A comprehensive general simulation of small twin screw displacement hull boats is developed as a tool to estimate ship and actuator responses in support of developing and tuning of control systems. The general form of this model allows many small displacement hull vessels to be rapidly simulated by specifying a set of 17 vessel parameters. The response of the vessel in the surge, sway and yaw degrees of motion are estimated from parametric equations and data compiled from literature and, when not found in literature, inferred from sea trials. Model validation and tuning of the coefficients not found in literature is accomplished using manoeuvring and response data obtained in a series of sea trials. The parametric model proved to be accurate and, when compared with the sea trial data, model estimates have rms errors over the vessel operational range of 0.09 m/s and 0.02 m/s for surge velocity when moving in a straight line in forward and reverse, respectively. For rotating on a fixed point, the simulation overestimates the rotational velocity by 7.6% and has an rms error of 0.27°/s. Open loop circle tests show that with both propellers rotating at 580 rpm, the simulated vessel has a minimum turning radius of 24.4 m and can complete a circle in 44 s, which is 18% smaller and 8% faster than during sea trials. Simulated rotation with the engines in opposite gears at similar RPMs of 500 result in a complete revolution in 68 s, 3 s faster than during sea trials.