JAXAshby wrote in message
...

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.

Lovely. I understand that. A plume with an enormous splay angle
converging into the prop. No need to do the 'rolling downhill
bit' for me.

Some of the plume being interfered with by various obstacles
(such as rudders and hulls).

At this stage you may be interested in the behaviour of my model
of smoking fag ends, bits of card and wire hinges, all mounted up
stream of the heater fan suggested by Derek Rowell.

First, there is a net force on the rudder, primarily exerted in
the direction of the fan.It has little lateral component, but
lots of fore and aft component (Those wire hinges were good for
resolving things). There's strong non-linear flow when the rudder
is deflected, but the net flow is an s bend zig-zagging around
it, going to the fan. Approach speed dropped markedly with
distance from the fan (as you comment - a cube relationship if
there are no constraints)

The rudder kicks hard over when it is allowed to pivot around its
forward vertical axis. Within the limitations of my crude
experiment, this seems to be caused mainly by the net effect of
the fore and aft component of force, not a lateral component.
This explains the rudder kick I've witnessed in astern in some
boats, and probably explains Derek Rowell's observation that the
rudder rotates when allowed to (If I understood his experiment
design right). My thanks to him for suggesting the idea of an
experiment. It's been great fun.

So I can now understand the mechanism whereby there's rudder
kick, but little or no lateral force. And I'm stuck with the
revelation that the yaw effect that my old skipper demonstrated
to me was wind, mirrors, inertia, prop walk and my hero worship.
Ah well.

But now I've got to get rid of those carpet burns before the wife
comes home.

JimB