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    Re: Prop-walk.
    From: Jared Sherman
    Date: 2003 Apr 23, 14:31 -0400

    Gary-
     
     Right on the first half. But consider this: What happens to a diver when they 
    go deeper in the water? The *diver* is under more pressure, and will be 
    eventually crushed from it. The fact that the water is incompressible has 
    nothing to do with the fact that the water is under more pressure as it gets 
    deeper.
    
     Because the deeper water is under more pressure (yet not compressed) means 
    that the edges of the prop are under more pressure, and because the deeper 
    parts are under more pressure the air does not readily "boil" off the bottom 
    half of the blade, even while it is boiling off the top--cavitating.
    
     Some interesting experiments with water pressure: Take a dime store snorkel, 
    use it. Now buy on of those 4-foot long models, and TRY to use it. The 
    pressure difference of the extra three feet makes it nearly impossible for 
    your chest muscles to bring in fresh air and use it. In fact, in SCUBA class 
    you would be taught never to come upright (from a horizontal position) 
    without exhaling, because the air in you lungs "down there" can expand 
    sufficiently to explode a lung only 4' higher up, when you "stand".
    
     The water doesn't have to be compressed, in oder to transmit pressure. That's 
    one reason hydraulic systems work so well.
    
     Incidentally, as you go deeper the water tends to be colder. Colder water can 
    hold more dissolved gasses than warmer water--which is why ice cubes made 
    from boiled water always have fewer bubbles than ice cubes made from tap 
    water. During the boiling process, the water can't hold the gasses and they 
    are driven off. Surface water in an ocean also will be able to hold less 
    dissolved gas than water even a foot or three down.
    
     This is why old tires migrate to the top of landfills. Literally--the tires 
    are "flowing" to the lower pressure at the top of the pile, where nothing 
    else is pressing down on them.
    
     Ain't magic, just odd science.
    
    But I still say bouyancy doesn't float boats, Poseidon's favor does.
    
    
    

       
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