The .present invention relates to a support structure for a cargo ship hatch cover carrying container load on its top plate, said structure comprising elongated support beams which are attached to the underside of said hatch cover, are aligned essentially parallel to the longitudinal axis of said containers and have a length essentially equal to the length of said hatch cover.

Such hatch covers are used particularly in cargo ships to close the hatchway to the ship's cargo space during a transport passage, and additionally, as a load-bearing part of the deck on which the on-deck cargo such as containers can be stowed. A plurality, e.g., 12 containers can be stowed on each hatch cover, stacked in two tiers, so that m each tier comprises three containers placed side-by-side by two containers placed end-by-end. This amounts to an extremely heavy load on the cargo hatch cover, which neces- sitates extra stiffening of the hatch cover by means of support beams individually welded to the underside of the hatch cover.

Such hatch cover support structures or beams are conven- tionally formed by mutually parallel aligned L- or T- sections which are attached by welding to the underside of the hatch cover top plate. Each supporting section is welded, typically aligned parallel to the long sides of the container and coinciding with the edges thereof, from both side edges of its one flange to the underside of the hatch cover top plate. Such L- or T-sections require a great amount of surface treatment including blasting, coating and welding. Moreover, the inner angles (90°) of the L- or T-sections tend to gather dirt and other debris thus causing an increased risk of corrosion.

It is an object of the present invention to provide a novel type of support structure for a cargo ship hatch cover carrying container load on its top plate, said support structure being capable of overcoming the above-mentioned drawbacks. The support structure according to the invention is characterized in that the support beam and the underside of the hatch cover top plate together form a structural element (box-section beam) with an essentially closed cross section formed by two side flanges and a bottom web plate and that the mutual distance between the side flanges of the box-section beam across the underside of the hatch cover top plate is essentially equal to the container width.

A preferred embodiment of the invention is characterized in that both side flanges of the box-section beam are aligned essentially parallel to the long sides of the container.

Another embodiment of the invention is characterized in that the side flanges of the box-section beam are essen¬ tially vertical and mutually aligned parallel to each other.

A still another embodiment of the invention is character- ized in that the side flanges of the box-section beam are aligned at an angle with the underside of the hatch cover top plate so that the flanges extending downward from the underside of the hatch cover top plate are convergent with each other toward their lower edges.

An advantage of the invention is that the surface treatment of the support beam will become easier than for convention¬ al beams, because the present structure is free from difficult-to-reach and dirt-collectmg inner angles that are subject to corrosion. A single box-section beam

according to the invention can replace the use of two conventional L- or T-section beams. Hence, the number of weld seams will be halved over the prior art. A further advantage of a closed box-section beam as a strength- imparting factor is its capability of equalizing stresses in the main structure of the hatch cover.

In the following, the invention is examined in greater detail with the help of a preferred embodiment by making reference to the appended drawings, in which:

Figure 1 shows schematically the conventional support beams of a hatch cover with containers stowed on the hatch cover;

Figure 2 shows an embodiment according to the invention in a similar view with the conventional arrangement illus¬ trated in Fig. 1;

Figure 3 shows an embodiment according to the invention in a perspective view obliquely from below.

Referring to Fig. 1, the end view of a hatch cover 1 is shown therein with containers 2 being shipped stowed thereon. The hatch cover 1 is conventionally supported and strengthened from its underside by mutually parallel, elongated L- and T-section beams 3, which are aligned essentially at the side edges of the containers 2. Each section is attached by welding to the underside of the hatch cover 1 so that a weld seam (not shown) is made at both side edges of the section.

Referring to Fig. 2, an embodiment according to the inven¬ tion is shown having each pair of the sections shown in Fig. 1 replaced by a single "box-section" beam 4, which is welded at the outer edges of its both flanges to the under-

side of the hatch cover 1. Thus, the open box-section beam together with the underside of the hatch cover 1 forms an essentially closed box-section structure, whereby accumula¬ tion of dirt in difficult-to-clean corners is prevented.

The open box-section beam 4 comprises two side flanges 5 and a bottom web plate 6 connecting them. The side flanges 5 are generally made vertical, but as can be seen from Figs. 2 and 3, they can alternatively be aligned at an angle with respect to the underside of the hatch cover so that the flanges extending downward from the underside of the hatch cover top plate are convergent with each other toward their lower edges. This slanting angle can be varied in the range of, e.g., 45-90°. Both side flanges 5 run parallel to the container side edges, and more advanta¬ geously, at said edges.

In short, the invention is characterized in that the width of the box-section beam is almost equal to the container width. In this context, the container width must be under¬ stood to refer to the mutual distance ^' of the center points of the corner strengthening plates (container support brackets), which are located at the container underside, from each other in the cross direction of the container. However, as the sizes of container support brackets may vary, certain variation in the size of the box-section beam can be allowed. The specification of an essentially equal beam width must not in this context be understood as to match the container width to the closest full millimeter, but rather, a certain tolerance is allowable within the scope of the invention. Most preferably, the width of the box-section beam 4 is in the range 1800-2500 mm. The length of the box-section beam 4 is essentially equal to the length of the hatch cover 1, which typically varies in the range 10-15 m. The height of the box-section beam 4, that

is, the distance of the bottom web plate 6 from the underside surface of the top plate of the hatch cover 1 is advantageously in the range 600-900 mm. The box-section beam 4 according to the invention is advantageously made from sheet steel with a material thickness of 7-20 mm.

The bottom web plate 6 of the box-section beam 4 is essen¬ tially horizontal, or, as shown in the diagram, the bottom web plate 6 is essentially horizontal at its longitudinal center part with ends slanted upward closer to the beam ends. In other words, the latter embodiment comprises a box-section with a tapering height toward its ends. In the latter case, the side flanges 5 are correspondingly shaped with a tapering height toward the beam ends.

Now referring to Figs. 2 and 3, therein is shown an embodi¬ ment having two box-section beams 4 attached at a distance from each other under a hatch cover. Without departing from the scope of the invention, also alternative embodiments are possible. Herein the ends of the hatch cover are shown provided with transverse end beams 7 extending essentially across the entire width of the hatch cover 1.

To those versed in the art it is obvious that the invention is not limited to the embodiments described above, but rather can be varied within the scope of the annexed claims.