  Arctic icebergs, unconstrained sea ice floes, oil slicks, mangrove drifters,
lost cargo containers, and other flotsam are known to move at 2-4% of the
prevailing wind velocity relative to the water, despite vast differences in the
material properties, shapes, and sizes of objects. Here, we revisit the roles
of density, aspect ratio, and skin and form drag in determining how an object
is driven by winds and water currents. Idealized theoretical considerations
show that although substantial differences exist for end members of the
parameter space (either very thin or thick and very light or dense objects),
most realistic cases of floating objects drift at $\approx$3% of the
free-stream wind velocity (measured outside an object's surface boundary
layer). This relationship, known as a long-standing rule of thumb for the drift
of various types of floating objects, arises from the square root of the ratio
of the density of air to that of water. We support our theoretical findings
with flume experiments using floating objects with a range of densities and
shapes.
