The Carriage of Goods by Sea Act
Distribution of Weights
Vertical Distribution
Longitudinal Distribution
Transverse Distribution
Local Stresses - Excessive Pressure - Design Limits
Longitudinal Distribution of Weights - Stresses
Stowage Factor - Efficiency
Angle of Repose
Handling Equipment - Gear
Prevention of Cargo Movements
Ship Stability
Cargo Density - Moisture
Cargo Density - Expansion due to Heating
Container Ships
General Practice
Other Factors

The Carriage of Goods by Sea Act provides that a shipowner shall:

"Properly and carefully load, handle, stow, carry, keep, care for and discharge the goods carried."

As we can see, stowing goods on a ship, "properly and carefully", is a primary responsibility for a shipowner and this is not an easy task. Certain factors and regulations have to be borne in mind so that goods can be safely carried throughout the voyage.

Two of the above mentioned factors are: weight and density of the goods. Determining the location for each cargo unit in the ship's hull(s) according to its weight/density, is a very complex task which must be based on regulations and some practical principles.

Distribution of Weights

Cargo stowage affects the stability of a ship, sometimes dramatically. In general: the cargo must be disposed throughout the vessel as to ensure that she is in a stable condition at all times, or can be kept in that position by the use of ballast tanks as the voyage progresses.

The total weight of the cargo has to be distributed by the centerline (forward/after, port/starboard, up/down) in such a way as to:

a) obtain the appropriate trim before proceeding to sea, b) reinsure the ship is not subjected to any heel and, c) produce safe stability for the ship.

Vertical Distribution

If too much weight is in the upper decks of the ship, she will have a small amount of stability (tender).If too much weight is in the lower decks of the ship, she will have an excess of stability (stiff).Being excessively stiff or tender is not desired and so appropriate vertical distribution of weights must be achieved in order to produce safe vertical stability.

Longitudinal Distribution

Weights must be disposed longitudinally in such a way as to produce the required and safe trim for the ship to proceed at sea. This means that an even distribution of weight has to be achieved longitudinally so that the ship will be evenly trimmed (although usually a slight trim by the stern is more appropriate).

The equal distribution over all holds of heavy cargoes like ore, etc., is, however, to be avoided, as that would bring too much to the ends, making the vessel sluggish in rising to head sea and liable to undue straining in heavy weather; with such cargoes the end weights should be kept down.

Transverse Distribution

The same provisions must be taken in respect to the transverse distribution of weights. In this case, weights have to be evenly distributed on both sides (port/starboard) along the centerline so that the ship will not be subjected to heel at any moment.

In all cases (longitudinal, transverse and vertical distribution of weights), slight variations or deviations from the desired condition, can be overcome by using and adjusting the double bottom tanks in order to produce the appropriate trim and avoid any heel.

Local Stresses - Excessive Pressure - Design Limits

Special consideration has to be given when loading heavy cargo on tank tops and on 'tween decks. The structural and local stresses must be within the design limits. Exceeding the maximum allowable stress loading (tonnes/m2) will cause structural deformation or even failure which in turn might lead to damaging the goods, spills, contamination, etc.

Maximum Cargo to be Loaded at any Lower Hold

The maximum number of tons of cargo loaded in any lower hold should not exceed:

db (3L + B) / 165 ,

where d = ship's summer load draught

b = the average breadth of the lower hold

L = the length of the lower hold

B = the ship's maximum moulded breadth

(all measurements are in feet).

The same consideration has to given when goods are stowed one upon the other (over-stowage): the weight imposed upon one cargo from all the super-imposed must be calculated and officers have to make sure that the bottom cargo will withstand the total pressure without being damaged (e.g. fragile goods should not be over-stowed; they should always be placed on top) and that the tank-top or 'tween deck can withstand the pressure from the total weight.

Where over-stowage is necessary, dunnaging must be generally used in order to avoid excessive concentrated stresses.

Over-stowage is a very complex stowing solution because different cargoes are discharged/loaded at each calling port and so planning has to be done continuously and with provision for future cargoes placed upon old cargoes.

High-density cargo, such as plate iron, which is heavy but has not great volume (small dimensions), must be placed on tank-top or 'tween deck which will withstand the concentrated pressure without suffering deformation or damages.

