Why fast liners did not always zigzag at sea

Did fast WWII liners zigzag to defeat submarines?

Not always in the mid-Atlantic: their speed and routing often made a firing solution impractical, while zigzagging mattered more on slower approaches to port.

The answer distinguishes high-speed transits from approaches to port. At more than thirty knots in the Atlantic, a liner could pass beyond a submerged submarine's useful firing opportunity before the attacker reached position. Intelligence and route changes also helped send fast ships away from reported U-boats. Continuous zigzagging would reduce the liner's speed and could therefore increase rather than reduce danger in mid-ocean. Near ports, where traffic controls forced ships to slow and follow predictable approaches, zigzagging again became more relevant.