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    Re: Prop-walk.
    From: JC Sutherland
    Date: 2003 Apr 23, 22:28 +0100

    George et al;
    
    Just to muddy the waters!
    When I was learning to fly a single engined biplane, I had to remember
    that when opening the throttle on take off I must compensate, for the
    Yaw created by Engine torque, by extreme rudder until there was enough
    air speed for the rudder to act normally.
    I believe also that when the towards the end of the war the high
    performance Spitfires and Typhoons, fitted with Griffon engines, there
    was so much engine power that they were prone to ground loop on take
    off. To combat this both these aircraft were later fitted with
    contra-rotating props.
    
    I agree that the effect of water density on the thrust of a ship
    propeller is negligible but the effect of pressure is not. I do not
    believe that the shape of the hull per se is important.
    My understanding is this:
    When from astern of a boat, looking forwards, towards a clockwise
    propeller, driving ahead,
    1. The extreme upper and lower portions of the propeller rotation
    produce transverse forces which cancel out and therefore have no effect
    on swing.
    2. The upwards moving propeller blade produces a wash of water with an
    upwards component of motion. This in turn produces a 'Hump' of water on
    the port side of the stern, adjacent and aft of the propeller. This
    extra height of water creates pressure against the hull, pressing the
    stern to move to starboard. Likewise,
    3. The downwards moving blade sucks water down from the surface
    producing a 'Hollow' on the starboard side. This creates a reduction in
    pressure on the starboard side of the hull sucking the stern to
    starboard.
    Consequently the boat yaws to port.
    When the propeller is driving astern the opposite effect occurs. Except
    this time the hump and hollow will occur forwards of the propeller and
    somewhat nearer to the Centre of Lateral resistance. This results in a
    rather smaller yaw to starboard.
    Once the boat gathers speed the rudder becomes more effective in
    steering the boat and the yaw disappears.
    
    Of course, every boat will be different;
    
    Clive Sutherland.
    
    jcs@boltblue.com
    
    oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo
    
    >>>
    Walter Guinon said-
    
    
    >Let me first say that I like prop walk, it makes landing port side to
    much
    >easier.
    
    George comments- I agree. I berth port side to also. When I engage
    reverse,
    the stern shifts sideways to port and snuggles nicely into the berth. My
    propellor is righthanded, turning clockwise (seen from aft) when going
    forward.
    
    
    George Huxtable
    
    >>>
    
    
    

       
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