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Dave-
 If you take a toy fan (because a real one would be way too dramatic) and 
place it on a carpeted floor, then turn it on, what happens? The blades spin, 
but the fan itself "walks" the other way on the blades.

Effectively a screw is just a like wheel (which also happens to produce axial 
thrust) and as the shaft turns, the wheel rolls sideways. That's all it takes 
to "walk" your stern sideways. The fact that water is being thrown sideways 
as it slips off the blades just enhances this prop walk.

The boat pivots sideways simply because there is no opposing force to prevent 
that end of it from walking. The keel, rudder, and hull are providing only 
lateral resistance when the boat is not in motion, so they just slow the walk 
down.

   

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