USING AIDS TO NAVIGATION WHEN TURNING
As the ship proceeds up the channel, numerous aids to navigation are available to the pilot and master. While the purpose of these aids is to determine the ship's position relative to the axis of the channel, there are other ways in which these aids can be used. A buoy at a turn can be used as a rate of turn indicator by aligning the buoy with a fixed point on the ship such as a stay, stanchion.
1. If the relative bearing changes toward the bow, so the buoy is moving forward relative to the reference point, the ship is turning at a rate that will bring her closer to that buoy. She will be closer to the buoy at the completion of the turn than she is at present. If the rate at which the buoy is moving ahead of the reference point is increasing, then the rate at which the ship is turning is increasing.
2. If the buoy is remaining steady relative to the reference point the ship is turning at a fixed rate. She will maintain her present distance from the buoy as she makes the turn. The ship will actually be a little farther from the buoy when the turn is completed, since she is sliding sideways during the turn at a rate relative to the speed at which she is moving. The ship can be considered to be maintaining a constant distance from the buoy during the turn.
3. If the bearing is opening away from the bow so the buoy is moving aft relative to the reference point, then the ship's distance from the buoy is increasing. If the rate at which the buoy is opening is increasing then the rate of turn is decreasing.
Using a buoy in this manner is effective when turning in a strong current since it is the vessel's movement relative to the buoy and channel (the net movement resulting from the combination of vessel momentum, swing, and current effects) that is of interest to the shiphandler. This motion is apparent when the ship is turned using a fIxed reference. By adjusting the rudder to alter the buoy's relative movement a shiphandler can position the ship in a turn with great accuracy.
The angle between the lines of buoys marking the port and starboard sides of a channel can be used with great accuracy both to predict the ship's future position in a reach into which she is turning, and to know the vessel's position relative to the centerline after she has steadied up in that reach. The rate at which a vessel is sliding laterally can be determined by watching the change in angle of those buoys during a turn.
A range can be used to determine the ship's position relative to the channel, but do not overlook the fact that the rate at which a range is opening or closing is of equal importance. This is used in the same manner as the change in angle of a line of buoys to position the ship in a reach or channel.
