US3698347A - Hatch covers and ventilating system - Google Patents

Abstract

A vessel of the OBO type in which the holds are serviced by a pair of transversely disposed hatches and a central longitudinal girder forms a ventilating trunk extending lengthwise of the vessel and a support for the adjacent hatch coamings of the transversely disposed hatches. The hatch covers are raised and lowered by jacks and one of the pair of hatch covers of the transversely disposed hatches is movable transversely of the vessel to underneath the other of the pair of hatch covers, and both of the pairs of hatch covers are movable piggyback transversely of the vessel.

Description

HATCH COVERS AND VENTILATING SYSTEM 1 Oct. 17,1972

FOREIGN PATENTS OR APPLICATIONS 1,196,791 7/1970 Great Britain ..1 14/202 [72] Inventor: Norman V. Laskey, 3641 Aylmer Street, Montreal Quebec, Canada Primary Examiner-Andrew Farrell [22] i Jam 5, 7 Attorney-Jerry B. Cesak A vessel of the OBO type in which the-holds are ser- [30] Forms Apphcauon Pnomy Data viced by a pair of transversely disposed hatches and a May 19, 1970 Great Britain ..24,09l/70 central longitudinal girder forms a ventilating trunk extending lengthwise of the vessel and a support for [52] US. Cl. ..ll4/202 the adjacent hatch coamings of the transversely [ITI- Cl. disposed hatches The batch covers are raised and 1 Field of Search 14/202; 49/ 8 lowered by jacks and one of the pair of hatch covers 2 of the transversely disposed hatches is movable transversely of the vessel to underneath the other of the [56] Reterences C'ted pair of hatch covers, and both of the pairs of hatch UNITED STATES PATENTS covers are movable piggyback transversely of the vessel. 1,768,701 7/1930 MacGregor ..l14/202 1,849,801 3/1932 MacGregor ..1 14/202 12 Claims, 11 Drawing Figures [13! 1 1 e r i e i L 56 1 A 4Q 1 35? 31; 57 -23 rm 5; 45-) p t I 29 L\ g 25 25 g 24 45 3' I PATENTEnucI 11 m2 3. 698.347

sum 3 0F 5 I F/Q INVENTOR N. v. LASKEY PATENTEBncnnszz 3.698 347 V snmuur s INVENTOR N. V. LASKEY PATENT AGENTS FATE "TED 17 I973 3.6 98 347 sum 5 or 5 INVENTOR N. V. LASKEY PATENT AYJENTS 1 HATCH COVERS AND VENTILATING SYSTEM This invention relates to vessels of the 080 or Oil- Bulk-Ore type and particularly to the Hatch Covers and Ventilation systems employed in such vessels.

Over the past years, single deck bulk carriers of the self-trimming type, comprising upper-wing or saddle ballast tanks have become popular and shipowners continue to order them in large numbers. The reason for this is not far to seek, because it is predicted that seaborne trade will increase at the rate of 6.5 percent per year over the next 15 years while the demand for shipping will be in the region of 7.25 percent. Also, dry bulk cargoes to betransported in ships will increase at the rate of about 9 percent each year. i

It is of interest to note that the original design intent of the single deck bulk carrier was to provide a vessel suitable for handling all types of free flowing bulk cargoes such as grain, coal, phosphate rock, potash, bauxite, alumina as well as manganese and iron ores in the natural, concentrated or pelletized forms. However, these vessels are being used with success in transporting automobiles, forest products including logs, scrap and finished steel products.

In the design of bulk carriers, the continued use of a single hatch with a width equal to 50 percent of the beam of the vessel is in with upper-wing or saddle ballast tanks has, today, outlived its usefulness. In the past, this was an essential feature of a single deck bulk carrier in order to provide self-trimming features for the carriage of grain in bulk and to eliminate the need for center-line shifting boards. This arrangement is also alleged to permit an adequate quantity of ballast to be carried during a ballasted voyage. I-Iowever,'with such an arrangement, the underdeck cubic in-way of the holds available for the carriage of a bulk cargo is reduced by the cubic invested in the upper-wing or saddle ballast tanks.

On many bulk carriers designed with the machinery at the after end, the hogging stress in the ballasted condition, using upper-wing or saddle tanks, will be at an inadmissible level. Sea-keeping performance in the ballasted condition will be greatly improved if water ballast is carried in a large number of the cargo holds instead of in upper-wing or saddle tanks.

