### Abstract:

The in-line force power spectrum is predicted by the frequency domain maximum force coefficient [C[subscript μn](z,f[subscript n])] vs. Keulegan-Carpenter number [K[subscript n] (z,f[subscript n])] relationship. The frequency domain Keulegan- Carpenter number is defined as the velocity from the amplitude spectrum at f divided by the respective frequency and diameter of the cylinder [K[subscript n](z,f[subscript n]) = u[subscript n](z,f[subscript n])/(f[subscript n] x D)]. When K[subscript n](z,f[subscript n]) is small, the frequency domain maximum force coefficient closely follows the relation C[subscript μn](z,f[subscript n]) = 2π²/K[subscript n](z,f[subscript n]) for a smooth cylinder. The force power spectrum can be estimated from a given wave spectrum by using linear wave theory and the above relation for C[[subscript μn](z,f[subscript n]). The non-dimensionalized rms errors between measured and predicted spectra are used to evaluate the predictions. The force time history can be predicted from the wave profile and an inverse Fast Fourier Transform method. This method is valid when K[[subscript n](z,f[[subscript n]) is less than 10 for smooth cylinder. Force time histories predicted by using this method compare reasonably well with measurements.