It's actually not a particularly "good" question.

Both forces are at work, along with water density, angle of drive shaft,
differences in drag in forward vs. reverse, etc. For a RH (clockwise) prop
movement in forward, the front of the boat tends to turn to port, all things
being equal. In reverse, the stern turn to port. On my Sabre, they designed
the engine (and shaft) to be mounted off center (shaft exists the boat
slightly to the port) in order to reduce/eliminate forward prop walk. From
the manual:

OFFSET PROPELLERS: Sabre yachts are built with propeller shafts offset to
port or starboard depending on engine rotation. Engines with clockwise
rotation (right hand) have shafts offset to port.
After extensive research it was found that for this hull type an offset
propeller benefited engine performance and handling characteristics.

Sabre rudders are partially balanced with the rudder shaft about 15% aft of
its leading edge. This feature reduces the load on the helm and makes it
more comfortable to hold while under sail. The powerful wash of a
centerline propeller causes the helm to veer to one side or the other after
a turn is initiated. The offset propeller wash to flow to the side of the
rudder. A second benefit is that the angle between the shaft and the fore
and aft axis of the boat is less than would be possible with a centerline
installed shaft.

Here's a decent link that describes the forces involved:
http://www.sailingislife.com/id28.html.

"Joe" wrote in message
...
What is the primary cause of propeller walk - torque or thrust?

Good question. I would think thrust. Touque would only tend to roll
the boat or cause it to tip.
A cavitating prop will not walk.

Joe



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"j" ganz @@
www.sailnow.com