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Letter to Editor: Regarding Drag Reduction

May 11, 2000  - John Waring

I have noticed a great deal of misinterpretation concerning the effect of drag reduction on final time reduction. In short, a 10% reduction in drag coefficient does NOT translate into a 10% reduction in a swimmer's final time.

I am an engineer, a swimmer, and a coach. As an engineer, I am ethically bound to tell you that I am under contract with Speedo America. Specifically, I invented a vortex modification to swimwear worn by Jenny Thompson and Lenny Krayzelburg when they set their WRs at the 99 PAN PACs in Australia.

That being said, my purpose here is to clear up misconceptions in the swimming community concerning the effect on final time of ALL manufacturers reduced drag swimwear. Consequently, the information below is applicable to any manufacturer's claimed drag reduction.

The power requirement for a swimmer goes up approximately as the cube of velocity. This means that to go TWICE as fast a swimmer must work EIGHT times as hard. However, power varies directly with drag coefficient. Doubling the drag coefficient requires a swimmer to work TWICE as hard to hold the same velocity. In effect, the power required to swim faster far outstrips the power reduction due to a "faster" suit.

I will not go into a detailed derivation here. However, if one assumes a constant power output (i.e. the swimmer is working as hard as they can), the change in final time can be estimated as follows:

(New Final Time) = (Old Final Time) x [cubed root of](1 - drag reduction)

For example if a swimmer who's best time over 100m is 60.0 dons an ACME Drag-Be-Gone suit with a claimed drag reduction of 4% (note that 4% = 4/100 = 0.04), his new final time would be: (New Final Time) = 60.00 * [cubed root of](1 - 0.04) = 59.19

Which is an improvement in the final time of 1.3%

This is a very rough estimate and neglects, amongst other things, the effect of surface swimming versus underwater kicking. It does, however, give a much more accurate idea of the effect of a reduced drag suit.

In essence, while a better suit has an effect, it is far less than the effects of improved technique and fitness (i.e. showing up for workout and listening to your coach).

John Waring
Head Coach
Carleton University Swim Team