C
Yv Yr m' is positive, the nominator almost always negative. Thus C' determines the sign of r'. Positive C' indicate yaw stability, negative C' yaw instability. Yaw instability is the tendency of the ship to increase the absolute value of an existing drift angle. However, the formula is numerically very sensitive and measured coefficients are often too inaccurate for predictions. Therefore, usually more complicated analyses are necessary to determine yaw stability. If the transverse velocity in...
Resistance and propulsion
3.1 Resistance and propulsion concepts 3.1.1 Interaction between ship and propeller Any propulsion system interacts with the ship hull. The flow field is changed by the usually upstream located hull. The propulsion system changes, in turn, the flow field at the ship hull. However, traditionally naval architects have considered propeller and ship separately and introduced special efficiencies and factors to account for the effects of interaction. While this decomposition is seen by many as an...
0067
The correlation coefficient cA differs fundamentally from the correlation coefficient for the ITTC 1957 method. Here cA does not have to compensate for scaling errors of the viscous pressure resistance. ITTC recommends universally ca 0.0004. The Hughes-Prohaska method is a form factor method. The form factor 1 k is assumed to be independent of Fn and Rn and the same for model and ship. The form factor is determined by assuming ct F4 Figure 3.7 Extrapolation of form factor Figure 3.7...
44 Numerical prediction of ship seakeeping 441 Overview of computational methods
If the effect of the wave amplitude on the ship seakeeping is significantly nonlinear, there is little sense in investigating the ship in elementary waves, since these waves do not appear in nature and the non-linear reaction of the ship in natural seaways cannot be deduced from the reaction in elementary waves. In these non-linear cases, simulation in the time domain is the appropriate tool for numerical predictions. However, if the non-linearity is weak or moderate the seakeeping properties...
2 Vkq
The body boundary condition is here explicitly given for the radiation problem of the body in heave motion. This will serve as an example. The other motions sway, roll and the diffraction problem are treated in a very similar fashion. The body boundary condition for heave is then n2 is the z component of the two-dimensional normal vector n. The radiation condition for 0 is derived by differentiation of the initial radiation condition for 0 with respect to y and z, respectively. The resulting...
Info Spr
The sum of contributions 1 to 4 constitutes the total body force For steady traversing or pure yaw motion without forward speed, only terms listed under 4 above are relevant. The yaw stability is very sensitive to small changes in the body forces. Therefore a reliable prediction of yaw stability based on the slender-body theory or regression analysis of model tests is not possible. Substantial improvements of theoretical calculations seem possible only if the flow separation around the hull is...
34 Simple design approaches
In early design stages, the power requirements have to be estimated to judge the weight and volume requirements of the main engine and fuel. As this has to be done repeatedly in design loops, model tests are no suitable solution for reasons of time and costs. Instead, simple, largely empirical methods are employed which only require a few global design parameters. These methods are discussed in more detail by Schneekluth and Bertram 1998 . The main approaches are estimate from parent ship, e.g....
References
Abbott, I. and Doenhoff, A. 1959 . A theory of wing sections. Dover Publ. Allison, J. 1993 . Marine waterjet propulsion. Trans. SNAME 101, pp. 275-335 Anep II-I 1983 . Standardized wave and wind environments for NATO operational areas. Allied Naval Engineering Publication Bertram, V. 1992 . Wellenwiderstandsberechnung f r SWATH-Schiffe und Katamarane. Jahrbuch Schiffbautechnische Gesellschaft, Springer, pp. 343-351 Bertram, V. 1998a . Marching towards the numerical ship model basin. Euromech...
21 Introduction
Ships are predominantly equipped with 'simple' screw propellers. Special means of propulsion include surface-piercing propellers contra-rotating propellers almost exclusively for torpedoes These are discussed in more detail by Schneekluth and Bertram 1998 . In the past, paddle-wheels played a large role for river boats, but have been largely replaced now by propellers or waterjets. We will limit ourselves here to ships equipped with propellers. Waterjets as alternative propulsive systems for...
12 Model tests similarity laws
Since the purely numerical treatment of ship hydrodynamics has not yet reached a completely satisfactory stage, model tests are still essential in the design process and for validation purposes. The model tests must be performed such that model and full-scale ships exhibit similar behaviour, i.e. the results for the model can be transferred to full scale by a proportionality factor. We indicate in the following the full-scale ship by the index s and the model by the index m. Geometrical...