It's like a car. When you hear the horn faintly in the distance, you may still cross the street, with a bit of luck. When it booms right near your ear, then it's about to crush you.
In other words, the sound goes from faint to very loud as the ship is closing by. When you start distinguishing the individual screws in the general whoop-whoop noise, it means it's really close - about 1000 meters. And BTW, you can hear the dc's splash in the water.
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