The art of model testing: Using CFD to adapt traditional tank testing techniques to a new era of wind propelled shipping

Hybrid testing is an experimental technique that can be used to test ships and marine structures when both hydrodynamic and aerodynamic effects are important, for example for wind powered or wind assisted ships and sailing vessels. SSPA is currently developing an experimental method using hybrid testing involving fan forces added to ship decks to simulate sails to assess the course keeping, seakeeping and manoeuvring performance of a wind powered ship. For conventional motor ships there are well established test methods and knowledge on how to scale the results from model to full-scale. For a wind propelled ship however, the driving force is no longer located at the propeller shaft but high above deck and at another longitudinal position that could vary with true wind angle and speed. Moreover, there is a large side force coming from the aerodynamic forces of the wingsails that needs to be counteracted with lifting surfaces underwater. The side-force and yaw moment are much more prominent than in conventional vessels. The combination of those factors will influence the manoeuvrability and course keeping, especially in waves. Having built up the research tools for predicting and simulating the behaviour of a full-scale vessel, making the model sail in a similar way as predicted for the full-scale vessel remains a challenge because of the difference between Froude scaling and Reynolds scaling applicable for the hull and lifting surfaces respectively. Using Computational Fluid Dynamics (CFD) to understand the scale effects in model tests for a wind powered ship and developing a methodology for determining the fan parameters that correctly model the ships behaviour and performance are the key objectives of the research study. The art of model testing encompasses the need to learn from different techniques to ultimately achieve the best agreement between model tests and full-scale results in terms of accuracy, repeatability, cost, and speed. Learning from preliminary experimental tests, through studies on CFD and ultimately paving the way to new testing methodologies is the main aim of the current paper.