Practical computational procedures for predicting steering and braking forces of escort tugs

Two distinct computational procedures based on semi-empirical and Reynolds-Averaged-Navier-Stokes (RANS) computations are presented to estimate towline forces applied by tractor type escort tugs. The crucial part of both techniques is to calculate the hydrodynamic forces generated by the hull and appendages of the escort tug. The escort performance is characterised by the components of the towline force, namely steering and braking forces, which can be estimated easily from the quasi-static equilibrium condition. In the semi-empirical approach, the hydrodynamic forces are represented by a set of manoeuvring derivatives, which are obtained by regression equations based on model test measurements or full-scale test results. This technique is quite simple and only requires basic design parameters but ignores the details of the hull and appendage geometries and only suitable to be used for the yaw angles up to the stall angle. The RANS-based approach, however, is complex and requires detailed hull and appendage geometries but takes the local flow characteristics into consideration. These techniques are proposed to be used at the different stages of the design process depending on the availability of hull and appendage data.