Hatchways

Hatchways in the majority of dry cargo ships extend across the deck for approximately one-third of the beam. In special types of ship, e.g. container ships, colliers etc., much wider hatchways are fitted as will be seen from the separate cross-sections of these types.

Special arrangements must be made to compensate for the structural discontinuities caused by these large openings; insert plates of increased thickness may be required at the hatch corners as shown. The arrangement of hatch coaming and adjacent structure with rounded hatch corners is illustrated. Note that the hatch coaming should be extended beyond the corner to form a bracket. The deck opening corners should be well-rounded to a reasonable radius, or eliptical or parabolic in shape, to avoid a concentration of stress at these points.

The deck plating forms an important structural member, especially at the strength deck, in resisting longitudinal stress, but only the plating clear of the hatches can be considered in this respect. If hatchways are made unduly wide, the effective width of this plating is reduced and the thickness has to be increased in order that the cross—sectional area of the steel is maintained.

In addition to the plating, beams will also be cut at hatchways and the ends of the half beams will be connected to the hatch coamings and supported by deck girders. The deck girders are usually integral with the hatch coamings as illustrated. At the ends of the hatchways, in the case of transverse framing and at the sides in the case of longitudinal framing deep hatch end beams will be fitted to support the coamings. A pillar will often be placed near the hatch corners at the intersection of the deck girder and a strong beam.

DOUBLE BOTTOM TANKS

Volume enclosed between the ship's bottom up to the turn of the bilge, abaft the collision bulkhead and forward of the aft bulkhead and tank top in the cargo holds is termed as double bottom tanks. Double bottom tanks are identified numerically from forward to aft.

Double bottom tanks do not extend in way of the peak tanks and Main Engine room space.

Double bottom tanks are sub-divided longitudinally to form Port and Starboard Tank by water tight girders or walls of the duct keel, Double bottom tanks are divided by water tight floors.

Double bottom tanks are strengthened by bracket floors, solid floors, tank side brackets, inter coastal girders, center girder, longitudinal members which are located under the tank top and on the inner surface of the bottom plating, all these members jointly contribute in strengthening longitudinal and transversely the bottom structure of the ship.

Tank side brackets are fitted along the length of the double bottom tanks in way of the turn of the bilge.

Double bottom tanks are generally used for ballast water, fuel, fresh water / boiler feed water.

Every double bottom tank must be provided with air pipes, sounding pipes, filling and suction pipe line arrangements. 

SHELL AND DECK PLATING

General: Seams, or ‘Edge Laps’, are joints which run fore and aft; that is, along the longer edges of the plates.

Butts, or ‘End Laps’, are joints which run athwartships, or vertically; that is, along the shorter edges of the plates.

Strakes are continuous, fore and aft, lines of plates.

Garboard strakes are the strakes of shell plating next to the keel on either side (i.e. the ‘A Strake’).

Sheerstrakes are the upper strakes of shell plating on either side, next to the upper deck. (The ‘J strake’ in the sketch opposite.)

The Deck Stringer is the outboard strake of deck plating, which is connected to the sheerstrake. (Strake ‘E’ on the deck plan, opposite.)

Stresses on Plating: The obvious purpose of plating is to keep out water and to tie together the ship’s framework. It also plays an important part in resisting longitudinal bending stresses, so it needs to be stronger amidships than at the ends, particularly at the deck and bottom. In long ships, it may

also be necessary to strengthen the shell plating against shearing stresses at about the half-depth of the ship, in the region of about one-quarter of the ship’s length from either end.

Shell Expansion and Deck Plans: These are plans which show all the plates in the hull, drawn to scale. They also show many other details, including frames, floors, deck edges, stringers, etc. The partial plans shown in the plate, opposite, are simplified and are merely intended to illustrate the fitting of shell and deck plating.

Identifying Plating: Strakes of shell plating are distinguished by letters from the keel outwards, the garboard strake being strake ‘A’. The plates in each strake are usually numbered from aft to forward. For example, plate D5 would be the fifth plate from aft in the fourth strake from the keel.

Strakes of deck plating are lettered from the centre line, outboard; whilst deck plates are numbered from aft to forward.

Stealer Plates: The girth of the ship decreases toward the ends and so the width of plates must be decreased in these parts. To save making the plates too narrow at the ends of the ship, it is usual to run a number of pairs of adjacent strakes into one. This is done by means of a stealer plate.

This can be seen in the upper shell expansion plan, given here; in which Plate B4 is a stealer, since it runs the B and C strakes into each other. Note how the stealer and the plates beyond it, always take the name of the lower of the strakes which are run together.

CONTENTS OF GENERAL ARRANGEMENT PLAN

A copy of the G.A plan is always displayed in conspicuous locations accessible and frequented by the ship’s crew. The general layout is displayed through a side profile, mid-ship section and Arial view of decks on board a ship. 

The general arrangement plan will familiarize the seaman with the Following information:

1. No. of Hatches, No. of cargo compartments on the ship.
2. Location of various double bottom tanks, topside tanks, peak tanks, Cofferdams etc.,
3. No. and location of fuel oil tanks.
4. Location of various decks, engine room, saloon and store rooms etc.
5. Salient features of lift gear.
6. Location of emergency machinery i.e steering gear, generator, fire pump etc.
7. Location of Crew and Master’s accommodation.
8. Location of Navigation Bridge, chart room, monkey island etc.
9. Salient of deck fittings, lifting gear, deck machinery, gangway etc.

 More details of any particular space, fittings must be refereed to appropriate plan.