The effects of surface finish on drag : coatings of racing shells

The purpose of this project was to design a lightweight surface coating for racing shells that reduces the drag due to surface friction, allowing them to move more quickly through the water. To achieve this goal, it was necessary to choose which materials and what coating recipe would be most effective in reducing drag, scale a test piece to coat, build a testing facility, and develop an effective testing procedure to gather the most accurate data possible. To some extent, all of these tasks were completed. I decided to explore the theory that superhydrophobic surfaces reduce drag by fabricating a Nafion-Aerogel film recipe and coating an appropriately scaled flat plate on which to test the film. While Nafion, a sulfonated tetraflouroethylene based fluoropolymer-copolymer manufactured by DuPont and used primarily in fuel cells, is not even hydrophobic, let alone superhydrophobic, it was possible to fabricate superhydrophobic films by adding superhydrophobic aerogel material to the Nafion. During a month-long study of these films, a recipe was developed and later replicated and painted on a test surface exhibiting a contact angle with water of 162°, well within the superhydrophobic region. In addition, with the help of recent graduate Andy Krauss, a circulating water tank was built in which the drag on test pieces was measured using a dynamometer. Finally, tests were run to obtain preliminary data that supports the theory initially being investigated. The data acquired from the current testing facility shows a slightly reduced drag on the superhydrophobic surfaces. At velocities of about 0.8 m/s, the drag on the superhydrophobic test piece is approximately 0.125 N and is 0.138 N on the uncoated piece. At Reynolds numbers around 140,000, the superhydrophobic plate has a drag coefficient of about 0.028 while the uncoated test piece has a drag coefficient of about 0.031. While this data supports the original theory that superhydrophobic surfaces reduce drag, there is uncertainty associated with the readings that must be considered. Due to the irregularity and volatility of the flow within the tank, at velocities of about 0.8 m/s and Reynolds numbers above 140,000, the standard deviation of the data is approximately to 70%. With improvements to both the design of the tank and the recipe and application of the films, I believe more substantial and encouraging results can be obtained.

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