A method for estimating the linear hydrodynamic forces acting on unmanned untethered submersibles is investigated. Two models are selected as examples, one has a slender body with a purpose to lower drag as in the case of a laminar flow aerofoil section, and the other has a body of an aerofoil section suited for gliding performance. The estimated values of stability derivatives are examined and compared with the measured values by PMM test, and are proved to be satisfactorily accurate. To analyze dynamic motion characteristics, simulations of stability and control response are carried out. The advantage of the proposed method of estimation are demonstrated for the stage of initial design.