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Wedge Effect

It is sometimes apparent that a ship when using stern power in the close proximity of solid jetties, banks or shallow water will "cut" the wrong way. There are two possible causes for this occurrence and only a pilot's local knowledge is likely to pinpoint them.

The first is a phenomenon known as the "Wedge Effect". This occurs when the ship with a fixed pitch right-handed propeller has a solid jetty or other vertical obstruction close to its starboard side. If excessive stern power is used, the wash created is forced forward between the ship and the obstruction.

If we look at the above figure , it can be seen that if the flow of water is restricted then a force is exerted on the ship forward of the pivot point. This is particularly apparent when the ship is stopped or making sternway. The force may be of sufficient strength to kill normal transverse thrust and sometimes generate a swing of the bow to port.

It will be worse if the ship has a bow-in aspect or is landlocked forward of the berth, thus increasing the entrapment of water flow. Whilst a disadvantage in some respects it can be turned to advantage in some parts of the world. Using the "wedge effect", a ship can be lifted bodily off a solid jetty when backing out avoiding dragging the bow along the dockside.

The second possible cause of a "cut" the wrong way may be attributed to the vicinity of shallow water. The flow of water from the fixed pitch right-handed propeller working astern as we have seen is up and on to the starboard quarter, but down and away from the port quarter.

If the ship has a small under keel clearance it is possible that, in addition to such factors as cavitation and restricted flow into the propeller, the flow of water on the port side is being deflected off the bottom and back on to the hull. This clearly gives some prior indication that the response of the ship may be unpredictable in shallow water and, once again, the bow may swing the wrong way.l

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