The theoretical study on the critical navigation of a ore carrier (L_<pp>=247m, C_b=0.824) has been carried out from the viewpoint of structural seaworthiness in longitudinal strength. The navigable wave conditions were obtained by the same procedure for the single screw container ship, previously reported. From the results, it is shown that whipping of full ship due to slamming grows rapidly at lower ship speed in head sea condition because of large impact pressure coefficient and flat fore bottom. And, wave bending moment in following sea condition is larger than in head sea condition, so that the operations of full ship in heavy seaways should be dicided with attentions not only to slamming but also to wave bending moment. From the comparison of navigable wave condition between the present ore carrier and the previous container ship, it is found that the difference of structural seaworthiness in longitudinal strength between them is not significant in seaway of longer average wave period.