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In this paper, we present the concept of Crabster (Fig. 1) including the main missions of the robot. The main focus is on the modeling of drag and lift forces on the legs of the robot, which makes main difference of dynamic characteristics between the on-land and underwater. Presented model is described as closed-form dynamic equations based on L-E formulation. Since the closed-form equations of motion are advantageous to apply easily in analysis and controller design purpose, the presented model guarantees the same advantages in underwater legs. Generalized drag torque is successfully derived as a function of generalized variables and its first derivative, even though the arm has roll joint and twist angles between joints. To verify the proposed model, we conducted drag torque simulations with simple scara robot example.