In an earlier contribution (Francis 1956), a report was made of the speed of small solid bodies floating in the upper layers of a free surface stream. The bodies were cylinders of several diameters floating with their axes vertical and ballasted to the same draft. All the floats travelled at the same speed in the long, wide stream which was of constant depth and was strongly sheared, and this speed was the mean speed of the upper layers of the stream. It was pointed out that this result is unexpected, since the solid boundaries of the floats prevent turbulent interchange there, so that the floats are not locked to the stream in the same way as the corresponding volume of turbulent water would be locked. Thus the floats would be expected to travel a little faster than the stream, large diameter floats travelling faster than small floats.
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