On my Westerbeke 44B Four, with a fixed 16" three blade Campbell Sailor Prop, it can develop about 500+/- pounds of reverse thrust at 80% of max throttle, when in reverse. This data comes from Campbell and their prop calculator which is based on the Michigan Wheel calc but adjusted for the Campbell.

As you can see my 44HP engine, at 80% of wide open throttle, can't even replicate 30 knots of wind.... Using low idle RPM barely replicates 15 knots on most sail boats.

Of course if your boat is already moving backwards, and this is not done from a dead stop, you'll also have the inertia weight to stop..