A twin-hatch open ship of the type herein shown can be certified for the carriage of grain in bulk without shifting boards in full compliance with the requirements of SOLAS 1960. In view of this, provision can be made to carry water ballast in all the cargo holds if need be so as to ensure better sea-keeping performance in the ballasted condition.

The provision of twin hatches per hold, oriented transversely across the vessel, applies equally as well in vessels of the 080 type where upper-wing or saddle tanks are eliminated. In fact, with twin hatches, the free surface correction to be deducted from the GM owing to the presence of a liquid within the square of the hatch will be less with an open ship arrangement than with a conventional bulk carrier arrangement having upper-wing or saddle tanks.

It is, therefore, an object of the present invention to provide a vessel of the open ship concept, serviced by pairs of hatches on the main deck.

It is a further object of the invention to provide a pair of hatch covers for each hold opening, with one of the pair of hatch covers being elevated to a height to enable the other of the pair of hatch covers to be elevated a short distance to clear its sealing joint and be transported sideways under the first mentioned elevated hatch cover.

A further object of the invention is to provide means whereby adjacent pairs of hatch covers can be transferred piggy back transversely of the vessel for ease of working both sides of the hold.

A further object of the invention is to provide a centerline longitudinal structure which is used to accommodate a part of the cargo hatch transportation mechanism, to provide a ventilation duct and to control ullage below deck.

A further object of the invention is to provide a positive gas seal for each of the pairs of hatch covers.

A still further object of the invention is to provide a system for ventilating the cargo holds incorporated in the hatch coamings of the side-by-side hatch openings.

These and other objects of the invention will be apparent from the following detailed specification and the accompanying drawings in which FIG. 1 is a plan view at deck level of a vessel incorporating the present invention.

FIG. 2 is a plan view under deck level showing a proposed hold arrangement.

FIG.3 is a transverse section taken on the line 3-3 of FIG. 1.

FIG. 4 is a partial transverse section of the vessel showing a pair of side-by-side hatch covers in the locked-shut position on the hatch coamings.

FIG. 5 is a view similar to FIG. 4, but showing one of the pair of hatch covers elevated above the level of the other of the pair of hatch covers.

FIG. 6 is a view similar to FIG. 5, but showing the other of the pair of batch covers slid transversely under the first elevated hatch cover, leaving one hatch completely open.

FIG. 7 is an enlarged vertical sectional view taken on the line A-A of FIG. 4 showing the lifting rail and hatch cover in the raised position.

FIG. 8 is an enlarged vertical, sectional view on the line B-B of FIG. 5 showing the lifting rail in the lowered position and the hatch cover in the locked-shut position.

FIG. 9 is an enlarged view of the area at the ships centerline as seen in FIG. 5 with one hatch cover elevated to a maximum height and the other hatch cover elevated sufficiently to clear its seal with the hatch coaming.

FIG. 10 is a cross-sectional detail of the drive rack shown in FIG. 9.

FIG. 11 is an enlarged view at the centerline of the ship showing the adjacent hatch coamings and ventilating air duct.

Referring to the drawings, a typical vessel, incorporating the invention of this application, is indicated by the numeral 5.

The vessel shown is of the double skin type, having relatively narrow side tanks 6 and a double bottom 7 as shown more clearly in FIG. 3.

As shown particularly in FIGS. land 2, the vessel 5 is provided with cargo holds 6, 7, 8, 9, l0 and 11. Each of the holds 7 through 10 are of approximately similar size and capacity, while holds 6 and 11 follow the form lines of the vessel fore and aft.

The open ship concept shown in the drawings has an arrangement of hatch openings and hatch covers whereby the hatch openings and hatch covers are arranged in pairs transversely of the vessel with a minimum of space between the pairs of hatch openings and covers, and between the hatch openings and the sides of the vessel.

The hatch openings are indicated by the dotted lines 12 on FIG. 1 and the hatch covers are indicated at 13.

v The hatch openings 12 are bounded above deck level by the coamings 14, and the coamings are supported by suitable brackets 15 for a purpose which is more clearly illustrated in FIGS. 4 to 8.

The hatch openings 12, as shown more clearly in FIG. 3, are defined below the deck level by the outside longitudinal tanks or passageways l6, and amidships by the trunk 17.

The hatch covers 13 are of single piece pontoon construction, having deep peripheral edges 18, including an inverted channel 19 around their lower peripheral edges into which a seal member 20 is inserted.

