A flexible method for the initial prediction of tugs escort capability

Preliminary prediction of escort tug capability aims to identify the maximum steering and braking forces the tug could deliver during operations at sea. To this end, the adoption of a steady-state equilibrium approximation ensures attaining a quick and enough accurate results for the initial design stage. Diverse techniques are suitable to resolve the equilibrium, with different simplification levels and related limitations. In this study, a method based on a nonlinear thrust allocation algorithm is improved, including propulsors interactions. Moreover, the addition of the transversal equilibrium modelling of the tug allows evaluating the compliancy with stability and safety Regulations for escort class. The coupling between enhanced equilibrium resolution and a simplified approach to estimate the hydrodynamic forces results in a flexible prediction method for tug escort capability, which is requiring few inputs, without excluding the possibility to use higher fidelity hydrodynamic loads. The calculation framework, implemented with different levels of confidence for hydrodynamic loads, is tested on different tug types, highlighting its flexibility compared to more traditional and widely accepted approaches for the initial design stage.