In the portion of a river where the channel narrows, the ship begins feeling the steep bank close on the starboard hand. She wants to sheer to port as the combination of suction on the starboard quarter and, to a lesser degree, the cushion of water built up between the ship's starboard bow and the bank become more strongly felt. The problem is compounded when it is necessary to slow the ship, the rudder loses some effectiveness as the flow of water is reduced, while the suction at the stern, which is primarily a factor ofthe ship's speed through the water, remains strong. The ship must be moved further from the bank and the rudder angle increased. But what if the ship gets so close to the bank that she starts to sheer across the channel? Don't reduce engine speed, as at this point the rudder needs to be as effective as possible and any reduction in the ship's speed that might result from a change in engine revolutions will be negligible in such a short period of time. Rather, let the head fall a few degrees off course across the channel while maintaining some rudder angle toward the near bank, and increase engine speed so the flow of water past the rudder increases significantly.
When the heading has changed a few degrees toward the center of the channel, increase the rudder angle toward the near bank to first check the swing, and then to bring the ship back on course as she reaches the center of the channel, or at least reaches a distance from the closer bank that will allow her to be steered safely. Only after getting away from the bank should the engine speed be reduced so the ship loses some headway and the tendency to take a sheer is reduced. It is obvious that a ship should not proceed in a narrow channel at full maneuvering speed, since she would not have any revolutions in reserve should they be required.
Remember too that a ship wants to move closer to a bank, due to the increased flow of water and the resultant reduced pressure along her side closest to the bank. For reasons explained by Signore Bernoulli, a ship wants to move laterally toward a closer bank even though her heading is parallel to it. Either keep the ship headed at some small angle away from the bank or hold her in the center of the channel when not meeting another vessel. The forces acting on a ship can often be put to your advantage, making an aid of what seems to the nonmariner to be a hazard. When making a turn, a bank can be put close enough to the quarter to cause the ship to sheer in the direction of the turn and make a turn that she otherwise could not navigate, or at least could not navigate at the speed that is possible utilizing these forces.
As an example, there are turns in the Gaillard Cut in the Panama Canal that theoretically cannot be negotiated by many ships without tug assistance and yet ships have been making these turns easily throughout the life of the Canal with the aid of bank suction. If your ship should transit the Panama Canal or another narrow waterway, watch the rudder angle indicator and the position of the ship and you will find the ship going around many turns with the rudder amidships. A seaman routinely uses learned skills to turn potential hazards into aids, both in close water and offshore. This same bank suction can also assist a ship to pass another vessel in a narrow channel, to locate the center of a channel in times oflimited visibility, or to make routine maneuvers, as long as it is planned and allowed for. Keep in mind, though, that speed must be restricted so the ship can come ahead if the sheer should become greater than desired.
