To reduce the surface friction of a body moving in a fluid the skin is formed with regularly spaced cup‐shaped indentations 2 which, as shown, are of generally oval form, and arranged in staggered rows 3, 4, 5, … spaced apart by approximately half the pitch of the indentations, such that at the desired relative speed clastic waves set up in the fluid by the motion are reflected into and out of successive indentations, whereby the fluid layer adjacent the surface is maintained in oscillatory movement. The passage of the body sets up discontinuities in the layers of fluid adjacent to the skin to produce a wave‐reflecting surface 11 (fig. 1b) so that a wave 10 set up by the front 9 of the body is reflected successively by the surface 11 and the indentations 2′, 2″, … as shown at 10, 10′, 10″ The staggered arrangement of the indentations ensures that adjacent layers of fluid do not interfere to any material extent. The resonant vibrations set up over the whole body reduce the drag by the action of the waves in breaking up the eddies which would otherwise be produced.
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