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Propeller walk
"Wilbur Hubbard" wrote in message anews.com... "Charles Momsen" wrote in message ... What is the primary cause of propeller walk - torque or thrust? Propeller walk is a term used to describe the results of the bottom of the propeller operating in a more dense environment than the top of the propeller. The primary factor is neither of the above. Drag, due to the propeller's interaction with a denser medium, is increased at the bottom half of the propeller. A prop turning clockwise as the vessel proceeds forward as seen from astern will cause the vessel's stern to move to starboard due to increased drag. Whereas thrust in a vessel is a description of fore and aft forces, prop walk is a description of transverse forces caused by drag differential. That is the easy answer that seems to satisfy most people but if you think about it do you really believe that water gets measurably denser because it is about 18" (say) deeper at the bottom of an 18"(say) propeller than the water at the top? Of course it does not. Water is hardly compressible at all except at very high pressures. .. Density will only be a factor if air is being entrained from the surface. You will find boats displaying propeller walk even if the propeller is too deep for air entrainment to occur. My boat does. So the question is 'why does the opposite direction of the blade at the top not cancel the walk created by the blade at the bottom'? I suggest that this is because as the blades rise upward towards the top the water impinges on the hull and creates a reaction force which acts in the same direction as the walk force created at the bottom. This is because, in yachts especially, the blades pass very close to the hull which usually is somewhat vee shaped and deflects the water sideways. In your example water would be deflected to port but the reaction on the hull would be a starboard force. |
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