Previous controlled studies demonstrate clear effects of drafting. It reduces oxygen consumption by around 10 percent, lactic acid levels by 31 percent, and participants' perceived exertion (RPE) by 21 percent. These measurements were taken under submaximal, controlled conditions. While distance per stroke was unchanged, the conclusion is clear: if you've been trailing a pair of evenly-paced feet, you'll reach T1 significantly less fatigued.

A later study involving eight highly trained triathletes tested the impact of drafting over 400 meters. Participants swam either without a drafting partner or behind another swimmer maintaining a constant pace. They were encouraged to stay as close as possible to the feet in front and maintain the same stroke rate and length as in non-drafting attempts, with the option to tap the feet ahead to increase pace.

Each participant completed numerous repetitions (10-17 attempts), resulting in hundreds of measurements per person. The outcome: drafting increased performance by an average of 3.2 percent, translating to about 3 seconds per 100 meters in their 400-meter trials. This equates to a pace of 1.12 with drafting and 1.15 without drafting at the same perceived exertion. You gain more from drafting at lower speeds (see below), so in an Ironman swim, you can reasonably expect at least about a 3 percent improvement in performance — possibly up to 4-5 percent in some cases.

The passive resistance of water — its inertia that the swimmer must overcome — is often measured in a lab by pulling a swimmer through the water at a set speed using a line. In such setups, another person can hold the line in front to create a drafting position (see the figure above). This is useful when you want to isolate the passive drag. Note the word passive: it is not the same as the active position when the person in front is actually swimming.

If you want a deeper explanation of how drafting helps, there is a section titled “Explanation of the benefit of drafting” in this study. In short: the swimmer in front sets the water in motion, thereby reducing resistance for the one behind.

In the study on the eight triathletes, the passive resistance was 13 percent lower with drafting at 1.7 m/s (59 s/100 m) and 26 percent lower at 1.1 m/s (1.31 min/100 m). At lower speeds, legs and feet tend to sink a little more, which increases the frontal area and thus the total resistance. Therefore, slower swimmers have more to gain from following someone who creates turbulence and reduces resistance. Faster swimmers lie more horizontally and therefore don't gain as much from drafting.

It's crucial to distinguish between controlled passive resistance (where you're being pulled through the water) and active resistance (where you're swimming behind someone who is actually swimming). A study from 2009 showed that in a passive position, resistance is about 20 percent less when you're behind someone—but this is when the person in front is pulled and not kicking. In an active position, meaning behind a swimmer who is swimming, the savings are still apparent but less: about 9 percent. It's still 9 percent easier than swimming alone. The oxygen cost follows the same pattern.

Another interesting point: resistance increased by about 9 percent (passively) and 8 percent (actively) when participants swam next to each other. The conclusion is simple: feel free to swim behind someone, but avoid swimming beside them—resistance increases that way. The same goes when you overtake someone: do not swim too close side-by-side (besides risking getting your goggles knocked off), rather pass quickly. There is also a study that shows a reduction of 6–7 percent in "drag" if you stay 50–100 cm behind the swimmer's hands, a little diagonally behind. That way, you are approximately at the level of the leader's hands or even further back relative to the lead swimmer.

An earlier study from 1988 examined the impact of wetsuits on drag. While the advantages of wetsuits — improved buoyancy and a smoother surface with less friction — are well-known, how significant are these benefits? Note that this addresses wetsuits around 1988 (neoprene speedsuit from AquaMan, USA).

In the study, 12 triathletes were tested in more realistic scenarios, as drag was measured while they swam, rather than in passive setups with lines. The results indicated that wetsuits reduced drag by about 14 percent at a 1:20 pace and 12 percent at 1:07 min/100 m — consistent with previous findings: greater gains at slower speeds. More recent studies from 2008 reveal a performance increase of approximately 3.2 percent (similar magnitude to the earlier study on eight triathletes) due to a drag reduction of 6.2 percent (full-body suit) or 4.7 percent (half-body wetsuit).

I found an intriguing study that examined how different cap surfaces affect passive drag. Three caps were tested: a smooth silicone cap, one with small dimples (similar to a golf ball), and a wrinkled silicone cap.

In brief: there was no significant difference between the smooth cap and the one with small dimples. However, the wrinkled cap increased resistance by about 4.4 percent—but only when the head/cap was the primary object breaking the water. When both arms were above the head (as in proper freestyle), there was no difference between the caps.

In practice, freestyle often means you have one arm in front of the head, so the head is rarely the primary “nose” in the water. But the point remains: take good care of your cap. If a wrinkled cap gives 1–2 percent increased resistance in practical freestyle, that's 1–2 percent unnecessary waste—and we don't like wasting power.

Drafting behind someone helps — both physically and, for many, mentally. Stay within about 50 cm behind the leading feet; it seems to be the optimal distance according to a couple of different studies. With 50 cm between the leading feet and your hands in an extended position, you gain maximum advantage. If you're closer to an active swimmer, you lose some of the benefit because the kick from the feet can create additional resistance in your zone. At about half a meter's distance, you can expect a performance boost of about 3–5 percent — it could mean several minutes of winning time on the Ironman distance.

Gaining time without increasing effort is pure gold. A lower level of exertion and lower oxygen cost also enables the body to use fat as fuel more efficiently and save glycogen a bit extra — a win all around.

Practical advice:

• Use a smooth and intact wetsuit and take care of your swim cap.
• Practice swimming with good technique — take a freestyle course or two. It often provides the greatest impact.
• Keep about 50 cm behind the feet in front for maximum drafting effect, but avoid swimming beside someone.

Those extra minutes you save by positioning correctly and using equipment wisely can be crucial. Practice drafting, become comfortable with the position, and let your body work smarter, not harder.

To calculate your energy expenditure and how much you save by drafting, use our Energy Calculator, which is updated with a drafting algorithm for swimming.

For more swimming technique tips, listen to episode 59 where we interview the Swim Coach.