U-boat evacuated vessel making a sudden change of course (1-4)

I believe the answer is:

u-turn

'sudden change of course' is the definition.

I don't understand how the rest of the clue works.

(Other definitions for u-turn that I've seen before include "Embarrassing reversal" , "Complete reverse of car on public road" , "180 degrees change of direction" , "Sudden complete change of plan" , "Complete reversal of direction or policy" .)

I've seen this clue in The Independent.

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