Patent Publication Number: US-6209471-B1

Title: Door structure

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a door structure, and more particularly to a door structure for ships with low Radar Cross Section requirements. 
     In order to make a ship invisible or nearly invisible to radar, a ship is build such, that when a radar beam irradiates the ship the reflection of said radar beam is directed away from the radar source, as a result of which the ship is not visible on the radar. However, radar is still further developed and smaller objects can be detected by radar. Traditional door structures, which are commonly used in such radar insensitive ships, still have protrusions and/or openings, which are particularly visible for radar, 
     Any opening or recess larger than approximately ¼ inch is especially visible for radar and therefore the connection of the conventional door to the bulkhead is visible. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a door structure, which is in contrast to traditional door structures less or not visible for radar. 
     This object is achieved by a door structure according to the invention, which door structure includes a door, a door opening in a bulkhead portion and a stiffening member extending peripherally in said door opening, wherein said door is hingedly attached to said bulkhead portion and is movable between a position substantially covering said opening and a position substantially freeing said opening and said door comprises a flat plate with peripheral edge zones, which extend to at least one of said stiffening member and the edge zones of said bulkhead portion bounding said opening, wherein the distance between said peripheral edge zones and at least one of said stiffening member and said edge zones of said bulkhead portion is smaller than ¼ inch. 
     As a result of the flat plate there are no protrusions on said door, which can reflect the radar beam in the direction of the radar receiver. As the flat plate also extends to the edge zones of the bulkhead portion bounding said opening, this door structure according to the invention does not have any openings which would reflect the radar beam in the direction of the radar receiver. 
     In a preferred embodiment, the peripheral edge zones comprise a bent portion, which is bent towards said bulkhead portion, such that a distance between said peripheral edge zones and said bulkhead portions is reduced. Because of the flat plate, the radar reflection of radar beams, which irradiate the door structure in a substantially perpendicular direction, is reduced. 
     Flat vertical surfaces will reflect horizontal radar beams horizontally back to the radar source and this makes these surfaces visible to the radar source. Surfaces placed under an angle with the vertical, will return horizontal radar beams not to the radar source, where they become visible, but will direct the radar beams upwards and avoid the beams from reflecting horizontally to the source. This is an effective means to make large surfaces less—or invisibly for radar. Any openings or protrusions in the flat surfaces may reflect beams horizontally, back to the radar source and thus make these openings and protrusions visible. The current state of technology makes openings and protrusions smaller than ⅛ inch not or barely visible for radar. 
     A curved bent portion will always have a part which reflects the radar beam directly to the radar source and receiver. It is therefore preferred that the bent portion is substantially flat. However, due to manufacturing conditions it may be preferred to provide the peripheral edge zones with a beaded edge. 
     The invention can also be applied to the vertical opening that is normally visible between the two leaves in the middle of a double door. The invention also provides for the cover of this vertical opening and protrusion. 
     According to the invention, there is also a door structure provided, including two door sections wherein the edge zones of the flat plates facing each other are bent in a direction substantially perpendicular to the direction of said flat plate. 
     In this case, preferably the edge zones of the flat plates facing each other are bent in a direction substantially perpendicular to the direction of said flat plate and the distance between the facing edge zones is smaller than ¼ inch. 
     When a door structure is formed by two door sections, which can open independently of each other, it is necessary to reduce or eliminate the gap between the door sections. This can be done as described above, or by constructing the door sections such, that one door section overlaps another. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention as said forth in the claims will become more apparent by reading the following detailed description in conjunction with the accompanying drawings, where like reference numerals refer to like parts throughout the drawings, in which: 
     FIG. 1 shows a perspective view of a ship, provided with door structures according to the invention; 
     FIGS. 2 a,    2   b  and  2   c  show a first embodiment of a door structure according to the invention; 
     FIGS. 3 a,    3   b  and  3   c  show a second embodiment of the invention; 
     FIGS. 4 a,    4   b  and  4   c  show a third embodiment of the invention. 
     FIGS. 5 a,    5   b  and  5   c  show a fourth embodiment of the invention; and 
     FIG. 6 shows a fifth embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a ship  10  with a hull  12 , a deck  14  and a superstructure  16 . The superstructure  16  has several door structures  18 ,  78 ,  108  according to the invention. The ship  10  has preferably been designed to be invisible for radar. The design criteria are common knowledge in the art and include for example special angles of bulk head plates or the use of special materials. 
