The River Thames is beautiful in early summer, and the stretch just upstream of central London is full of birds: motionless herons watching for fish, bustling moorhens poking around the submerged plants, and watchful flocks of preening swans. But as my canoe club paddles along the river, we’re always looking out for our clear favorites: the mother ducks with a fluffy cluster of ducklings tagging along behind.
When they’re lingering at the shore, it all looks slightly chaotic, with baby birds dawdling and exploring as the mother duck keeps an eye on them. But when it’s time to move on, chaos shifts into order as the ducklings line up behind the adult and the convoy moves off together quickly and in perfect formation. It’s mesmerizing to watch, and it’s not accidental. This is all about saving energy.
Swimming is hard work, because water is dense and slightly viscous. At the surface, a large part of the resistance to movement comes from the waves that are produced behind whatever is moving: its wake. This is the familiar wedge-shaped wave pattern that we see behind ships, canoes and ducks alike, and all of those waves carry energy. If you move along the water surface, those waves are always continually generated and so the swimmer needs to put in enough energy to create them. This energy cost is felt as a resistance to movement. There’s also resistance because the water touching the ship or duck is pulled along too, creating additional drag. [Read more…]
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