In this paper, the authors report on the computer programs for principal dimensions study of ships, which have been developed and put into practical use at their ship yard. The programs, called P. D. S. programs, furnish a designer with brief charts to point out an optimum combination of principal particulars, after making cost and economic studies on the numerous ships designed automatically under a set of same design and operating conditions. P. D. S. programs can deal with tankers, ordinary bulk carriers, ore carriers and some typical combination carriers.