NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Prop-walk.
From: George Huxtable
Date: 2003 Apr 23, 01:16 +0100
From: George Huxtable
Date: 2003 Apr 23, 01:16 +0100
Some comments on prop-walk from George. I'm not convinced by any of the explanations so far. Dave Weilacher was certainly wrong when he said- >The bottom end of the prop is more efficient than the top half. The water >is more dense >by 3% of an atmosphere at the bottom than at the top. That simply just ain't so. Water is virtually incompressible. Any difference in density is infinitesimal. Perhaps list members may like to ponder on the following hypothetical situation. Take an immersed cylindrical submarine, ballasted so as to be neutrally buoyant (and so as not to be rotated by the prop-shaft torque). Will that show prop-walk? To me, simple symmetry implies that there will be none. Turn the sub through 90 degrees, so that its stern is vertically down. Now, will there be a force, at right angles to the prop-shaft? No, there's no way of choosing one direction over another: it's symmetrical. It's like using a propeller-type paint-stirrer, driven from an electric drill: the type which always ends up flinging paint around the garage. But before that happens, is there any sideways force on it, or on the paint-tin? Symmetry says no. Does anyone disagree so far? Go back to the horizontal cylindrical submarine. Attach a vertical fin to its tail, behind the prop (just where a rudder would be), extending exactly as much below the prop as above it. Again, this is symmetrical, so there's no overall sideways force. Now, remove the bottom half of this fin, destroying the symmetry. The water-flow from the prop is a backward-facing jet of water, but it also picks up a spiralling motion from the rotating propellor. If the propellor is turning clockwise (seen from aft) then the upper fin will be pushing the stern to starboard, because of the water-flow impinging on it from the propellor. That would have been exactly balanced by a corresponding pressure on the lower fin, pushing the stern to port, if the lower fin was in place. But now we have removed the lower fin, so that balance has been lost, and the result is prop-walk. That thought-experiment was designed to remove any influence of the nearby water-surface, or any ship-shape of the hull. Prop-walk can arise from surfaces of the vessel which intercept some of the spiralling outflow of water from the propellor. Now, back to surface vessels. For a vessel with a thin vertical sternpost, and a vertical rudder which extends well above and below the outflow from the propeller, I suggest that propwalk would be minimal. For a boat such as mine, with a sternpost and rudder-pivot at 45 degrees, I would expect to see a lot, because there's a much more hull-and-rudder-area behind and above the prop than there is behind and below it. If any vessel had its sternpost angled the other direction (going further aft as you descend), I suggest propwalk would be in the opposite direction: but I know of no such vessel. In reverse, a corresponding effect must occur. The spiralling outflow goes forward from the prop, embracing the hull, divided into two parts by the keel. With the propellor turning anticlockwise, water leaving the prop on the port side of the keel is free (to a large extent) to pass downwards underneath the hull without being intercepted. However, water leaving the prop on the starboard side of the keel, with a partly upward motion, becomes trapped between the hull and the surface, and has to be deflected by the hull before it can flow away. Try fixing your rudder in the straight ahead position, and with the boat lashed to its berth, put the engine into reverse. Give a few seconds for the resulting current flow around the boat to stabilise. If your boat behaves like mine does, and your prop is a clockwise-forward (right-handed) one like mine is, you will see a strong surface current leaving the starboard quarter, in a direction about 45 degrees clockwise from the bow: but on the port side, little or nothing. That strong flow direction must imply a corresponding reaction on the stern of the boat in the opposite direction, pushing the stern to port. I'm sorry not to be able to put numbers or experimental results into the arguments above. I just hope to have persuaded some of you that the detailed shape of boat and hull may have a large part to play in the effect of prop-walk. George. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================