Ship model basin

A ship model basin may be either a physical basin or tank used to carry out hydrodynamic tests with ship models, or the organization (often a company) that owns and operates such a facility.

In the second meaning, the company or authority is an engineering firm that acts as contractor to shipyards and provides hydrodynamic model tests and numerical calculations to support the design and development of ships and offshore structures. Its hydrodynamic test facilities include at least

• A towing tank: This is a basin, several metres wide and hundreds of metres long, equipped with a towing carriage that runs on two rails on either side. The towing carriage can either tow the model or follow the self-propelled model and is equipped with computers and devices to register or control, respectively, magnitudes like speed, propeller thrust and torque, rudder angle etc. The towing tank serves for resistance and propulsion tests with towed and self-propelled ship models to determine how much power the engine will have to provide to achieve the speed laid down in the contract between shipyard and ship owner. The towing tank also serves to determine the manoeuvring behaviour in model scale. For this, the self-propelled model is exposed to a series of zig-zag manoeuvres at different rudder angle amplitudes. A post-processing by means of system identification results in a numerical model to simulate any other manoeuvre like Dieudonné spiral test or turning circles. Additionally, a towing tank can be equipped with a PMM (planar motion mechanism) or a CPMC (computerized planar motion carriage) to measure the hydrodynamic forces and moments on ships or submerged objects under the influence of oblique inflow and enforced motions. The towing tank can also be equipped with a wave generator to carry out seakeeping tests, either by simulating natural (irregular) waves or by exposing the model to a wave packet that yields the RAOs for all frequencies in one single test.

• A cavitation tunnel to investigate propellers. This is a vertical water circuit with large diameter pipes. At the top, it carries the measuring facilities. A parallel inflow is established. With or without a ship model, the propeller, attached to a dynamometre, is brought into the inflow, and its thrust and torque is measured at different ratios of propeller speed (number of revolutions) to inflow velocity. A stoboscope synchronized with the propeller speed serves to visualize cavitation as if the cavitation bubble would not move. By this, one can observe if the propeller would be damaged by cavitation. To ensure similarity to the full-scale propeller, the pressure is lowered, and the gas content of the water is controlled.

• Workshops: Ship model basins manufacture their ship models from wood or parrafine with a computerized milling machine. Some of them also manufacture their model propellers. Equipping the ship models with all drives and gauges and manufacturing equipment for non-standard model tests are the main tasks of the workshops.

Some ship model basins have further facilities like:

• A manoeuvring and seakeeping basin. It is wide enough to investigate arbitrary angles between waves and ship model and to perform manoeuvres like turning circles, for which the towing tank is too narrow. However, some important manoeuvres like the spiral test still require even more space and still have to be simulated numerically after system identification.

• An ice tank: To develop ice breaking vessels, this tank fulfills similar purposes as the towing tank does for open water vessels. Resistance and required engine power as well as manoeuvring behaviour are determined depending on the ice thickness. Also ice forces on offshore structures can be determined. Ice layers are frozen with a special procedure to scale down the ice crystals to model scale. Also tests in brash ice are performed.

Addiyionally, these companies or authorities have CFD software and experience to simulate the complicated flow around ships and their rudders and propellers numerically. Today's state of the art does not yet allow to replace model tests by CFD calculations. One reason, but not the only one, is that elementization is still expensive. Also the lines design of some of the ships is carried out by the specialists of the ship model basin, either from the beginning or by optimizing the initial design obtained from the shipyard. The same applies to the design of propellers.

The ship model basins worldwide are organized in the ITTC (International Towing Tank Conference) to standardize their model test procedures.

Some of the most significant ship model basins are the David Taylor Model Basin in the United States and the HSVA in Hamburg, Germany.

de:Schiffbau-Versuchsanstalt