The top peripheral edge 21 of the coamings 14 is provided with an upwardly projecting peripheral compression bar member 22'adapted to sealingly engage with the seal member 20 in the hatch covers 13 when they are lowered into place over the hatch openings 12.

The hatch covers 13 are reinforced internally in order to maintain their peripheral edges against distortion and to ensure that the seal provided by the mating members 20 and 22 will be uniformly gas tight around the whole periphery of the covers and thereby assist in a proper controlled ventilation of the holds of the vessel via the ventilating trunk 17.

Referring now to FIGS. 4, 5, 6, 9 and 10. In FIGS. 4, 5 and 6 it will be observed that one hatch cover 13A of the two single piece, pontoon type hatch covers serving each of the holds of the vessel is elevated by four high- Iift hydraulic jacks 23, while the other hatch cover 13B is lifted clear only sufficiently to break the gas-tight joint by four short-lift hydraulic jacks 24.

The hydraulic jacks 23 and 24 are located one pair on the forward facing side and one pair on the after facing side of each of the hatch covers 13A and 138.

The high-lift hydraulic jacks 23 are mounted on brackets 25 on the sides of the coamings 14A, the brackets 25 providing a housing into which the shafts 26 of the jacks are lowered in stowed position. The adjacent side walls of the hatch covers 13A are provided with blocks 27 with which the shafts 26 engage when the hatch covers are to be lifted.

The low-lift hydraulic jacks 24 are mounted on brackets 28 on the sides of the coamings 14B and the shafts 29 of the jacks are provided with a short length of lifting rail 30 for engagement with the rollers 31 mounted for rotation on the adjacent side walls of the hatch covers 138.

Between the adjacent longitudinal coamings 13C and 13D there are mounted, for rotation, a pair of cog wheels 32, supported on the brackets 33 spanning the space between the midship longitudinal coamings 13C and 13D.

On each of the longitudinal sides 34 and 35 of the hatch covers 13B, there are pivotally mounted pairs of latch dogs 36 and 37. The latch dogs 36 engage with the teeth of the cog wheels 32 when the hatch cover 138 is in the position shown in FIGS. 4 and 5, while the latch dogs 37 engage with the cog wheels 32 when the hatch cover 138 has been moved to the position shown in FIG. 6. When the hatch cover is in the closed position, sealing the hatch opening, the latch dogs 36 and 37 can be rotated to lie against the side walls 34 and 35 respectively of the hatch covers and be secured in that position by any suitable means so as not to interfere with passageway on the deck alongside the hatches.

The cog wheels 32 are mounted on the shaft 38 which in turn is rotated by a suitable drive, preferably by a hydraulic motor 29 as shown in FIG. 9.

Secured to the lower edge of each of the transversely disposed sides 40 of the hatch covers 133 is an inverted angle 41 and a series of equally spaced pins 42 straddle the space between the member 43 of the angle 41 and the adjacent surface of the wall 40 of the hatch cover. These pins 42 are spaced apart so as to be engaged by the teeth of the cog wheels 32 for movement of the hatch cover 13B horizontally when the shaft 38 is rotated. A series of pins 44, similar to and spaced at the same pitch as are the pins 42, are set in the latch dogs 36 and 37 In order to provide for easy transition of the hatch cover 133 from the position shown in FIG. 5 to the position shown in FIG. 6 and vice versa, there is provided rails 45 set on the upper edge of the transversely disposed walls 46 of the coamings 14A and 148. The rails 45 are aligned with the short length of rails 30 of the short-lift jacks 24 when these jacks are elevated to disengage the seal between the hatch cover 13B and the hatch coaming 148.

By first engaging the latch dogs 36 with the cog wheels 32 and then, on rotation of the shaft 38 in an anti-clockwise direction, the hatch cover is moved transversely of the vessel from the position shown in FIG. 5 to take up the position shown in FIG. 6, with the latch dog 37 finally engaging with the cog wheel 32 at the end of the movement of the hatch cover.

The jacks 23 under the hatch cover 13A can now be lowered to bring that hatch cover to rest on top of the hatch cover 138.

The half of the hold on the right hand side of the vessel, as shown in FIGS. 3, 5 and 6, can now be worked.

When it is desired to work the other half of the hold, i.e., the left hand side as seen in FIGS. 4, 5 and 6, the two hatch covers 13A and 138 can be moved together on the rails 45 in piggy back fashion, transversely of the vessel from left to right and remain supported on the rails 45.

