What is GA or General Arrangement Plan and Information Provided in a GA?

Introduction

Once the Main Dimensions and Hull Form have been fixed consideration can be given to the General Arrangement.

Normally this will be done by means of the drawing of a small scale General Arrangement plan. A scale of 1 to 200 is quite suitable although a larger scale may be more appropriate for small ships. The only boundaries which have been fixed so far are the hull surface and any deck lines which affect freeboard. The remaining space boundaries in the ship remain to be fixed.

A major decision is to determine the position of the machinery space. In a light condition the density of the machinery space and the accommodation, taken together, is greater that the rest of the ship's length.

In a loaded condition the reverse is truer. This is important when considering trim.

GA plan depicts the division and arrangement of the ship:

Side View.
Plan Views of most important Decks.
Cross-sections.

The views and sections display:

Division into compartments (tanks, engine room, holds)
Location of bulkheads
Location and arrangement of superstructure
Parts of the equipment (winches, loading gear, bow thruster, life boats).

Basic data included in the GA Plan:

Dimensions (Length, Breadth, Draft)
Volumes of the holds (Cargo Capacity Grain & Bale)
Tonnage
Deadweight
Engine power
Speed (Service Speed)
Class

Important Decisions for Planning of General Arrangement Drawing

For general arrangement drawing some important decision is to be determined about

1. Trim

2. Location of Machinery Space

3. Storage of Liquid

4. Cargo Holds

5. Hatchways

6. Accommodation/Superstructure Arrangement

7. Length of Machinery Space

GA of a dry cargo
Crew accommodation plan

1: Trim

A level keel trim is usually specified for the full load condition with homogeneous cargo. This is mainly to make the best use of the available depth of water in port - usually a restrictive item.

Some designs either specify a design trim or must accept one. There are ships in which the weight distribution is so extreme that balance between the LCG and LCB can only be achieved by using trim to make a radical adjustment to the sectional area curve.

Tugs and fishing vessels are common examples where the need for propeller immersion also plays its part and warships often have this feature.

As ships tend to trim by the bow relative to their static trim when running at normal speeds, no bow trim at all can be permitted at rest.

Usually cargo is disposed to ensure some stern trim in most sea-going conditions. Steering and directional stability can be upset by bow trim.

In the initial design stages trim is mainly controlled by the location of the machinery space relative to the cargo holds. Provision of ballast spaces including the peak tanks gives some control over trim but carrying ballast is a waste of deadweight and may impose undesirable stresses.

Reasonable trim must also be maintained during cargo working at intermediate ports. In the case of the traditional general cargo vessel this was no easy thing to do unless the machinery space was amidships.

2: Location of the Machinery Space

The most common position for the machinery space in modern cargo ships is completely aft. Trim problems are severe in general cargo vessels and cannot be solved without ballast tanks forward to use in the light condition. Sometimes it is difficult to avoid bow trim when loaded. This location is suitable for, and typical of, ships which carry homogeneous cargoes such as tankers and bulk carriers, especially when the cargo is denser than seawater since their weight distribution can be controlled to solve trim problems. While the best part of the ship is given to the cargo holds the machinery space may require more

length than expected in order to accommodate the auxiliary machinery. The use of segregated ballast tanks in tankers or a floodable hold in bulk carriers provides control of draught, trim and bending moment if carefully sized and located.

In large high-speed and high-powered container ships the machinery space is often situated in the three-quarters aft position. Although this splits the container stowage area into two parts it allows the machinery to be installed in a fairly full part of the ship. Trim can be kept under control with only a modest requirement for water ballast and bending of the hull girder between loaded and light condition may be minimized.

3: Storage of Liquid

One the size and position of the machinery space has been decided then attention can be turned to rank spaces. Normally these are confined to double bottoms but deep tanks may be arranged for additional water ballast in the fore and aster peaks for trim or near amidships to control hull girder bending. Fuel reserve storage may be arranged at the double bottom, but there should have a service/ ready use (RU) tank for continuous consumption. Few other tanks are required for storing dirty/contaminated oil, gray water or black water as per new IMO conventions.

Once the Position and size of the machinery space has been decided then attention can be turned to tank spaces. Normally these are confined to double bottoms but deep tanks may be arranged for additional water ballast in the Fore and After Peaks for trim or near amidships to control hull girder bending.

Engine Room double bottoms will first be allocated to Lubricating Oil storage, drain and sump tanks together with cofferdams to ensure there is no Lub. Oil/Salt Water interface which could leak and cause contamination.

While main propulsion engines will be happy running on fairly heavy fuel oil, diesel generators normally require the lighter Diesel Oil. This should be stowed reasonably close to the generators. Ideally, the tanks for fuel oil can then be allocated with a view to ensuring that the LCG of the fuel is forward of the LCG of the loaded ship so that as fuel is consumed the ship will not tend to trim by the bow.

Modern practice, driven by pollution control requirements, discourages the use of double bottom tanks for fuel storage. Thus the only way to have control over fuel LCG is to fit deep tanks forward and aft of the cargo holds and accept long filling and supply lines to/from the forward tanks. The alternative of only having fuel tanks aft has the consequence that a significant stern trim in the Departure condition will be followed by a significant bow trim in the arrival condition.

In addition to the storage tanks, space also needs to be found for settling tanks and daily service tanks to satisfy the needs of the main and auxiliary machinery. These are usually located within the boundaries of the machinery space. Actually finding the space for them may not be a task for the Naval Architect but the Weight & centre of gravity of their contents is a legitimate concern.

Water ballast is required to give adequate propeller immersion in the lightest seagoing condition and to ensure that the minimum draught forward is sufficient to avoid excessive slamming.

