Ship Stationary One Screw Going Ahead, Other Screw Backing (Single Rudder) The rudder will have little effect until head or sternway has been gained. The turning of the two screws will be the same, but it may not be large when your shafts are close together. If the ship has no deadwood,(meaning she is light) she may turn easily. In narrow waters the two screws should be operated at speeds that the ship does not gain headway or sternway when going ahead or backing at one-third or two-thirds speed. This balancing of forces will let the captain move the ship ahead or astern as desired by varying the speed of the backing or ahead engine. As a rule, with the speed of engine ahead the same as that of engine astern, the ship will slowly make headway. The rudder may be used to increase the swing when some steerageway has been gained.
Twin Rudders on Twin-Screw and Multiple-Screw Ships Twin rudders, which vary in position, shape, and size, are on many vessels, large and small. This improves the maneuverability of ships.
The general rule when handling twin-screws with one rudder has been to order the proper rudder after the ship has gained headway or sternway. The installation of twin rudders has changed this rule, and the captain must now order right or left rudder before the engines are moved. The rudder should be put over to take advantage of the flow from the ahead propeller. This flow acts against the forward side of its rudder and creates a powerful force to turn the ship.
The improved turning characteristics of ships are really noticed in narrow channels or when going alongside a pier. The maneuvers to leave a pier under different conditions of wind and tide also helps.
The key to all these difficult maneuvers is the effect of the discharge flow from the ahead propeller on the rudder astern of it. If the other propeller is operated astern, it may do so without affecting the turn because the water passing through its disc comes from the free surface and does not on the rudder to any great extent. The ship can be turned to port from dead in the water,by ordering "left full rudder, ahead two-thirds" on the starboard engine and "back, two-thirds" on the port engine. The speed of the port engine can be varied to allow the ship to gain steerageway. The rudder and the starboard engine need not be changed until the turn is completed.
