Steven Shelikoff wrote in message
...
On Wed, 31 Mar 2004 10:20:59 +0100, "JimB"

So I hung the spatula just behind the fan. Lo and behold, the
same
thing happens but just a little less. When I rotate the
spatula to the
left, there is a noticable *left* motion to the blade... i.e.,
it's not
only drawn forward into the blade but it also moved to the left
from
where it was when the spatula blade was perpendicular to the
fan. When
I turn it to the right, the spatula swings to the right.

Steve, that was the experiment I first did. Then I realised that,
to yaw the boat, I had to look solely at lateral force. To do
this I had to constrain the card so that it could only hinge
laterally (no fore and aft motion permitted). This is where the
bits of wire came in. The card had a bit of wire attached rigidy
to the top, sticking at 45 deg horizontal angle to the card. The
card end of the wire bent down to stop the card swinging around
the wrong end of the wire. I hung the card (your spatula I
guess!) through two loops (hinges) first mounted parallel to the
centre line of the fan, then at right angles.

This gave a different result, very little lateral swing, lots of
fore and aft swing. Of course (a weakness in the experiment) it
didn't check for any lateral force effects on the fan of changes
in airflow, nor was it a very good representaion of relative
sizes of prop and rudder.

That proves to my satisfaction that if the rudder is close
enough to the
prop, it's direction will have some effect on the motion of the
boat
when you throw it in reverse even before the boat starts making
sterway.

My initial conclusion too, until I changed the hinging
arrangement.

JimB