     FIGS. 2 a - 2   c  show a first preferred embodiment of a door structure  18  according to the invention. This door structure  18  is arranged in an opening in a bulkhead portion  20 . In order to strengthen the opening in the bulkhead portion  20  and to facilitate the installation, a strengthening member  22  with a T-shaped cross section is welded in the opening. A door panel  24  is hingedly attached to the frame  22 , which is welded into the bulkhead  20  of the ship, by means of hinges  26 . 
     The door  24  consists of a flat plate  28 , which is strengthened by web plates  30  and a frame portion  32 . 
     The peripheral edge  34  of the flat plate  28 , is bent, substantially flat and extends over the frame  22  towards the bulkhead portion  20 . As a result, the flat plate  28  covers the whole opening in the bulkhead portion  20 . 
     In the frame portion  32 , a gasket  36  is arranged, which cooperates with a knife blade  38 , which extends peripherally from the frame  22 . In a closed position of the door  24  the gasket  36  and knife blade  38  provide a watertight closure of the opening. 
     Furthermore, the door structure  18  comprises a conventional bolting mechanism, which can be manually operated by a handle  40 . The door  24  has a grip  42  in order to be able to manually put the door  24  in a open or closed position. 
     Although the handle  40 , the grip  42  and the hinges  26  are arranged on the outside of the door and will increase the visibility for radar, they are constructed in this way as a result of other construction requirements, like that a door should be operable from the outside. 
     FIGS. 3 a - 3   c  show a second embodiment according to the invention, which is similar to the first embodiment. Corresponding parts of the door structure  18  are referenced by the same reference numerals and are not described further. 
     This door structure has a door  60 , which is hingedly attached to the bulkhead portion  20  by hinges  62 . The door  60  has a flat plate  64 , which is strengthened by web plates  30  and a frame portion  32 . The edges of the flat plate are beaded. 
     Preferably, the distance between the beaded edge  66  and the bulkhead portion  20  is smaller than ¼ inch. 
     FIGS. 4 a - 4   c  show a third embodiment of the invention. This embodiment is similar to the second embodiment according to FIGS. 3 a - 3   c.  Similar parts are referenced by the same reference numerals and are not described further. 
     The door  120 , which is hingedly attached to the bulkhead portion  20  by hinges  62  has a flat plate  122 , which is strengthened by web plates  30  and a frame portion  32 . The peripheral edges  128  of the flat plate  122  extend to the strengthening member  22 . Preferably the distance between the peripheral edge  128  and the strengthening member  22  is less than ¼ inch. 
     FIGS. 5 a - 5   c  show a fourth embodiment of a door structure  78  according to the invention. This door structure  78  is a double door with two door panels  80 , which are each hingedly attached to the bulkhead portion  20  by hinges  82 . Each door section  80  has a flat plate  84 , which is strengthened by web plates  86  and a frame portion  22  on the outer periphery of the door. The facing edges  101 ,  102  of the flat plates  84  are bent and the distance therebetween is preferably less than ¼ inch. 
     On the inside of the door structure  78 , several bolting mechanisms  90  are arranged in order to bolt the door sections  80  in a closed position. Each bolting mechanism  90  has a handle  92 , which is rotatably arranged in the frame portion  88  by a bearing  94 . The handle  92  has a wedge like part  96 , which cooperates with a wedge like part  98 , which is arranged on the frame portion  88 . When operating the bolting mechanism  90 , the wedge like parts  96 ,  98  are shifted along each other, as a result of which a knife plate part  100  is pressed into a gasket  102 . The hinges  82  are provided with a elongated slot to enable the hinge  82  to have a small displacement, in order to ensure a good cooperation between the knife plate part  100  and gasket  102  near the hinges  82 . 
     Similar bolting or dogging mechanisms may be used on other positions and/or other types of door structures according to the invention. 
     FIG. 6 shows a fifth embodiment of a door structure  108  according to the invention, which is a combination of the third preferred embodiment according to FIGS. 4 a - 4   c  and a door structure according to FIGS. 2 a - 2   c  or FIGS. 3 a - 3   c,  in which the bulkhead portion  20  is formed by a door section  80 . 
     With reference to the drawings it is note that FIGS. 2,  3  and  4  show three alternative arrangements and structures for eliminating the described radar reflecting properties of openings in single door structures. FIGS. 5 and 6 show that these three different structures can also be used in double door structures and in “door-in-door” structures, respectively.