A displacement vessel — obviously — displaces a (large) amount of water. In open and deep navigation areas this water can travel almost without any restriction underneath and along the ship’s hull. In restricted and shallow waterways, however, the displaced water is squeezed under and along the hull. These bathymetric restrictions result in increased velocities of the return flow along the hull. The resulting pressure distribution on the hull causes a combination of forces and moments on the vessel. If generated because of asymmetric flow due to the presence of a bank, this combination of forces and moment is known as bank effects.
By far the most comprehensive and systematic experimental research program on bank effects has been carried out in the Towing Tank for Manoeuvres in Shallow Water (cooperation Flanders Hydraulics Research – Ghent University) at Flanders Hydraulics Research (FHR) in Antwerp, Belgium. The obtained data set on bank effects consists of more than 14 000 unique model test setups. Different ship models have been tested in a broad range of draft to water depth ratios, forward speeds and propeller actions. The tests were carried out along several bank geometries at different lateral positions between the ship and the installed bank.
The output consists of forces and moments on hull, rudder and propeller as well as vertical ship motions. An analysis of this extensive database has led to an increased insight into the parameters which are relevant for bank effects.
Two important parameters are linked to the relative distance between ship and bank and the ship’s forward speed. The relative position and distance between a ship and an arbitrarily shaped bank is ambiguous. Therefore a definition for a dimensionless distance to the bank will be introduced. In this way the properties of a random cross section are taken into account without exaggerating the bathymetry at a distance far away from the ship or without underestimating the bank shape at close proximity to the ship.
The dimensionless velocity, named the Tuck number (Tu), considers the water depth and blockage, and is based on the velocity relative to the critical speed. The latter is dependent on the cross section (and thus the bank geometry) of the waterway.