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Why do boat trails last so long?
Les' question was put to Cambridge University's Chris Smith and Hugh Hunt...
Hugh - Well, the wake that comes off a boat is turbulence. It's swirl that's generated by the boat and the only way you can get rid of swirl is by that swirl meeting another solid object because conservation of angular momentum says that that swirl doesn’t disappear. Now, if the water is quite deep and the boat was away from a shore and there weren't other big boats around then that swirl will last for a long time. It's the same problem you have when a turbulence left from a plane that's just taken off sticks around for quite a long time.
Chris - One other thing to bear in mind is that when a boat goes across the surface of the water, it disturbs or perturbs the surface of the water, and there's a lot of proteins and surfactants which are a bit like washing up liquid in their action chemically in seawater. That's because of plants and other microscopic organisms and sewage. And the effect that that has is to reduce the surface tension of the water so it makes making bubbles much easier. And so, you tend to see little layers of foam or bubbles where boats have gone. And so, one of the things that Lez may also have been noticing is a little trail of spume or the white material that you see as a raft of little bubbles and disturbance on the surface.