What propels boat south -- oarsmen? (5)

I believe the answer is:

screw

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'boat south oarsmen?' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for screw that I've seen before include "shot in pool hall" , "Helical thread" , "made tight by driver?" , "Tapered fastener with a slotted head" , "Prison guard, colloquially" .)

I've seen this clue in The Telegraph.

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