such complex motion dynamics while at the same time providing predictions in real-time, including the effect of control inputs by the operator. A balance had to be struck between ‘accurate enough’ and fast computation times. The method also had to be robust to cope with a wide variety of possible conditions and inputs, and also provide realistic motion predictions at all times.

XMF Based on recent experience a compu- tational model was built up from the ground within XMF consisting of a number of key building blocks. A module was developed based on added mass planing theory (or ‘momentum theory’) as was pioneered in the 1930s by Von Karman and Wagner. Experience from recent research in the DROPSIM tool devel- opment programme into modelling the im- pact of free falling lifeboats into the water surface also contributed. In this approach the ship is split into a number of 2D transverse sections and the impact force of each of the sections is determined based on the impact velocity and wetted shape of each section

at each time instant. The method is based on first principles and proves to be very adequate in dealing with both planing in calm water and impact forces on fast vessels in waves.

Another building block deals with the hydrostatics and wave forces, computed on the actual submerged geometry. The resistance was also modelled as a function of ship speed and actual submerged wetted surface. A detailed engine model is included to simulate the dual Z-drive setup, including the effect of trimming the Z-drives. A coefficient- based manoeuvring model to cope with the horizontal plane motions and additional damping coefficients complete the model.

Navy input An important step in real-time simulation is the tuning of the various components and coefficients in the underlying computational building blocks. For the FSSS project, MARIN made use of the know-how that it has been building up over the past years about small, fast craft and RHIBs. This

includes various model tests with fully remote controlled models of high-speed small craft in the Seakeeping and Manoeuvring Basin and the development of advanced computational methods such as PANSHIP.

In the development phase of this project, RHIB instructors from the Royal Netherlands Navy were invited to give their feedback on the behaviour of the computational model in combination with the motion system. During various workshops, the instructors have been testing every aspect of the simulator, including steady trim in calm water, steering and throttle response, turning circle diameters, the roll angle during turning and the motion behaviour in waves from every direction. Even aspects like the forming of spray on the visualisation, friction between the RHIB and vessels during boarding operations and the engine sound were discussed and improved. This approach makes this project truly unique, resulting in a valuable simulation and training tool for operators of RHIBs and other fast small craft.

report 25

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