For engine seating a plate floor is required every frame  Main engine seatings are in general integral with this double bottom structure.  Inner bottom in way of the engine foundation has a substantially increased thickness.  Often the machinery is built up on seatings forming longitudinal bearers which are supported transversely by tripping brackets in line  with the double bottom floors, the longitudinal bearers being in line with the double bottom side girders.  There are transverse plate floors at each frame. The thickness of the engine seating is governed by the power, weight, and length of  the unit  
Marine Engineering exam notes for Seafarers