To assure the operability of ships and the safety of ship and crew the manoeuvring requirements of ships have to be specified. Minimal non-binding requirements are given in the IMO regulations but may be subject to further specifications in the contract. More demanding requirements are especially important for certain ship types such as tugs, ferries, offshore supply vessels or underwater vehicles. The manoeuvrability is generally investigated experimentally but numerical methods increasingly contribute vital information to the manoeuvring predictions.
The benefits for the client are:
- Assessment of scale effects
- No limitations, in contrast to the confined testing facilities
Besides the possibility to carry out direct manoeuvring tests, different approaches exist to determine the manoeuvrability. The approach generally carried out at the SVA Potsdam is based on the formulation of a mathematical series to describe the external forces and moments acting upon the ship in the equations of motions. Once the unknown hydrodynamic coefficients in the mathematical model are determined arbitrary manoeuvres can be simulated.
Numerical methods (e. g. RANSE solvers) can be applied to calculate the hydrodynamic coefficients, having the advantage that the numerical simulations are not influenced by any limitations imposed by the available testing facilities and that the calculations can be carried out in full-scale.
Manoeuvring investigations can be classified as static and dynamic tests.
Static tests include :
Static rudder test, static yaw test, static pitch test (e. g. with submarine models) and resistance tests.
As the rudder still is the most common steering device, some examples of static rudder tests are given below. The following figure (left) compares calculated and measured rudder forces of a semi-balanced rudder in homogeneous inflow conditions. Scale effects are revealed when the model scale results are compared
with the corresponding full-scale measurements, as in the
figure on the right.
In behind ship conditions the rudder operates in an inhomogeneous flow field which can lead to flow separation, as shown in the following model scale animations of the pressure distribution on the rudder and stream lines of the hub vortex for a rudder angle of δR = 20°.
| Pressure distribution on the rudder, rudder angle δR = 20°, port side | Pressure distribution on the rudder, rudder angle δR = 20°, starboard side |
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Results of a static yaw test are shown below:
Dynamic tests:
The dynamic tests are simulations in the time domain. In the following the calculated force coefficients for different dynamic tests are compared exemplarily with the corresponding measurements.
