How
about an explanation of those phenomena for a numerate old
thickie?

each blade the prop (in reverse) pushes water forward towards the bow of the
boat (also sides, but ignore that). Because "water flows downhill" water fills
in behind each blade as it pushed water forward

(note, if you watch an underwater prop turning in a stationary position, you
will see the level of the water surface fall behind the prop and rise forward
of the prop)

Now, the water that fills in behind the blades comes from whereever there is
water "uphill" of the blade. This is not just behind the prop, but also to the
sides and top and bottom, in more or less a hemisphere (an over simplification.
water actually comes from the "high pressure" side of the blade, the side
towards which the water is pushed.)

The flow "through the prop circle" hs the greatest velocity, with "all that
water" aft just waiting for its chance to "roll down hill". the greater the
distance from the prop, the slower the speed of the water rolling down hill
(lots of water available so it doesn't roll very fast. The speed at which the
water rolls towards the prop is inversely proportional to 4/3rd the distance
cubed (volume of sphere) is all aimed at the center of the prop.

People "think" the water flow towards the prop is straight at the prop, but it
isn't. it is from all edges of the hemisphere aft of the prop. Left, right,
up down, back. all edges.