Athwartships

Athwartships means in the transverse direction, across the ship from one side to the other, perpendicular to the fore-and-aft centerline. It is the axis of beam, of transverse stability (the metacenter and GM are transverse properties), and of roll. Structural members running athwartships include floors, transverse frames, and deck beams, which resist the transverse bending and racking the hull sees in a seaway. The term pairs with longitudinal (fore-and-aft) and vertical to fix any of the three orthogonal ship axes; a transverse waterplane moment of inertia, for instance, is taken about the longitudinal centerline to give the athwartships BM used in initial stability.