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Why can two ships moving close together be drawn toward each other?

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differential manometer, Torricellian barometer, and siphon — fluid mechanics Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.

When two ships move in close proximity, they can be drawn toward each other due to the principles of fluid dynamics, specifically Bernoulli’s theorem.

As the ships travel forward, the water flowing through the narrow channel between their hulls is constricted. According to Bernoulli’s principle, when a fluid is forced through a restricted space, its velocity increases. This increase in fluid speed results in a corresponding decrease in fluid pressure in that region.12

Because the water pressure between the ships becomes lower than the pressure acting on their outer sides, a net force is created that pushes the ships toward one another. This phenomenon is a practical application of the Venturi effect, where the pressure exerted by a fluid is lowest where the cross-sectional area of the flow is smallest.1