A metal plate assembly such as a vessel hull (10) is fabricated by securing longitudinal edges of aluminium hull plates (12, 14, 16) in position to prevent outward movement thereof. Adjacent longitudinal edges of the hull plates are welded together with sufficient heating to cause the welded hull plates to bow outwardly between the longitudinal edges. The convex outer surfaces (24) are placed in compression when the welded hull plates (12, 14, 16) cool to ambient temperature and ambient pressure on the inner and outer surfaces of the plates is equal. The resulting hull (10) can be curved into deeply bowed curvatures, and it provides the advantages of an outer surface in compression.