Downing the Resistance

As winter turns into spring and summer, we rowers experience pleasant changes. Not only can we reduce our intake of vitamin D supplements, as our bodies produce this vital nutrient through exposure to sunlight, but also our boats glide through the water faster as if by magic, without our having to put in any extra effort.

This is not only because we reduce the weight we carry in the boat by getting rid of the layers of clothing we needed in cold weather but also because higher temperatures reduce the resistance forces that slow us down.

From my own experience as a coach and athlete, I’ve noticed time and again that splits get better as the temperature rises. The differences are by no means tiny. You row at your steady-state pace and a week later you’re moving with the same effort but you can see on your SpeedCoach that your splits are faster. Since this can’t be attributed to any significant change in training, it must be related to warmer temperatures.

Only a few studies exist that measure speed changes at different temperatures. When the temperature increases from 62 to 82 degrees F., the famous boatbuilder Klaus Filter showed a speed improvement of 0.66 percent, and Jacques Bex calculated an advantage of 0.97 percent. Training at the higher temperatures, a crew rowing 500 meters in 2:10 would cover the same distance in 2:09.1 or 2:08.8, respectively. While the differences are small, a crew certainly would notice.

How can this be explained? Every liquid exerts drag forces when an object moves through to it. Because an object has to push the liquid molecules aside in order to move forward, the less densely packed the molecules, the easier it is. This is what happens as the temperature rises. Since each molecule takes up more space without changing its mass, the density of the liquid decreases. Result: an object moving in warmer liquid encounters fewer molecules and experiences less resistance.

Density is one of the factors in the formula used to calculate the drag force of an object moving through to a fluid. Assuming that such other factors as drag coefficient, cross-sectional area, and object speed remain constant, the resistance encountered by an object as it moves through a liquid or gas changes linearly with the temperature of the liquid. For our sport, this means that the drag forces decrease at the same rate as the density as the temperature increases.

For a long time, I considered only water resistance as a variable that decreases at warmer temperatures. On closer inspection, however, I found that the change in water density is so small that we see hardly any difference in speed. For example, the density of the water changes only by 0.2 percent from 62 to 82 degrees F., which means that the crew mentioned above would experience a meager speed uptick of 0.3 of a second over 500 meters, which would be undetectable with a SpeedCoach.

While there’s no doubt that we experience discernible increases in speed as the water gets warmer, the density changes of water cannot be the sole reason. This is especially true when we consider that as density decreases, our boats sink a little deeper in the water to achieve the buoyancy needed to keep us afloat. This, in turn, increases the wetted surface area of the hull and affects drag.

Another reason for the increase in speed as temperatures rise is the air resistance of the rowers, boat, and oars. The drag forces that slow us down are generated by water and air, and air drag in rowing is significant, ranging from 20 to 40 percent. Our bodies, the boat with its riggers, and, above all, the oar shafts and blades form a relatively large surface area. Temperature changes the density of air more than it does water—by 3.4 percent from 62 to 82 degrees F.

If we factor in the change in air density, we can see why  speed increases noticeably when we row in warmer weather. If air resistance accounts for 20 percent of the total when rowing, we get the figures given by Filter and Bex. If air resistance accounts for 40 percent, there’s an additional improvement of 0.6 of a second for the crew in the example above.

So temperature does indeed influence rowing speed and therefore race times. And because air is such a big part of overall drag, streamlined features such as riggers, skinny shafts, and close-fitting clothing are of tangible value when it comes to gaining an advantage in a race.

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