Steven Shelikoff wrote in message
...
On Tue, 30 Mar 2004 08:04:41 -0700, Keith Hughes
wrote:

enormous snip

Sorry to interrupt this thread, but you may remember a little
earlier I said I'd go away and play with fans, bits of card, bits
of wire and smoking fag ends to get my brain around this. The
idea was suggested by a very reasonable post from Derek Rowell.

I'm trying to square Jax's flat 'nada' with rudder kick I've
observed, and an impression that the rudder direction affects
boat yaw when in reverse and not moving,

I fixed a card to a bit of wire which I could bend at various
angles to the card (rudder angle). I then hung the card upstream
of the fan so that it could pivot only along the fore and aft
axis (above the fan) and again so it could pilot only along the
lateral axis, and again so that the rudder could rotate around
the vertical axis of its front post. Smoking fag ends came later,
with a rigid mount. Fan was turned on.

Forces were observed by noting the degree of card deflection
around the relevant hinge. There was a net force on the rudder,
primarily exerted towards the fan. It has little lateral
component, but lots of fore and aft component.

Smoking fag ends showed strong non-linear flow when the rudder
was deflected, but the net flow is an s bend zig-zagging around
the rudder towards the fan and two carpet burns. Smoke speed
dropped markedly with distance away from the fan.

The rudder kicked hard over (either way) when allowed to pivot
around its forward vertical axis. Within the limitations of my
crude experiment, rudder kick is probably caused by the net
effect of the fore and aft component of force, not a lateral
component.

I think this explains the rudder kick I've witnessed in astern in
some boats engaging astern gear, and probably explains Derek
Rowell's observation that the rudder rotates when allowed to (If
I understood his experiment design right).

However, the zig-zagging airflow proves to my satisfaction that
the rudder may not create a net lateral force, so 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