Why a boat's wake is always a 39° V: Kelvin's wedge
30seconds
Behind every boat spreads a V of ripples, and it is always about 39° wide, whether the boat crawls or speeds. Lord Kelvin worked out why in 1887: only waves that keep pace with the boat add up, and deep-water waves carry energy at half their phase speed, which pins the edge at arcsin(1/3), about 19.47° either side.
Build the wake yourself, drag the wave direction to see why nothing ever goes wider, then switch to sound-like waves and watch a jet-style cone narrow with speed instead.
Deep dive · 4 min
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