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How to Draft Like A Dolphin

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Drafting: you've may have seen it in ducks and geese and you may have even seen it in cyclists and ra
 

Swimming In Research

February 2 · Issue #3 · View online
Be the smartest person on deck.

Drafting: you’ve may have seen it in ducks and geese and you may have even seen it in cyclists and race-car drivers, but what about swimmers? Can they benefit from the same principles while in the water?

Steady Hydrodynamic Interaction Between Swimmers
Aim
The aim of this research was to quantify wave drag reduction of trailing swimmers and understand how to take advantage of a their waves
Main Points
  • Drag is one of the biggest factors affecting a swimming performance and it consists of 3 components. First, there is wave drag which is drag from swimmer-generated waves. The next component is skin-friction drag when the water immediately adjacent to the skin slows down as it passes close to the body. Lastly there is pressure drag which is created from the pressure differential on the body ie. high pressure at the head/low pressure at the feet. Think of the swirling eddies you see as a body moves through water. The eddies are a result of the low pressure.

  • This study claims that previous studies have grossly underestimated the contribution of wave drag at 5% and 12% because based on naval knowledge wave drag can rise to as much as 70% of total drag depending on speed and depth.

  • It is important to note In this study wave drag is not calculated indirectly by subtracting skin-friction drag and form drag from total drag, but by borrowing a concept from the naval industry: discounting fluid viscosity and simply solving the Laplace equation. (see pages 2 & 3)

  • When swimming slow on the surface (less than 38s for 50m) wave drag is of a similar magnitude to frictional and pressure drag. When swimming speed increases (~ 38s to 27 high for 50m) wave drag becomes the dominant drag making up a whopping 50-60% of all drag. At high swimming speeds (27s and faster) the wave drag actually decreases while the other two forms of drag continue to increase until they account for all of the drag.

  • In open and calm water, the position with the least amount of resistance is when the drafting swimmer’s head is almost in contact with the lead swimmer’s feet. In this position we see a wave reduction of 125%, but swimming in this position isn’t realistic. Interestingly enough, when the drafter is very close to the leader we also see the leader’s wave drag diminish.

  • The relationship between wave drag and distance from the leader is not a linear one. Instead it was observed that some positions presented more drag than others and when plotted it became obvious that these drag-reduced positions came in intervals…just like waves!

  • After looking deeper into into this it became apparent that the waves produced by the lead swimmer is essentially being ridden by the drafting swimmer. Furthermore, the best position to ride the wave of the leader is with the upper part of the body in the trough and the lower part in the crest as we see dolphins doing when riding waves behind a boat. In this position a wave drag reduction of up to 64% can be experienced. (this was without lane ropes)
Dolphin riding the Boat waves.
  • These results beg the question: in the middle of a race how does a swimmer position themself halfway in a wave? Well according to the researchers: feelings. Competitive swimmers should be able to sense feel the spots of minimum drag.

  • As the the distance between draft and lead swimmer increases the amplitude of the waves decreases and so does the wave drag reduction. Once that distance is greater than 0.5m “the drafter’s bow wave will not have any influence on the leader’s wave drag.”

  • Another important factor affecting wave drag is wave interference. When a the drafting swimmer’s waves are in sync or in phase wave drag increases however when the leader and the drafter’s wave are out of sync they cancel out and drag is minimized.

  • In open water swimming if a swimmer were to trail in the right position behind two leading swimmers such that the drafting swimmer is at the bottom of the V shape, the total amount of drag experienced can be reduced up to 50% (depending on the distance).

  • When drafting behind two leaders it is possible for the drag reduction to be so great (more than 100%) that it turns into a thrust force which propels the swimmer forward.

*The effects of lane ropes were not taken into account for this study.

Which swimming topics would you like to see research on? Send me a note and I’ll see what I can find.
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