NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Prop-walk.
From: Jared Sherman
Date: 2003 Apr 22, 15:32 -0400
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".