By no fluke does this slow the boat down (3,6)

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

Rufus likes his nautical references, and this is a fairly technical one: a fluke is a kind of blade on an ship’s anchor, designed to catch the ground. A sea anchor doesn’t have flukes, but works by using the drag of the water to slow the ship down.