Reducing Drag During Glides
Does a swimmer's depth in the water make a difference in glide speed?
Not too long ago, the conventional start technique was a flat entry. The athlete was told to jump out as far as he could by reaching straight out, and then land flat on the water's surface. In other words you were to do a belly-flop. In fact, if the coach didn't hear a loud slap when you entered the water, then you weren't doing the entry correctly. When the swimmer's skin was red from head-to-toe, then the entry was executed correctly.
(Aren't you glad this technique isn't used anymore? No wonder starts were not practiced very much and only at the end of the work-out. Ouch!)
Today, we encourage the athlete to jump up-and-out for the maximum time in the air. We encourage a clean entry, defined as an entry through a single hole of the water's surface, first made by the hands and then the rest of the body following through the same hole.
Now the question is: After entering the water how deep should the dive be?
The following table presents two studies, side-by-side, with seemingly opposite conclusions. Which one makes more sense?
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J. Jiskoot and J. Clarys "Body resistance on and under the water surface" In J. Terauds & W. Bedingfield (Eds.), International series on sport sciences, SWIMMING III, Vol. 8,Baltimore: University Park Press. |
Andrew Lyttle (University of Western Australia, Australia)
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| Average to good males (N = 43) were towed in a 200-m tank on the surface and at a depth of 60 cm at speeds ranging from 0.7 to 2.0 m per second. Both tests were repeated some months apart with the first test being conducted in a water temperature of 18 and the second at 24 degrees Celsius. | 31 experienced male swimmers towed in a prone position through the water using a pulley system to determine the optimal depth for the glide and push-off phase. |
| The resistances increased in both conditions upon the retesting. This could be due to the water temperature change. | Drag force acting on each swimmer was measured in trials performed at four depths (surface, 0.2, 0.4, and 0.6 m). |
| Resistance increased in the vicinity of 20+% when towing velocity was increased from 1.5 to 1.9 m/s. That amount is far in excess of previously published values of 11%. | For velocities of 2.2 to 3.1 m/s, the drag at 0.2 m underwater was significantly higher than the drag recorded at 0.4 and 0.6 m, but no significant difference was found between 0.4 and 0.6 m underwater. |
| Resistance was found to be greater when totally submerged than when swimming on the surface. Under water resistances are due to total frictional resistance plus total eddy resistance while on the surface there is wave resistance, and partial frictional and eddy resistance. | The drag at the surface was significantly higher than at depth for all towing velocities (range from 1.6 m/s to 3.1 m/s). |
| Implication. It is preferable to perform on the surface rather than under the water because of the resistances incurred in swimming. | Conclusion: it may be beneficial for swimmers to perform their glides at a depth of approximately 0.4 m to gain maximum benefit from reduction of resistance. |
Pretty confusing, isn't it? But consider this:
Frontal Area Is Difficult To Estimate
Frontal surface area during crawl stroke was measured and compared to methods which estimate frontal surface area.
National caliber crawl stroke swimmers (N = 10) swam a maximal 50 m sprint. Frontal surface area for one complete stroke cycle was digitized by assessing the trunk only, and the body (arms, trunk, legs). Frontal surface was also calculated using the methods of Clarys (1979), Yeadon (torso calculation, 1990), and Kolmogorov and Duplishcheva (1992).
All methods produced significantly different results except for the digitized trunk area and Clary's method as well as trunk and Yeadon's method.
Implication: Estimating frontal surface area is not an accurate procedure. Caution should be exercised in calculations for active or passive drag in swimming when frontal surface area needs to be considered. Unless digitized values can be obtained, calculations should not be entertained.
Cappaert, J. M., Gordon, B. J., & Frisbie, K. (1997). Frontal surface area measurements in national caliber swimmers. Medicine and Science in Sports and Exercise, 29(5), Supplement abstract 712.
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Our conclusion: The best way to find out how deep your entry should be is personalized technique development with "qualified, experienced guidance". At the PowerStarts Start-Training Clinics, you will discover the technique that produces the quickest results that work for you. |