After both sides of the vessel have been worked and the hatch openings are to be closed, the two hatch covers 13A and 13B are moved together to the left into the position shown in FIG. 6. The jacks 23 are then elevated to raise cover 13A clear of the hatch cover 13B. The hatch cover 138 is then moved to the right from under the hatch cover 13A to bring the wheels 31 over the short lengths of rail 30 and the jacks are lowered to bring the hatch cover into sealing engagement on the hatch coaming.

The jacks 23 can now be lowered to bring the hatch covers 13A into sealing engagement on their hatch coaming. If should be noted that the jacks 23 and the blocks 27 on the hatch cover 13A are spaced outwardly of the hatch cover sufficient to permit the hatch cover 13B to be moved freely on the rails 45 and under the hatch cover 13A when the jacks 23 are elevated as shown in FIG. 5.

The hydraulic jacks 23 and 24 and the hydraulic motors 39 can be individually controlled from a remote position to enable the operation of lifting and transporting of the hatchcovers automatically and only inspection by a qualified person is required.

In order to give adequate support for the above described structure, it should be noted that the peripheral walls of the coamings 14, as shown in FIGS. 3, 4, 5 and 6, are extended downwards below deck level. The outer longitudinal walls of the coamings are extended downwards at 50 to form the inward facing walls of the longitudinal compartments 16 which are a part of the double skin of the vessel and the inward longitudinal walls of the coamings are extended downwards to form the longitudinal side walls 51 of the longitudinal ventilating trunk 17.

The transverse walls of the coamings 14 are extended downwards below deck level to form the transverse girders 52 extending between the outer longitudinal compartments l6 and the centerline longitudinal ventilating trunk 17.

The strengthening of the peripheral area of the coamings 14 in the manner shown ensures that even under the most adverse conditions, the gas-tight seal between the hatch covers and the hatch coamings will be maintained.

By ensuring a positive gas-tight seal of the hatch covers, a more positive control of ventilation of the holds of the vessel can be effected through the ventilating trunk 17.

Due to the relatively narrow space between the inwardly facing longitudinal walls of the coamings, a short pipe connection can be fitted between the ventilating trunk 17 andthe hold served by the two hatch openings 12.

Referring to FIG. 11, a short length of pipe 53, open at its lower end to the interior of the trunk 17, is topped by a reducer 54 and a cross tec 55, and branch connections are made to each of the hatch openings 12 through the control valves 56 and the pipes 57.

The longitudinal ventilating trunk 17 extends lengthwise of the vessel, as shown in F IG. 1, to serve all of the holds 6 through 11 and is used to convey air pumped into it by supply fans 58 located at the fore and aft ends of the vessel.

By this arrangement a common centerline longitudinal structure is used to strengthen the vessel, to provide adequate support structure for the hatch covers and the means to move two adjacent hatch covers transversely across the vessel, and to provide for short ventilating air connections through the hold coamings at above deck level.

With the above described structure, including the "open ship hull structure and hatch openings with the hatch cover sealing means and the ease of movement of one or a pair of transversely adjacent hatch covers, the vessel can be rapidly converted to carry bulk cargo of either fluid or dry type and has the advantage of a double skin without the use of upper-wing tanks whereby the holds can be used for maximum cubic capacity. In addition, where fluid cargo is carried, the centerline ventilating trunk extends far enough down below deck level as to provide for an effective division of ullage below deck level to reduce the effect of roll of the vessel on the fluid cargo.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Hatch structure for twin hatch, open ship type vessels, the said vessel having a main deck and twin hatch openings in the deck disposed transversely of the vessel over a common hold, a hatch coaming about each of said twin hatch openings above deck level and longitudinal and transverse reinforcing about each of said twin hatch openings below said deck, the longitudinal reinforcing structure under the midship longitudinal sides of the said coamings being common to the adjacent midship sides of said hatch coamings, a pair of pontoon type hatch covers, one for each of said twin hatch openings, high lift hydraulic jacks located adjacent two opposite transverse sides of a first of said pair of hatch covers adapted to effect elevation of a first of said hatch covers, low lift hydraulic jacks located adjacent two opposite transverse sides of a second of said pair of hatch covers adapted to elevate said second hatch cover a distance sufficient to break the seal of the hatch cover with its coaming, a pair of rails disposed transversely of the vessel and supported one on each opposite transverse sides of the said coamings at the upper edge thereof, means to slide said second hatch cover transversely of the vessel to a position underneath an elevated said first hatch cover to uncover one of said twin hatch openings, and to slide the said pair of hatch covers transversely of the vessel touncover the other of said twin hatch openings.

