When you are just getting under way the ship is stationary or just starting to move, the wake does not exist or is negligible. The top blade may break the surface and lose some of its transverse force to port. If it does not break the surface, air may be drawn down with the same effect. The lower blade still acts to force the stern to starboard. The rudder even when amidships, gets the helical slipstream at an effective angle high up on the port side. If the rudder has a larger area above the axis of the propeller than below, that force tends to move the stern to starboard. The result of these forces may be that the stern will move to starboard.
When the ship is backing, the forces caused by the top and bottom blades and are reversed. The top blade may break the surface, but the bottom blade will dominate. Since no helical slipstream is thrown against the rudder and most of the water that passes through the propeller disc comes from the free surface, the rudder can exert no steering force until the ship gains sternway. The upper part of the discharge flow from the propeller strikes the starboard underwater body of the ship at a good angle, the lower part strikes the keel on the port side at a poor angle. It is probable that the force caused by the upper part dominates; at any rate these forces tend to push the stern to port.
Steering a twin-screw ship, single rudder a twin-screw ship has two propellers, one on either side of the centerline. The starboard one is right-handed and the port one left-handed. They turn in opposite directions to balance the propeller forces and make the ship steer a straight course with no rudder.
A multiple-screw ship normally has four propellers, two on a side, outturning, and so controlled that those on a side go ahead or astern as a unit. The action of a multiple-screw ship is similar to that of a twinscrew ship.
The steering of a twin-screw ship is simpler than that of a single-screw ship. The tendency of the single-screw ship to back stern to port does not hold with the twin-screw, and a twin-screw backs with well in either direction, barring the effect of wind, waves, and currents.
The various forces affecting the action of the single-screw ship are still present to a degree in the case of a twin-screw. They are less because the forces from one screw are balanced by similar but opposite forces from the other screw. There is a new force caused by the movement of the screws around the centerline. It will be seen that with one screw going ahead and the other astern, tends to throw the bow to the side of the backing screw.
One powerful force should not be overlooked. It is the momentum of the ship, ahead or astern, acting through the center of gravity. If a twin screw ship is going ahead and one screw is backed, two opposing forces are set in motion, the force of the backing screw acting in one direction at a certain distance from the centerline and the weight of the ship acting in the opposite direction. These are in addition to the forces caused by the action of the wake on the rudder if it is "put over."
