Design and dimensioning of seaweed farms at sea are dependent on knowledge of forces exerted on the farm due to the e.g. sugar kelp being exposed to current and waves, and numerical methods that can simulate the sea farms when subjected to design conditions. Results from measurements of drag forces and tensile forces on cultivated sugar kelp from towing experiments in a closed test facility containing sea water showed a clear relation between the size of the sugar kelp and the drag forces. Numerical methods have been developed and are available as part of SINTEF Ocean's software FhSim for simulation of seaweed farms.