2. Hatch structure as set forth in claim 1 in which the said second hatch cover is fitted with two pairs of wheels for rolling engagement with said pairs of rails when said second hatch cover is elevated by said low lift hydraulic jack.

3. Hatch structure as set forth in claim 2 in which the said low lift jacks include a short length of rail for engagement with the said wheels during elevation and lowering of the said second hatch cover.

4. Hatch structure as set forth in claim 3 in which the said short lengths of rail is elevated into alignment with the said pair of rails when the said low lift jacks are elevated.

5. Hatch structure as set forth in claim 1 in which the said pair of rails are in sections whose end positions are spaced apart on either side of the vertical axis of the said low lift jacks and the said low lift jacks include a section of rail, the said section of rails occupying the space between said rails when the said low lift jacks are elevated.

6. Hatch structure as set forth in claim 1 in which the said means to slide said second hatch cover includes a pair of latch dogs pivotally mounted on each of the longitudinally disposed sides of the said hatch cover, a series of equally spaced pins mounted on the transversely disposed sides of the said hatch cover and on said latch dogs, and a pair of cog wheels are mounted for rotation between the adjacent longitudinally disposed sides of said first and second hatch covers when the said latch dogs are pivoted into engagement with said cog wheels.

7. Hatch structure as set forth in claim 6 in which the said cog wheels are mounted on a shaft disposed longitudinally of the vessel, the said shaft being rotatable in brackets bridging the space between the coamings of said twin hatch openings, and a drive motor is connected to the said shaft.

8. Hatch structure as set forth in claim 1 in which the seal between the said hatch covers and the said coamings is gas tight and includes an elastomeric seal member seated in a peripheral channel around the lowermost peripheral edge of the hatch cover, and a peripheral member projecting upwards from the uppermost peripheral edge of the said coamings.

9. Hatch structure as set forth in claim 6 in which the said low lift hydraulic jacks are elevated a distance sufficient to lift the said second of said pair of hatch covers clear of the peripheral member projecting upwards from the coaming.

l0. Hatch structure as set forth in claim 1 in which the said longitudinal reinforcing structure at midships of the vessel is a ventilating trunk extending lengthwise of the vessel, and a ventilating connection is located between each adjacent pair of hatch coamings, the said ventilating connection being open to said trunk and through said coamings to the interior of the hold at said hatch openings.

11. Hatch structure as set forth in claim 10 in which the said ventilating connection includes a pair of shutoff valves, the said shut-offvalves individually closing the connections from said trunk to the hold at said hatch openings.

12. Hatch structure for twin hatch, open ship type vessels, the said vessel having a main deck and twin hatch openings in the deck disposed transversely of the vessel over a common hold, a hatch coaming about each of said twin hatch openings above deck level and longitudinal and transverse reinforcing about each of said twin hatch openings below said deck, the longitudinal reinforcing structure under the midship longitudinal sides of the said coamings being common to the adjacent midship sides of said hatch coamings, a pair of pontoon type hatch covers, one for each of said twin hatch openings, means to form a seal between the said hatch covers and the peripheral edge of the corresponding hatch coamings, high lift hydraulic jacks located adjacent two opposite transverse sides of a first of said pair of hatch covers adapted to effect elevation of a first of said hatch covers, low lift hydraulic jacks located adjacent two opposite transverse sides of a second of said pair of hatch covers adapted to elevate said second hatch cover a distance sufficient to break the seal of the hatch cover with its coaming, a pair of rails disposed transversely of the vessel and supported one on each opposite transverse sides of the said coamings at the upper edge thereof, means to slide said second hatch cover transversely of the vessel to a position underneath an elevated said first hatch cover to uncover one of said twin hatch openings, and to slide the said pair of hatch covers transversely of the vessel to uncover the other of said twin hatch openings.

