Ship's primary means of propulsion using blades? (5)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

A sort of clue-as-definition – we think it must be: S (first or ‘primary’ letter of ship) CREW (means of propulsion using blades -as in rowing) with SCREW referring to the propeller on an engine-driven ship, which itself has blades. All rather clunky, and not one of Serpent’s better clues in our opinion