Guideline for calculating the draught loaded
The available fairway depth, and therefore the possible draught loaded, is a crucial economic criteria for the cargo shipping business. Higher draughts loaded, and therefore a better load factor of the ship units employed, reduce the transport costs per ton significantly. Below you can find a compilation of the most important fairway parameters including the relations between the individual parameters.
A water gauge measures the gauge height which corresponds to the height of water at a certain point in the reference profile of a body of water, i.e. the water level. It has to be kept in mind that the water level measured at a water gauge does not necessarily coincide with the actual water depth of a river and hence with the current fairway depth. This is due to the fact that the gauge zero, i.e. the lower end of a gauge staff or the reference altitude of a gauge, usually does not align with the location of the riverbed. Up-to-date information on water levels essential for Danube navigation can be found here.
Reference water levels
As the water level at a gauge changes continually, reference water levels have been defined. Reference water levels are statistical reference values for average water levels, which have been registered at a certain gauge over a longer period of time. The most important reference water levels for inland waterway transport are:
- Low navigable water level (LNWL)
- Highest navigable water level (HNWL)
Fairway and fairway depth
The fairway is the area of a body of a waterway for which maintenance of certain fairway depths and fairway widths is aimed at for navigation purposes. For the Danube, the fairway depth determined for a "minimal" cross section refers to low navigable water level (LNWL). The width and the course of the fairway are indicated by fairway markings.
Water depths available in the fairway determine how many tons of goods may be carried on an inland cargo vessel. The more cargo loaded on board of a vessel, the higher is its draught loaded, i.e. the draught of a ship when stationary and when carrying a certain load.
Squat denotes the additional sinkage of a ship in motion due to hydrodynamic effects compared to its stationary condition. A loaded vessel has a squat within a range of about 20 to 40 centimetres.
Safety clearance (keel clearance)
The safety clearance is termed „keel clearance“ and is defined as the distance between the bottom of a vessel in motion and the highest point of the riverbed, i. e. safety clearance = fairway depth – total draught (= draught loaded + squat). In general, the safety clearance is chosen by the shipmaster and depending on the section passed (gravel or rock), the type of ship, the type of propulsion and motorisation, the relation of the journey (downstream or upstream) as well as other factors.
In the following calculation, all previously described parameters are exemplarily shown for the Danube section east of Vienna.
This calculation is based on exemplarily assumed values. The currently available fairway depth and draught loaded are based on the minimum fairway depth at the shallow section, which is currently decisive for a particular stretch of the Danube. Up-to-date information is published by viadonau under following address: https://www.doris.bmk.gv.at/en/water_levels_low_sections. The final decision regarding the chosen draught loaded is subject to the responsibility of the captain. Thus, any claim of liability based on this calculation example provided by viadonau as service feature is by any means whatsoever excluded and void.