Plunges bobs (5)
I believe the answer is:
ducks
This clue might be a double definition.
'plunges' is the first definition.
(ducking is a kind of plunging)
'bobs' is the second definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Maybe they are linked in a way I don't understand?
Can you help me to learn more?
(Other definitions for ducks that I've seen before include "Evades" , "Swimming birds with scores of nothing?" , "Birds with webbed feet" , "Water birds; zero scores" , "Eg, mallards" .)