- In this paper, the diffraction theory originally developed for N bottommounted vertical circular cylinders is reviewed and generalized to be applicable to deep-draft truncated vertical cylinders, such as the columns of a tension-leg platform (TLP). More importantly, the complementary radiation problem for N vertical circular cylinders is solved. Radiation potentials as well as added mass and wave damping for six-degree-of-freedom motions are obtained in closed forms. To demonstrate the usefulness of the present method, it is applied to the computation of wave loads and hydrodynamic coefficients for four columns of the ISSC TLP and a row of 19 bottom-mounted vertical cylinders. The present method quickly produces reasonably accurate solutions and precludes the laborious convergence test and grid generation, and hence should be a valuable tool for the preliminary design of deep-draft multicolumn structures, especially when the number of the columns is large. ASCE.