This paper deals with a method of generating a desired sea state in a wave tank. The time duration in which all wave groups are present at the test section and before reflected waves reach the test section is maximized for a given length of wave tank. The duration is found to be dependent on the number of frequency bands chosen to represent the spectrum. The more frequency bands chosen and consequently, the better the spectrum is approximated, the shorter the duration. Each frequency band is approximated by two sine waves of equal amplitude. The frequency bands must be chosen so that they can be adequately represented by two waves.