The fur of seals may help them swim efficiently by maintaining laminar flow.
"…the short wiry hair or feather coats of such good swimmers as seals and penguins seem to be a great advantage for the maintenance of laminar flows. This is borne out by technical experiments with fine wire on surfaces in a flow field (M. D. Kramer, 1938), as well as by indirect clues. The splendid fur of the seal, for instance, provides no protection against getting wet; it gets soaked through in the water. And, when exposed to the air, wet fur presents a considerable risk of a chill. We must assume that wet fur represents an advantage for swimming. And, like the seal's coat of hair, the feathers of birds may have a favorable effect on the boundary layer." (Tributsch 1984:49)
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