Claims (12)

1. Hatch structure for twin hatch, open ship type vessels, the said vessel having a main deck and twin hatch openings in the deck disposed transversely of the vessel over a common hold, a hatch coaming about each of said twin hatch openings above deck level and longitudinal and transverse reinforcing about each of said twin hatch openings below said deck, the longitudinal reinforcing structure under the midship longitudinal sides of the said coamings being common to the adjacent midship sides of said hatch coamings, a pair of pontoon type hatch covers, one for each of said twin hatch openings, high lift hydraulic jacks located adjacent two opposite transverse sides of a first of said pair of hatch covers adapted to effect elevation of a first of said hatch covers, low lift hydraulic jacks located adjacent two opposite transverse sides of a second of said pair of hatch covers adapted to elevate said second hatch cover a distance sufficient to break the seal of the hatch cover with its coaming, a pair of rails disposed transversely of the vessel and supported one on each opposite transverse sides of the said coamings at the upper edge thereof, means to slide said second hatch cover transversely of the vessel to a position underneath an elevated said first hatch cover to uncover one of said twin hatch openings, and to slide the said pair of hatch covers transversely of the vessel to uncover the other of said twin hatch openings.

2. Hatch structure as set forth in claim 1 in which the said second hatch cover is fitted with two pairs of wheels for rolling engagement with said pairs of rails when said second hatch cover is elevated by said low lift hydraulic jack.

3. Hatch structure as set forth in claim 2 in which the said low lift jacks include a short length of rail for engagement with the said wheels during elevation and lowering of the said second hatch cover.

4. Hatch structure as set forth in claim 3 in which the said short lengths of rail is elevated into alignment with the said pair of rails when the said low lift jacks are elevated.

5. Hatch structure as set forth in claim 1 in which the said pair of rails are in sections whose end positions are spaced apart on either side of the vertical axis of the said low lift jacks and the said low lift jacks include A section of rail, the said section of rails occupying the space between said rails when the said low lift jacks are elevated.

6. Hatch structure as set forth in claim 1 in which the said means to slide said second hatch cover includes a pair of latch dogs pivotally mounted on each of the longitudinally disposed sides of the said hatch cover, a series of equally spaced pins mounted on the transversely disposed sides of the said hatch cover and on said latch dogs, and a pair of cog wheels are mounted for rotation between the adjacent longitudinally disposed sides of said first and second hatch covers when the said latch dogs are pivoted into engagement with said cog wheels.

7. Hatch structure as set forth in claim 6 in which the said cog wheels are mounted on a shaft disposed longitudinally of the vessel, the said shaft being rotatable in brackets bridging the space between the coamings of said twin hatch openings, and a drive motor is connected to the said shaft.

8. Hatch structure as set forth in claim 1 in which the seal between the said hatch covers and the said coamings is gas tight and includes an elastomeric seal member seated in a peripheral channel around the lowermost peripheral edge of the hatch cover, and a peripheral member projecting upwards from the uppermost peripheral edge of the said coamings.

9. Hatch structure as set forth in claim 6 in which the said low lift hydraulic jacks are elevated a distance sufficient to lift the said second of said pair of hatch covers clear of the peripheral member projecting upwards from the coaming.

10. Hatch structure as set forth in claim 1 in which the said longitudinal reinforcing structure at midships of the vessel is a ventilating trunk extending lengthwise of the vessel, and a ventilating connection is located between each adjacent pair of hatch coamings, the said ventilating connection being open to said trunk and through said coamings to the interior of the hold at said hatch openings.

11. Hatch structure as set forth in claim 10 in which the said ventilating connection includes a pair of shut-off valves, the said shut-off valves individually closing the connections from said trunk to the hold at said hatch openings.

12. Hatch structure for twin hatch, open ship type vessels, the said vessel having a main deck and twin hatch openings in the deck disposed transversely of the vessel over a common hold, a hatch coaming about each of said twin hatch openings above deck level and longitudinal and transverse reinforcing about each of said twin hatch openings below said deck, the longitudinal reinforcing structure under the midship longitudinal sides of the said coamings being common to the adjacent midship sides of said hatch coamings, a pair of pontoon type hatch covers, one for each of said twin hatch openings, means to form a seal between the said hatch covers and the peripheral edge of the corresponding hatch coamings, high lift hydraulic jacks located adjacent two opposite transverse sides of a first of said pair of hatch covers adapted to effect elevation of a first of said hatch covers, low lift hydraulic jacks located adjacent two opposite transverse sides of a second of said pair of hatch covers adapted to elevate said second hatch cover a distance sufficient to break the seal of the hatch cover with its coaming, a pair of rails disposed transversely of the vessel and supported one on each opposite transverse sides of the said coamings at the upper edge thereof, means to slide said second hatch cover transversely of the vessel to a position underneath an elevated said first hatch cover to uncover one of said twin hatch openings, and to slide the said pair of hatch covers transversely of the vessel to uncover the other of said twin hatch openings.

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