with a different total body shape
This Shape while moving in the water will generate resistances. All of these resistances are called « shape resistance » (ifl : resistance de forme). It is the sum of three resistances identified by Counsilman (1968) :
– Frontal resistance
– Friction resistance
– Drag resistance
These resistances are called passive resistances, they are the result of the action of water on the body passively undergone by the swimmer.
It is the resistance created by the surface of the body presented while moving forward. This surface is called the frontal area, it will create resistances when penetrating the water.
As the swimmer moves through the water, the water molecules closest to the body (skin or wetsuit) adhere to the body. The speed of these molecules is therefore equal to zero. As one moves away from the body, the speed of these water molecules increases until they reach the speed of the other water molecules in the « outside flow ». The thin Layer of water in which speed increases is called the « boundary layer » (Jacques Lachnitt 1978). Because two infinitely adjacent layers of water have different speeds, this results in high viscosity forces. We are talking about Friction resistance.
It is a vortex resistance also called tail suction. This resistance to forward movement is extremely disadvantageous : it causes a suction effect at the back of the body.
NB : Between frontal resistance and drag resistance, which of these two resistances is the most penalizing ? Reading the result of the experiment that follows provide an interesting answer. When 4 objects (a, b, c, d) of the same weight and frontal area are dropped in the water (considering that they keep their trajectory during their fall) their order of arrival on the ground will depend on the importance of their frontal resistance and drag resistance.
– Sphere « a » comes first : it has less frontal resistance and drag resistance than the other three objects.
– Disc « d » comes last : it has the most frontal resistance and drag resistance than the other three objects.
– Half Sphere « b » comes before Half Sphere « c »
Conclusion : drag resistances are more penalizing than frontal resistances.
Wave resistance is related to movements close to the water surface. As the swimmer moves, he or she creates an area of turbulences causing waves : the frontal wave in front of the body and the tail wave in the back. These two waves hinder the swimmer’s progress because they form zones of high pressure. To minimise this resistance the mono-finswimmer creates with the position of his hands an additional wave at the front of the body which interferes with the bow wave to give rise to a reduced resultant wave, this is called the Bulb effect. At the front of some boats there is a cylindrical bulge called a Bulb. Its function is to create an additional wave at the bow where the trough falls where the bow wave should be. The two waves interfere negatively and tend to cancel each other out, which considerable reduces resistances. During a DYN event, a freediver may face this resistance if he gets to close to the surface. To completely avoid this resistance it will be necessary to be at a depth greater than 3 times the diameter of its body throughout the dive.
Synthesis of passive resistances
If we seek to reduce passive resistances, we will also seek to create active resistances that will serve as propulsive supports : we then speak of propulsive resistances. It is through the creation and maintenance of propulsive resistances that we move in the water.
The term « propulsive drag resistance » is used for any resistances resulting from or contributing to propulsive support.
Synthesis of passive and active resistances
Swimming with a monofin is the fastest of the swims, it is the « Formula 1 » of fins sports. The swimming technique that has evolved over the last 50 years is the one that best adapts to the reduction of swimming resistance. It produces the propulsive motion with the lowest propulsive drag resistance ratio.
Hydrodynamic law applicable to the monofin
Overpressure and Low pressure
The monofin like an airplane wing ?
It can be considered at least theoretically that the monofin blade under the effect of the water flow (laminar flow) which passes under and over the monofin blade, creating a high pressure zone (overpressure) and a low pressure zone (underpressure), is assisted in its horizontal replacement. Areas of overpressure push, areas of underpressure attract.
Resistance and Speed : R=KSV^2
It is interesting to understand the equation R=KSV^2, which includes the mechanical components of the swimming resistances (R = Resistance, K = Coefficient corresponding to the Total Body Shape, S = surface of the Frontal area, V = Speed).
NB : R=KSV^2 is true for a rigid body. The swimmer has a variable geometry and tries to minimize resistances. Physilogical measurements show that for a swimmer the exponent is less than 2. Di Prampero gives an exponent of 1,2 which seems to be suitable for a swimmer underwater. For surface swimming an exponent of 1,5 would be more realistic.
In this corrected equation : R=KSV^1,2 the speed increased by an exponent shows that it is a determining component.
If the speed is zero then the swimming resistance is zero. On the other hand, this factor has a major influence on the swimming resistance when moving in the water.
If the Speed is 1m pers second, we have : R=KxSx1
If the Speed is 2m pers second, we have : R=KxSx2,3
In Freediving we often speak of a speed of about 1m per second. The swimming resistances is therefore not increased by the Speed component, but the fact remain that if your swimming style is not hydrodynamic (K and S not efficient), then throughout your performance you will produce a greater effort than necessary to move forward.
As an indication, a finswimmer can reach 3,6m per second (R=KxSx4,65). Swimming fast is really confronting different types of resistances. A good lap time implies an ability to reduce swimming resistances. Each time you improve your time, then you will have objectively improved your style.
Swimming with a monofin for a freediver with an efficient style is not an impossible goal. Most of the technical defects that I obeserve in freedivers come mainly from a bad learning or outright non-learning of the technique of swimming with a monofin. This results in significant passive and active resistances which, if reduced, would result in being able to perform more easily, especially at the end of an effort, i.e. : less fatigue, less oxygen consumption, slowing the CO2 production, less accumulation of lactates, more serenity, more lucidity, more efficiency, less risk of loss of motor control, less risk of blackout.
You must therefore optimize your style by working on technique to overcome the swimming resistances. Remember to put regular (timed) speed series in your monofin technique training pool sessions (using a front snorkel for example or by practicing speed apnea). It’s essential !
Next post : Monofin technique
– Approche Scientifique de la Natation Sportive, 1992 Didier Chollet
– La mécanique des fluides, 1978 Jacques Lachnitt
– La science de la Natation, 1968 James E. Cousilman
– The Energy Cost of Human Locomotion on Land and in Water, 1986 Di Prampero (equation N°17)
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Theo-Patrick FOURCADE – bewaterfreediving.com ©
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