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I don't buy this description of the effect. It seems to me that under circumstances where there is little forward or backward motion, then the largest influence on a boat is the combined effect of the propellor wash and its action on the rudder. Indeed I am pretty certain that when the propellor is going astern, the effect on the rudder is reversed so that the rudder needs to be turned the opposite direction from what would be expected.--Rjstott 05:23, 12 December 2005 (UTC)[reply]

I'm afraid that the article is quite succinct and accurate in its description. Prop walk is a direct result of the rotational energy introduced into the water and the fact that the bottom half of the propeller disk is marginally more efficient than the top half. The rudder has nothing to do with prop wal per se. Jmvolc 03:23, 27 May 2006 (UTC)[reply]

But wrong. Here is a better and scientific response which I will summarise in the article

"MARINE PROPELLER BIAS

Marine propeller (or screw) bias is a well-known effect which occurs at all speeds and in all water depths. Also known as the “paddle wheel” effect, it results in the propeller trying to move laterally when under way. In a single screw ship, this causes the stern to move laterally, requiring a small amount of rudder to keep a straight course with an otherwise course-stable ship. As twin-screw ships generally have propellers of opposing rotation, screw bias is a phenomenon not usually noticed on such vessels, until a “twin screwing” manoeuvre is undertaken.

The phenomenon has been studied extensively and features, for example, in most good ship manoeuvring simulations because it is such a well-known feature of low speed ship handling. Various explanations have been given, the most popular (and most incorrect) postulating that its cause stems from the water being more dense at the bottom of the propeller than the top. This cannot be the cause because the density gradient over such a distance is negligible.

Perhaps the best explanation is that arising from some Japanese research carried out in the 1960s. This identified three primary causes of screw bias in deep water:

1. Upward oblique flow at the propeller location. 2. Vertical wake distribution at the propeller. 3. Unbalanced lateral forces on the rudder (when set amidships) arising from the propeller slipstream impinging on the rudder blade.

When a ship moves through water, a flow around the hull is generated, the results of which form the “wake” at the stern of the ship. This is where the propeller is located and the flow regime is anything but uniform. Flow comes round the sides of the hull and, in deep water up from the flat bottom and into the propeller disk. In fatter ships, such as tankers, the flow at the top of the propeller disk can be very much slower than that at the bottom.

This disparity of flow between the top and bottom of the propeller disk affects the inflow angles to the blades, creates an asymmetry in the lift forces from the blades leading to forces in all three directions – longitudinal, lateral and vertical. The last of which these small, but contributes to the change in trim experienced when a ship is propelled, as distinct from towed at a given speed.

So items 1 and 2 in the above list arise from the wake distribution. Japanese tests in deep water, using single screw ship models without rudders, showed that:

• Screw bias caused the stern to move to port and the ships to sheer to starboard. The opposite occurs with the rudder in place • The “fatter” the ship model was, the more it turned

The third item on the list is a consequence of the first two. It is due to the angled flow from the propeller slipstream differing at the top of the rudder from the bottom, thereby causing a lateral force which usually reverses that arising from wake effects.

In shallow water the upwards flow from under the vessel becomes much less strong and ultimately disappears. Model tests carried out by one of the progenitors of BMT showed that, at a very small underkeel clearance, screw bias caused a ship to sheer to starboard (rather than port) when moving ahead and that there is an intermediate depth where the sheer from bias is neither one thing nor the other.

Finally, when moving ahead with the propeller moving astern, flow into and around the propeller is very confused. Generally the overall result for a single screw ship when stopping is a sheer to starboard, but this is not always guaranteed; sometimes it may go the other way, depending often on any yaw rate on the vessel when the propeller starts to turn astern.

For further information on screw bias in shallow water, see:

Dand. I. W.: “Hydrodynamic Aspects of Shallow Water Collisions” Transactions of the Royal Institution of Naval Architects, Volume 118, 1976."

(unsigned)

This article really needs to be rewritten! The claim that the angle of the shaft is the only determinant is completely incorrect, as anyone trying to reverse a boat with a sail drive will know (it has a 90 degrees shaft gear below the hull, and thus a propeller axel horisontal in the water, but still has a significant walk in reverse). I doubt it is even the main determinant. The references to the bottom propeller half being more efficient than the top should be edited into the article, and the reference cited be supplemented.

The illustration has been cropped so as to hide some of the text in and (worse) below the illustration itself. Also, the reference to *boat* movement in the figure is actually misleading, as the walk effect is created by the propeller, and is at its very strongest when the boat is moving forward or still in the water while the propeller is reversing, i.e. to stop/reverse the boat's progress (since the rudder is then not effective and can not compensate for the walk).

It should also be noted that the references to reversing is strictly for a reversing gear, whereas if reversing by reversing the pitch of a variable pitch propeller, the walk would remain in the same direction as for forward. 89.11.232.88 (talk) 02:01, 17 May 2017 (UTC) G Halbo[reply]

False explanation

The explanation given in this article was totally wrong. Also the illustration tells the wrong story. Water has a fairly constant density, so water around the lower blades is not heavier than water around the top blades. The simple explanation for propellor walk, as can be seen by every yachtsman is the propellor water hitting the hull sideways and pushing the ship sideways (when in reverse). I have added two illustrations to show the effect. Unfortunately the false explanation of the phenonenon is repeated in many textbooks.Janhuisman42 (talk) 21:48, 11 October 2017 (UTC)[reply]

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