Gunwale bobbing is a phenomenon in which a person jumping on the gunwales of a canoe achieves horizontal propulsion by forcing it with vertical oscillations. The canoe moves forward by surfing the resulting wave-field.
After an initial transient, the canoe achieves a cruising velocity which satisfies a balance between the thrust generated from pushing downwards into the surface gradients of the wave-field and the resistance due to a combination of profile drag and wave drag. By superposing the linear wave theories of Havelock [1] for steady cruising and Helmholtz for an oscillating source, I demonstrated in a recent paper [2] that such a balance can be sustained. I calculated optimal parameter values to achieve maximum canoe velocity, and compared theoretical results to accelerometer data taken from an enthusiastic gunwale bobber (see GIF to the right) and to estimates from videos of other aficionados.
Interestingly, there are some similarities between gunwale bobbing and other examples of macroscopic wave-driven bodies (such as walking droplets), not to mention possible applications to competitive sports.
Have you ever wondered why a kayak or a rowing boat is pointed at both the front and back? Whilst the point at the back makes the kayak more streamlined (similar to the tapered end of an airplane wing), the point at the front helps the boat slice through the water with minimal wave disturbance. Each of these aspects of the design are made to minimise wasted energy that could otherwise be used to make the boat go faster. But because these two sharp points have origins in two different physical phenomena, there is no reason for them to be symmetric. In other words, why not design a kayak with some front-back asymmetry?
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