WAVE DRAG NEEDS ATTENTION
As a swimmer moves through a fluid, frictional (surface) and form (cross-sectional) drags (resitances) occur. When a swimmer is totally immersed in air or water, the action of forcing the fluid to divert produces waves. Waves occur and are at a minimum when completely covered and at a particular depth in water or height in air. For swimming, it is water immersion that is of primary importance. A depth of at least 0.4 m is necessary for a complete swimmer to incur minimal resistance from the production of waves. With complete immersion, water can easily escape under, above, and to the sides of the swimmer.
When a swimmer performs at the water's surface, that is, the interface between water and air, the body creates additional forces that cause increased waves. The reaction to those forces, called wave drag, is additional to frictional and form drags. When completely immersed to an appropriate depth, a swimmer moves more easily through water by incurring frictional and form resistances and minor wave resistance. On the surface, frictional and form resistances are reduced but wave resistance is increased dramatically because water can only escape below and to the sides of the swimmer. In some situations in swimming, such as after dive entries and turn push-offs, swimmers travel faster when appropriately immersed than when on the surface swimming with a full stroke. The effect of the increased wave production on the surface is much greater than that of fully immersed swimming. When the body is submerged there is an increase in frictional and form drags, which is more than offset by the reduction in wave drag relative to what is incurred on the surface. That is the reason for swimmers being able to propel themselves faster when fully submerged compared to what they can do on the surface.
The production of wave drag at the surface should be of great concern. As velocity increases, wave resistance increases as the cube power of the velocity. Thus, if a swimmer's velocity doubled, frictional resistance is doubled, form resistance is quadrupled, and wave resistance is octupled (eight times as great). This is one reason why a swimmer's comfortable velocity is not much slower than maximum velocity. The sum of the resistances at the surface mount very, very quickly as velocity is increased, to the point where the swimmer can produce no more propulsion (energy) to overcome the sum of the resistances.
Since wave drag increases so much faster than form drag and very much faster than frictional drag, it stands to reason that technique developments and changes should minimize the production of waves. The shape of the swimmer (e.g., long and thin or "cigar-like"), the posture of the swimmer (rigid and linear as opposed to relaxed and bendable), and the orientation of that posture (e.g., parallel to the surface, that is, flat or horizontal) will minimize wave production from the swimmer's non-propulsive body parts.
Body parts have to move to develop propulsive forces. A crawl stroke recovery over the water to almost full extension would incur and develop fewer resistances than one that knifed into the water alongside the head and reached forward under water to full extension. Once the arm is in the water, and waiting for the appropriate time to reposition and produce propulsive forces, it should have the characteristics of a non-propulsive body part, that is, extended, rigid, linear, and horizontal. The arm would also have to be at a depth that would allow water to flow over it to minimize the wave drag it creates.
Any unnecessary movements of the body in the vertical plane will cause waves to occur as the movements displace large and weighty volumes of water. The energy for the water movements comes from the body and is energy lost from propulsive potential. There are actions in crawl stroke, such as lifting the head to breathe and then snapping it back down underwater, that are wave-forming movements. Any sideways or vertical movements of the hips will also increase wave production as more water is diverted in unnecessary and unproductive directions.
The simplest indication of wave resistance when swimming on the surface is the size of the bow wave created by the swimmer's progression, although a bow wave is only one of the several waves formed. The relationship of the bow wave to the lane lines is a simple index. A technique change that reduces the bow wave size relative to the constant lane line size at a constant velocity is likely to be a good technique change.
The above explanation is for crawl stroke but, it also pertains to butterfly stroke and backstroke. It does not apply to breaststroke.
If a breaststroke recovery is made over the water, the vertical forces created by the arms, shoulders, and head crashing back into the water to set up for the next stroke causes a great production of energy-sapping waves. While many breaststrokers still persevere with the total or aspects of "wave-breaststroke", it is gradually being shown to be less efficient than flat, underwater breaststroke. The techniques of several women breaststrokers have been again oriented to being flat and underwater, and world-records have resulted. Male breaststrokers are lagging behind the women, although Kosuke Kitajima spends a good deal of his stroke in very good positions, his minor over-the-water recovery causes unnecessary wave resistance and disruption to good body and limb positions.
Wave reduction is an easy technical way to produce faster swimmers. Instead of being totally concerned with the most minor resistance in swimming, that of frictional resistance, wave resistance minimization is likely to yield better dividends. Ian Thorpe recounted that the Adidas body suit he selected over other body suits was primarily "about muscle compression: a suit so tight it stiffened the body so it would penetrate water like a dart . . Ian was never a hard-bodied athlete; . . " (Hunter, 2004, p. 164). It should be pointed out that this reasoning was sound but unfortunately, other aspects, such as increased frictional resistance, of the Adidas and other body suits more than offset this stiffening advantage.
Wave reduction is an overlooked factor in technique evaluation and instruction. The unfortunate marketing of full bodysuits has focused coaches on frictional resistance. Traditionally, most swimming coaches have been aware of the need to reduce form drag by minimizing frontal area through streamlining exercises and adjustments. Wave production has been of lesser concern when in fact, it should be the first concern because of its devastating effect on swimmer efficiency. However, technique changes usually do not only apply to one form of resistance. A reduction in wave drag is also likely to cause a reduction in form drag, and vice versa. The best approach for swimming coaching would appear to pay attention to both form and wave drag at the same time. In swimmers who improve technique by reducing both form and wave drags, if their velocity does not increase then their ability to sustain a velocity for an increased period should result. When energy expenditure is critical, and that is in all swimming races from 100 meters to beyond, wave reduction is of paramount importance.
Hunter, G. (2004). Ian Thorpe: The biography. Sydney, Australia: Pan MacMillan.
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