Propeller walk

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Propeller walk is the term for a propeller's tendency to rotate a boat as well as accelerating it forwards or backwards.

A right-handed propeller (which rotates clockwise when in forward gear) will tend to push the aft of the boat to starboard. When in reverse gear, the effect will be much greater and opposite. A right-handed propeller will now push the aft of the boat to port.

Knowing of and understanding propeller walk is important when maneuvering in small spaces. It can be used to one's advantage while mooring off, or it can complicate a maneuver if the effect works against you.

Propeller walk is a complicated effect which depends on ship geometry, direction of travel, propeller direction, vessel speed and depth of water. Three causes are identified for a vessel 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.

The first of these results from there being a measurable difference in speed of water flowing close to the hull and that at lower depths which has not been affected by the vessel's motion. At low speeds the last effect is most pronounced and when going astern has even more influence.

In shallow water the upwards flow from under the vessel becomes much less strong and ultimately disappears. Model tests carried out show 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.

Other terms for propeller walk are propeller effect, asymmetric thrust and asymmetric blade effect.