When I lived in New England, I often spent summers paddling around a lake in either a kayak or canoe. Every stroke was an opportunity to stare down into the dark water and watch how the flow curled around my oar. Here you see a bit of what that looks like from underwater.
The animation above shows a flat plate – twice as tall as it is wide – submerged about 20 mm below the surface and accelerated steadily from rest. As it starts moving, there’s a clear vortex ring formed and shed behind it. You can also see how the plate distorts the free surface into large depressions. Both of these cause extra drag on the plate. Eventually, though, the plate reaches a steady state.
All together, what you see here is a good representation of what’s going on when a rower first begins to accelerate their boat from rest. Hydrodynamically speaking, the best way to do that isn’t to dig in with a deep stroke. It’s to use a series of short, relatively shallow strokes to get the boat up to speed. This takes advantage of the efficiency of drag generation during acceleration to get the boat to its cruising speed quickly. (Image and research credit: E. Grift et al.)
