Mooring motions works on oceanographycal measurements such as acoustical position fixing, acoustical tomography, and so on. Therefore, acoustical measurements of mooring motions are required to obtain good accuracy on acoustical oceanographycal measurements. In this paper, a drag coefficient of chained glass balls to get a buoyancy force of a mooring system are determined on the basis of experimental data in the sea. Mooring motions are investigated with lumped mass method using its drag coefficient. A taut mooring motion draws a circle in a rotating current, on the other hand it draws a track like a numeral of 8 in an elliptically rotating current. Displacements of a taut mooring system can be estimated from current velocities based upon two kinds of simple dynamical equivalent models. This makes easier to measure mooring motions than acoustical method.