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    Re: Prop-walk.
    From: Jared Sherman
    Date: 2003 Apr 22, 15:32 -0400

    
    
    Yes George. Basic Newtonian physics, i.e. you send water to the left, and the 
    boat reacts by moving to the right, or vice versa. A screw is not an 
    efficient means of moving a boat, they were developed for the military 
    because paddlewheels were very vulnerable to cannon damage, and screws are 
    protected under the hull.
    
    The shape of the screw is such that some water moves aft, some moves 
    sideways--opposite the turning ("walking") of the blades. Place the screw in 
    a cylinder (a kort nozzle) and that ends as an "end plate" on the blades, so 
    there is less walk and more thrust. Place the screws up a pipe and you've got 
    a water jet engine instead--and no prop walk.
    
    I'm sure that if you really wanted to pursue this, you could obtain or develop 
    mathematical models of the blades in the water which would show you the exact 
    thrust vectors off the screws and allow you to build a larger model, similar 
    to a VPP program, which modeled the entire exact hull along with all wind and 
    current effects.
    
    But that would be the equivalent of investing a doctoral dissertation on 
    modeling the wind and current and prop walk for one single unique small 
    craft. Time that could be much more easily invested in just helming it, and 
    learning the way it responded for real. (Which you'd need to observe and 
    document anyway, to confirm the theories.)
    
    Try enquiring at a school of naval engineering, or aviation powerplant 
    engineering. I expect this has all been documented (or written off as "ignore 
    the details") for 75 years or so. What we call "prop walk" on a boat is 
    pretty much the same as "P-factor" on a prop-driven aircraft, except there it 
    is noted because it means the plane can peel out faster "this way" than "that 
    way".
    
    
    

       
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