Ferries and other craft crossing Sydney Harbour naturally leave a distinct wake, signified by trails of a lighter colour than the surrounding water. These sometimes persist for more than 30 minutes after the craft has passed through, and the surface takes on an appearance reminiscent of ski trails through virgin snow. How can this be so on a body of water that is in constant, albeit gentle, motion?
• The wake has two main components: the bow wave that spreads out at an angle behind the boat and turbulence created by the craft and its propellers, which produces eddies or swirls in the water. These eddies have angular momentum that is slow to dissipate, so the disturbance lingers and is visible for some time after the boat has passed.
Ships don’t just mark their paths on the water, but in the air too. Their routes can be traced by (condensation trails). Known as ship trails, these are strings of cloud induced by particulate pollution from the exhaust fumes of boats (24 February, p 24).
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David Muir, Edinburgh, UK
• The trails persist because there isn’t much mixing of the disturbed water in the wake by wind and waves. Water gets stretched a bit in one direction and compressed in another when deformed by waves, but each portion of water remains in about the same place relative to each other. And although wind may cause the surface water to flow in a particular direction, it doesn’t normally cause a lot of mixing.
Eric Kvaalen, Les Essarts-le-Roi, France
• Sydney Harbour is nearly surrounded by land. Its complex shoreline damps currents, wind and waves, leaving vertical oscillations that do little to scramble anything writ in water.
Other factors help trails persist. In clean water, bubbles soon escape, but pollutants that are distressingly common in such areas weaken the bubbles’ surface tension. The shearing action of high-speed propellers can then split bubbles so finely that some become microscopic and remain submerged long after the boat has passed. The frothiness of these small bubbles scatters light, giving a pale cast to boat trails.
Jon Richfield, Somerset West, South Africa
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