Manouevring Prediction of Foil-Assisted Vessels: A Mathematical Model Assisted by CFD

The competitiveness of the maritime sector together with energetic and environmental challenges have promoted a demand for more efficient marine vehicles. In response to this, research campaigns have been conducted, with the aim of developing new technologies. One of these technologies is the application of hydrofoils (e.g.: a Hull Vane) to marine vehicles. Although these allow a significant increase in efficiency, challenges and concerns rise due to their influence on seakeeping and manoeuvring performance. As a starting point, for a better understanding of the impact of hydrofoils on the dynamic behaviour of vessels, this research was focused on the: “Development of knowledge, methodologies and tools to perform manoeuvring predictions of vessels equipped with a Hull Vane.” For this, Virtual Captive Tests (VCT) were combined with a 4 DOF manoeuvring model, to perform manoeuvring predictions of vessels equipped with a Hull Vane. The convergence of the CFD solutions, for VCT, proved to be a challenging task with a lot of room for improvement. This is caused by the presence of complex flow features, e.g.: fore and aft-body vortices. In the end, an Adaptive Grid Refinement (AGR) algorithm, allowed an equilibrium between computational cost and discretization uncertainties. Two types of VCT were performed and compared: Oblique Towing Tests (OTT) and Planar Motion Mechanism (PMM) tests. Due to the strong non-linear nature of hydrodynamic forces and moments, a Discrete Spectral Method was developed for the analysis of the PMM tests. On one hand, PMM tests represent an advantage in the determination of unsteady forces/moments compared to semi-empirical models. On the other hand, virtual PMM tests are more complex and more vulnerable to sources of errors, e.g.: flow memory effects and higher numerical residuals. At last, this research showed that the gain in accuracy in the determination of unsteady effects, hardly compensates the loss in accuracy in the determination of steady effects. Furthermore, it was shown that the uncertainties of captive tests can be amplified by the manoeuvring model, which can significantly penalize the precision of manoeuvring predictions. For the validation of manoeuvring predictions, a 25 m patrol vessel equipped with a Hull Vane (RPA8) was considered, for which experimental data is available. By comparing the results of the manoeuvring prediction with the experimental ones, it was shown that the manoeuvring prediction process over-estimates the course stability. Resulting in 20% (≈20 m) over-estimation of the 35° turning circle diameter and 50% (≈0.4°) under-estimation of the 3°-3° zig-zag overshoot. Regarding the impact of the Hull Vane on the manoeuvring performance, it was concluded that the Hull Vane increases the course stability of RPA8, leading to 7% increase of the 35° turning circle diameter and 18% decrease of 10°-10° zig-zag yaw overshoot. This is the result of a combination between: interaction phenomena, an increase in yaw damping caused by the Hull Vane struts and an increase in Munk moment caused by the change in dynamic trim. Therefore, the extrapolation of these results for other vessels is not trivial, due to the interaction between these effects. With the aim of reducing the cost of manoeuvring assessments, a Linear Pressure Distribution Method (LPDM) was developed, to model the effects of the Hull Vane on manoeuvring. This model was successfully validated, using the results of an integrated approach, i.e.: hydrodynamic coefficients determined using captive tests with the Hull Vane. The results show about 1% (≈1 m) difference in the prediction of the dimensions of a 35° turning circle, and less than 11% (≈0.3°) difference in the prediction of the 10°-10° zig-zag yaw overshoot. Finally, this research provides knowledge, tools and methodologies for a better understanding of the dynamic behaviour of foil-assisted vessels, particularly vessels equipped with a Hull Vane. The results of this research provide a solid foundation for further research in this field.Marine Technolog