TRAFFIC LANE WIDTH ON RIVER BENDS
Dr.-Ing. Andreas Gronarz
(Versuchsanstalt für Binnenschiffbau e.V., Duisburg, Germany)
The inland waterway navigation is often characterized by a dense traffic, especially when encounter and overtaking manoeuvres are taking place. The capacity of the rivers and canals is limited by the width of the navigation fairway and the width of the traffic lane used by the vessels.
Starting with the absolute minimum lane width (the breadth of the ship) when running straight ahead it is enlarged by the course keeping activities of the helmsman. In river bends the ship has to follow a recommended path with a given radius. To enable the ship to pass this bend it has to be put into a turning motion, which normally can only be maintained by a certain drift angle. This angle increases the lane width significantly for long vessels, and even if it is zero, the traffic lane is still much wider in bends than on a straight course. When sailing downstream on a river the virtual curve radius is decreased due to the flow velocity overlapping the ship's speed and hence resulting in a bigger traffic lane width.
Standard inland waterway ships are normally manoeuvred by a stern rudder. With this configuration a drift angle is unavoidable for turning manoeuvres. The investigation deals with the application of lateral bow thrusters and bow rudders in order to minimize the lane width depending on the ship's speed, control settings, bend radii and flow velocity. For simulations based on hydrodynamic coefficients the forces from the bow control organs have been additionally modeled. Numerous calculations were carried out with variations also of the ships length and draught. The results are presented in graphs of traffic lane width versus curve radius, featuring the different parameters. They are extremely helpful for the decision making process regarding the improvement of the inland waterway capacity.
Waterborne transport is an excellent alternative to the transport on roads and rails. Traffic jams and high costs give the transport of goods on ships a chance to overcome the problem of lower speed (longer transport time). Particularly for bulk cargo like ore and coal there is no cheaper solution available than the cargo ship - either as single vessel or as barge train.
On the European inland waterways there is still spare capacity to increase the transport volume. The latter of the two possibilities - higher shipping frequency or larger ships is normally the cheaper alternative. One of the major problems with bigger transport units on waterways is the limited width of the river or the canal regarding the passage of bends. Particularly during encountering manoeuvres on narrow fairways the width of the traffic lane required by a vessel is of high importance.
2. TRAFFIC LANE WIDTH
The traffic lane width of a ship shall be defined as the area of the waterway which is covered by the vessel during (the manoeuvre) operation. If it is seen as a continuous film, the two envelopes of the outermost parts of the ship is the traffic lane and the shortest distance of this envelopes at a certain position is the traffic lane width. Fig. 1 illustrates a motor vessel with a barge passing a river bend in form of a plot.
Fig. 1 Traffic lane width on an inland river
2.1 Straight Course
The absolute minimum value of a traffic lane width is the breadth of the ship, but this is a purely theoretical value. Any external influences like wind or turbulences in the water require slight corrections to keep the straight course of the ship.
2.2 Human influence
These corrections are normally carried out by the helmsman or an automatic pilot. Depending on the adjustment of the autopilot or the state of the helmsman the rudder actions result in an enlargement of the traffic lane width, which may be up to 10-20% of the theoretical value. In Fig. 2 the rudder activity and the motion parameters of course and yaw rate are plotted for a 2 minutes interval, showing the work of the helmsman steering the vessel through a narrow passage.
Fig. 2 Rudder activity within a wide river bend
Apart from running straight ahead the normal motion of a ship is to follow the natural bends of a river or the artificial ones of a canal. In order to be able to enter a bend a zentripetal force is needed which normally is brought up by the lateral plane of the ship through a drift angle. This drift angle is created by the stern rudder, which initially creates a lateral force away from the center of the circular motion. Due to the drift angle a much bigger force directed inwards is created, which keeps the vessel in the turning motion.
This drift angle is the reason for the required enlargement of the traffic lane width when a ship with conventional rudder arrangement is moving through a bend. It can be concluded that the higher the yaw rate (assuming a constant speed) the bigger the drift angle will be, resulting in a smaller radius of the bend the ship is following.
On inland rivers normally a current can be expected. Its velocity is dependent on the topography and may vary between 1 and 8 km/h, even more, if the river is in full spate.
Assuming that the current direction is almost tangential to the center line of the river it can be said that the speed of the vessel over ground is composed of its own speed V (through the water) and the velocity of the current Vc. This results in a changed radius r of the ship's bend over ground, but not in a change of the drift angle of the vessel. This relation can be expressed in the following way (1):