This paper provides simulation results for water flow ratio in the waterjet attached to SUP (Stand Up Paddle) board fin. This simulation is a part of the project “Development of technical solutions for water transport transmission system”, grant number ZI-2020/3. The goal of the project is to attach the waterjet propulsion to the SUP board with maximum efficiency. To do it, the model of the person driving the board and waterjet were created in SolidWorks. First part of the simulation focused on appendage resistance calculations, because additional appendage increases drag force resulting from water resistance. The results are already accepted for publication in Latvian Journal of Physics and Technical Sciences and will be published in January. In this paper we focus on the water flow rate depending on different nozzle and duct design. There are two conflicting requirements for the propulsion duct design. On the one hand, duct must protect propulsor from stones and water plants, because SUP boards travel on river, sometimes very close to the stones. On the other hand, the efficiency of the waterjet requires maximum water flow rate available for possibly greater Inlet Velocity Ratio. Finally, there should be enough place for battery pack, which drives the waterjet, but it creates additional barrier for the water flow. The flow rate at the outlet depends on the nozzle and is also important for propulsor efficiency. The rotating flow after waterjet propeller should be simulated in SolidWorks Flow Simulations environment as a boundary condition for the inlet volume flow. Variable design options for the waterjet duct and nozzle would be proposed and tested in SolidWorks Flow simulation environment. Summarizing the simulation goals and results, optimal design will be selected for the prototype for 3D printing.