The ideal aim of the ship motion reduction by anti-pitching fin in waves might be a heaving-less and/or pitching-less state. Equations of motion at this state give conditions to be satisfied by fin's location, area and aspect ratio. These equations mean that wave-exciting force and moment of main hull are cancellated by fin's ones.
Calculating such fin's particulars with respect to a container-cargo ship, conclusions are :
(1) heaving-less and pitching-less state may be possible by two-pairs of fairly large fin installed at appropriate location.
(2) pitching-less or heaving-less state may be obtained by one-pair of fin. The fin area is smaller, the location be nearer F. P. in general. Optimum aspect ratio of the fin, especially for anti-pitching, is very small and lies out of assumption, so that another theoretical approach might be necessary.
The experiments in head waves are carried out and confirmed theoretical conclusions. The present results are of a container-cargo ship, there might not be much difference in general tendencies but in details for the other types of ship.