Monday, February 1, 2016

Are You a "Skilled" Swimmer? Part II

by Alan Couzens, MS (Sports Science)

With the onset of winter, many of you will be returning to the ever-frustrating mission of trying to make up some more ground on those elusive folk who perpetually lead you out of the water –- the ex-swimmers.

In a previous post, I provided a means of classifying yourself into three levels of swim skill -- skilled swimmer, triathlete and novice -- by simply comparing how fast you move through the water relative to your engine size (VO2max). The inference being that if you have a big engine and you’re not particularly quick in the water, you’re wasting a lot of the work produced by your engine in non propulsive activities, or, put another way, you’re not "skilled" when it comes to swimming. In reality though, the situation is a little more complex…

Did you ever wonder why rowing shells are shaped the way they are? If you’ve ever tried to get into/onto one, you’ll know that it’s certainly not for the stability that the shape provides! Nor, is the shape especially conducive to floating. If you compare a rowing shell with a swamp boat –- a vessel designed to stay on top of the water as much as possible, they’re quite different. So what’s the skinny on the long and skinny? And what implications does it have on our own aquatic vessels as swimmers?

The power that a swimmer must produce to achieve a given speed in the water is the sum of both their propulsive efficiency/"skill" (that is, how much power actually makes it to the water) and the drag that they must overcome. More drag means more power needs to be produced to achieve a given swim speed. Or, put another way, the more drag that a swimmer is dealing with, the fitter they must be to hold the feet of a more skilled swimmer.

This total drag is the sum of three separate sources of drag:

  1. Form Drag – The drag created by the cross-section of your body moving through the water.
  2. Friction – The drag created by the interface of your skin (or suit) on the water.
  3. Wave Drag – The drag created by the wave you pull along with you when when swimming on the surface.

So, in addition to maximizing our propulsive efficiency (skill), what can we do to minimize these retarding forces? Answer: not much.

We can shave down and (sometimes) wear wetsuits or skinsuits to minimize friction drag. We can also wear wetsuits, improve stroke timing and keep a horizontal body position to minimize form drag (to some extent) but when it comes to the dominant drag –- wave drag -– we’re kind of stuck with what we’ve got.
Going back to the rowing analogy, there is, indeed, a reason that row shells are long and skinny, just as there is a reason that elite swimmers are long and skinny and it comes back to wave drag and a guy by the name of Froude. Froude discovered that the resistance that a ship encountered was directly proportional to the velocity/length of the ship at the waterline (^1/2). In other words, as either the velocity increased or the length of the ship decreased, the resistance that the ship had to overcome went up. This is the reason that we see boats that are built to "slip" through the water at high speed are long and skinny and swimmers who are built for speed are the same.

In fact, if we compare the Froude number of an athlete who is 6’6” versus an athlete who is 5’4”, the shorter athlete will encounter 22% more drag and have to put out 10% more power to swim at the same speed! In general, an athlete will lose 0.6% speed (about 0.5s/100m) for every inch difference in height.

However, if you’re more row boat than row shell, don’t get too depressed. In my last post I used data from Holmer (1972) and Khort (1987) to show an approximate 20% difference in swim speed between "skilled" swimmers versus "moderately skilled" triathletes. In reality, only a relatively small part of this difference can be attributed to body type. The relative contribution of propulsive efficiency, wave drag and form drag for a 1.9m tall swimmer and a 1.6m triathlete is shown below (derived from Toussaint, 2005).

Or, put in real world terms, for two very fit ironman athletes of different stature (5’4" and 6’3”) and skill, but with comparative VO2max numbers of 4.8L/min, one with swimmer skill, one with triathlete skill, the swimmer will outswim the triathlete by 13 minutes for the same output over the course of an Ironman swim. Of this 13 minutes, eight can be attributed to differences in propulsive skill, while five can be attributed to differences in body size/shape. Of course, when these two athletes hit the bike, the situation is reversed, with an athlete with smaller frontal area but the same sized engine dramatically advantaged on the bike.

The larger points of the above illustration being:

  • As triathletes, many of us still have the most upside to improving skill in the water.
  • If you are a shorter triathlete with good skill, it will take more swim strength/fitness to keep up with more morphologically favored athletes in the water –- fitness that will take time to develop, time that may be better devoted to maximizing your natural strengths.

Train Smart

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