In this paper, a 9 degree of freedom seaweed harvester dynamical model is presented. The model presented describes the coupled dynamics of a vessel and a harvesting device, lifted by a winch, subject to sea waves disturbances. In order to find an appropriate harvesting path, which minimizes the harvester motions, the model response is simulated for different encounter angles. This 9 DOF seaweed harvester model is defined by merging the unified ship model, given in Fossen [1], Perez [2] ,

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... ng the seakeeping fluid memory effects, and the Nielsen [3], Fard [4] 9 DOF shipcrane model, which includes an elastic coupling between the vessel and the harvester, and computes the harvester hydrodynamics, as in [5]. For the ship-harvester elastic coupling, a linear stiffness matrix, based on Shashikala et al [6], has been computed. The model obtained also includes the effects of the vessel forward speed, the wave and current loads, the ship propellers and motion control forces, and the winch motor dynamics. The motor dynamic is based on the control-oriented models of Messineo et al [7]. A mathematical formulation is first presented, and its assumptions are discussed. Then, a model simulation, obtained by modifying the Fossen et al [8] Simulink MSS Toolbox, and its results, are discussed. The harvester position variance and the RMSE index are compared, for the simulation results of different harvesting paths, and a compromise solution is discussed.