NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Prop-walk.
From: JC Sutherland
Date: 2003 Apr 23, 22:28 +0100
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 >>>