Drafting is something that you hear about in a number of different sports. Basically, when you draft, you sit very close behind someone else (and by sit I mean paddle, swim, bike, run, etc). The idea is that you spend less energy than the person at the front. But how does this work?
A lot of effort is spent to overcome drag. Drag is primarily created by two forces:
- Friction – this is the friction between your body/equipment and whatever is surrounding you (air/water). All of the skin tight, lycra outfits are designed to minimize friction between your body and the the air/water. Basically, a smoother surface means that there are less bumps for the air to get caught on. Less bumps results in less turbulence and therefore less friction and less drag.
- Pressure – this happens because both air and water have individual molecules that are moved around by an object going through them. We see it even by just pushing our hand through the water. The molecules in front get bunched up, while the molecules behind spread out. As the molecules in front bunch up they become denser which makes them harder to move through.
When one athlete drafts behind another they are experiencing less pressure force because they are sitting in the pocket of less dense molecules right behind the other athlete. Less molecules to go through means lower density and less energy expenditure. But drafting is not an easy skill to execute, and other factors can come into play.
For example, a greater benefit to drafting was identified in speed skating (2 m between athletes reduced drag by 16% for the second athlete, 1 m between reduced drag by 23%) but in in line skating the technical challenges of staying in a drafting position, particularly on corners, resulted in lower benefits.
In cycling, I know from experience that riding behind another person feels like it decreases the energy I have to spend, but you need to be comfortable riding fairly close to another person, and you need to trust that they will warn you of upcoming hazards such as potholes (the Canadian winter wreaks havoc on our roads). If you’re not, then energy savings are not worth the extra stress of the ride.
I have also seen a team in dragon boating get sucked in by the reduced pressure created behind (or beside in this case) another boat and almost collide.
So, drafting then becomes one of the skills that athletes need to practice if they want the full benefits.
Brisswalter, J. & Hausswirth, C. (2008). Consequences of drafting on human locomotion: Benefits on sports performance. International Journal of Sports Physiology and Performance, 3, 3-15.