The purpose of this paper is twofold, (i) to describe what is the NRCC-IMD Marine Dynamic Test Facility (MDTF) and how we use it to measure the hydrodynamic coefficients for a towfish, and, (ii) to show how these coefficients can be used in numerical simulations to determine how this towfish will behave during towing, given a particular tow ship and seastate. The MDTF is a controlled-motion device that allows us to force a scale model of an underwater vehicle to undergo a series of simultaneously-imposed heave, pitch, sway, yaw and roll motions in a towing tank. It is attached to the towing carriage so that the sixth degree of forced motion is provided by modulating the towing speed. The model is attached to the MDTF via a sting-type mount. with an internally-mounted six-component balance, we measure the reaction forces and moments that are exerted by the water on the vehicle. From these measurements we extra the hydrodynamic coefficients. In March 1997 we performed a series of experiments with the Canadian Navy's new Mine Counter Measures (MCM) towfish. Subsequently we performed simulations with the program SHIPMO in order to compute the motions of a Maritime Coastal Defence Vessel (MCDV) in head, beam and quartering seas in seastate 3 using a specified wave spectrum. From these motions we computed the spectra of combined heave, pitch and roll motions at the ship's towpoint, where the two cable is attached. By dividing the spectral results into a number of discrete-frequency motions, we could use the output from the program to obtain the transfer functions of the towpoint motions. Finally we performed the tow cable and towfish simulations with the program DYNTOCABS. Given the mass and inertia properties and estimates of the hydrodynamic coefficients for the towfish, this program computes the position and motions of the towfish and the variation in tension along the two cable. By combining the discrete-frequency results into an equivalent spectra (of motions and tensions), we were able to provide statistical estimates of the extreme values of the pitch and roll motions and tensions for each towing condition. This paper summarizes the methodology that we use to predict the full-scale performance of a towship-towfish system, before either the towfish of towship have been constructed, given only their geometrical shape, mass and inertia characteristics.