While many ships now distil their own Fresh Water from sea water a limited storage capacity is necessary for use when the ship is in polluted or coastal waters where distillation is not possible.

Holding tanks for sewage and waste water are necessary to avoid marine pollution. They are small in a cargo ship but of significant size in passenger ships.

4: Cargo Holds

The number of holds is dictated largely by the size of the ship and the type of cargo. Requirements, which came into force in February 1994 for the damage stability and survivability of cargo ships, have brought flooding into consideration.

Holds in container ships will have lengths which are multiples of the container length (plus an allowance for the cell guides). A hold around 40 ft long can take either one 40 ft container or two 20 ft containers; a hold 60 feet long can take 3 at 20 ft or two at 30 ft or one at 40 ft and one at 20 ft depending on how the cell guides are set up.

In dry bulk carriers the usual of number of holds is a choice from 5, 7 or 9. Five holds are common in Handy Size vessels of around 25,000 tonnes deadweight; seven holds are the usual choice for a 75,000 tonnes deadweight Panamax vesssel; while nine holds are often found in the largest Capsize vessels of 150,000 tonnes and over.

The height from the double bottom to the upper deck will be divided by tween decks in accordance with the requirements of the trade. Thus none will be found in Bulk Carriers while Fruit Carriers and Banana Carriers will have the total depth of the hold divided into tween decks. The height of the tween deck may vary between 2.4m and 3.0m. The clear height in the hold varies immensely but it should be noted that some cargoes will crush if loaded too deeply.

5: Hatchway

Hatchway is a means of passing through a wall or floor, having a hatch (especially on a ship); a doorway with a hatch rather than a door. Large hatchway assists easy cargo working but hatch widths are restricted by the need to maintain not only the cross sectional area of deck material for structure reasons but also the shelf space at the tween deck levels. The ingenious use of twin hatches, side by side, can facilitate both good cargo working and the constrained by the length of deck taken up by cargo gear and hatch cover stowage.

Large hatchways assist easy cargo working but hatch widths are restricted by the need to maintain not only the cross sectional area of deck material for structural reasons but also the shelf space at the tween deck levels. The ingenious use of twin hatches, side by side, can facilitate both good cargo working and the containment of grain cargoes in a general cargo ship.

The length of hatches is constrained by the length of deck taken up by cargo gear and hatch cover stowage. General cargo ships usually have the capability of carrying some containers within the line of hatches and this will lead to hatch dimensions tending to be a multiple of container lengths and widths with an allowance for clearance between them.

Flush hatches are clearly desirable for ease of cargo working but in general load line requirements will prohibit or severely penalise the ship for their use on a weather deck.

6: Accommodation/Superstructure Arrangement

Usually the accommodation is sited above the machinery space and around the engine casing to minimize interference with cargo operations. The result is a short, high superstructure giving good forward visibility but possibly compromising stability. A good arrangement is largely a matter of common sense, experience and foresight. Any difficulties which arise in service should be noted and avoided in the next design.

Minimum manning scales and minimum standards for accommodation are laid down in regulations. Virtually every crew member nowadays will have a single cabin and officers may well have suites with dayroom, sleeping cabin, bathroom etc.

Automation has a continuing influence, gradually reducing crew numbers and further significant changes may take place in the coming decades.

Remember that the accommodation is where the seafarer lives out his/her life. It is his/her home for long periods as well as his/her place of work. There must be public space to socialize in and private space as a retreat from work.

7: Length of Machinery Space

Assuming that diesel propulsion is to be adopted (and it usually is) then the length of the machinery space is governed either by -

(1) The Main Engine, Gearbox (if fitted) and Thrust Block

(2) The Generators

An end clearance of one or two frame spaces should be added to the neat length. Care must be taken to ensure that there is sufficient space for the auxiliary machinery. Ideally these should be sited on the tank top, particularly those requiring a solid foundation to minimize vibration e.g. Generators and Compressors. Flats can be fitted to provide additional area but often cannot be made stiff enough to support major auxiliaries.

Technological change tends to make the machinery grow in complexity but to shrink in size and so machinery spaces tend to become smaller over time. However engine maintenance is an important consideration for the effective operation of the ship. Too compact an engine room may make maintenance more difficult and even more expensive. Access to the equipment and removal routes for parts from them should be adequate.

Minimum Requirements for Crew Accommodation

Segregation into Officers, Petty Officers and Ratings is still common in the Merchant Navy although it becomes harder to sustain as crews become smaller. A ship which had 30 of a crew thirty years ago would be designed to run with half that number now. Justifying separate facilities for each grade becomes very difficult.

a) Deck and Engineer Officers. In Single or double cabins (Master and Chief Engineer should each have an individual cabin). Bathroom with one bath or shower and one wash basin for every six persons. Separate dining saloon and smoke room.
b) Petty Officers. Cabins and Washing facilities as for officers. Separate messroom. (1m² per person)
c) Engine Room Hands. Separate sleeping and dining accommodation (but numbers sharing cabins not specified). Bathrooms as for officers.

The International Labour Organization (ILO) recommends minimum floor areas per person in sleeping rooms as -
3.75 m² in ships of 1000-3000 tons,
4.25 m² in ships of 3000 – 10000 tons and
4.75 m² in ships over 10000 tons.
(Tons are gross tons; a volumetric measure)

Where two ratings share a cabin the above figures are reduced by 1 m² per person.

d) Deck Hands. Cabins, dining accommodation and bathrooms as for Engine Room Hands. Crew’s smoke room shared with Engine Room Hands.

Many ships offer higher standards than these such that all crew have single cabins with ensuite bathrooms and perhaps double beds so that wives can travel on some voyages. An officer’s single cabin could reach 21 m² with bathroom and crew’s single cabin 16 m²also with bathroom.