"In order to reduce the risk of distortion or failure of 'tween structures in bad weather, due to concentrated loading of heavy cargo, such cargo should be so distributed that the intensity of loading in any part of a 'tween deck or hatch does not substantially exceed the weight of cargo that could be stowed thereon for the full 'tween deck height, at the rate of 50 cubic feet per ton."

When stowing heavy cargo it is preferable to avoid buckling tank top plating by bringing the weight of heavy packages to bear between frames, especially where floors are fitted on alternate frames, etc.

Weighty packages, such as cases of machinery, railway, bar or plate iron, blicks of stones, ore, billets, ingots or pigs of metal, etc., should always be stowed on the floor and the lighter cargo on top.

As a general rule fragile and light packages should be stowed in 'tween deck spaces (the ground floor of such being, if necessary or advisable, covered with weighty goods) where they will not be subjected to heavy topweight.

Longitudinal Distribution of Weights - Stresses

It is of utmost importance to produce a safe (but not always totally even) longitudinal distribution of weight.

Weight should not be concentrated:

i) at the ends - producing hogging bending stresses
ii) at the center - producing sagging bending stresses
iii) or at any point - producing shear forces.

At any of these cases unforeseen damages could be caused to the ship (distortion, deformation, structural failure, etc.), putting at risk the security of the cargo and the safety of crew and passengers. Hence, selecting the best stowing place for a given cargo must be done in accordance to the moments it will produce about the centerline so that the final longitudinal distribution of weight will be within acceptable limits.

Stowage Factor - Efficiency

Operating efficiently a cargo ship means that cargo space has to be exploited to the best advantage. This is where the stowage factor must be considered.

Stowage factor is the figure which expresses the number of cubic feet which a ton will occupy in stowage and should include a proper allowance for broken stowage (due to bulkheads, stiffeners, etc.) and dunnage: (if filled with cargo of the design S.F. it will be carrying its maximum weight -down to the max load line- and to max. stress on structure -decks- and there will be no empty space).

The S.F. varies according to the type of the cargo and its form (barrels, cases, drums, bales, sacks, bags, loose, bulk, casks, bundles, pallets, containers, rolls, boards, cylinders, cartons, baskets, etc.).

Maximum exploitation of space requires a careful examination of the S.F. of each cargo unit to be loaded and then the skill to be able to combine cargoes with high and low S.Fs. together in a single hull while having in mind the variety and characteristics of the goods and the sequence of calling ports.

Angle of Repose

High density bulk cargo should be loaded in the lower holds and trimmed appropriately as a precaution against shifts of the center of gravity of the cargo.

Handling Equipment - Gear

Heavy packages must be stowed as near as possible to the handling gear thus avoiding undue strain on the gear in case of lowering the equipment (derricks/cranes) to reach cargo located at long distances from the handling gear.

Heavy cargoes must be stowed is places which can be served by handling equipment, (derricks, cranes, heavy lifts) both on-board as well as ashore, of adequate capacity (in weight/volume).

Prevention of Cargo Movements

Heavy weighted cargo must be stowed in places where there is little possibility of side movement. Is this is not feasible, heavy cargo must be secured.

Any shift of a bulky and heavy item could cause:

a) severe damages to other cargoes,
b) damages to the ship's hull and,
c) dangerous shift of the center of gravity (G) forcing the ship to heel.

Hence, heavy cargo should be stowed in secure places or in positions near bulkheads, stiffeners, etc., where by using ropes, wires, slings, etc., lashing and securing would prevent any cargo shifts (caused by vibrations during the voyage, etc.).

Ship Stability

In general heavy cargo is to placed on lower 'tween decks while less heavier cargo is to be placed on upper decks, thus effecting the position of the center of gravity (lowering G) and determining the metacentric height (GM), which contribute to the ship's stability.

Cargo Density - Moisture

Cargo density must be considered especially in bulk cargoes. Bulk cargoes having low densities, such as grain, can become dangerous in cases of absorbing moisture. Moisture concentration might occur, and a shift of the center of gravity of the cargo is inevitable. This is reflected to the ship's center of gravity making the ship unstable (list).Hence, cargoes of this type must be stowed in places (hulls) being as best protected as they can be from rain, moisture, oil or water ballast leaks, etc.

Cargo Density - Expansion due to Heating

Another factor to be borne in mind when stowing high-density cargoes is the fact that some might incur expansion due to heating (when crossing the tropicals or when it is located next to machinery).The location of stowage must be carefully selected so that in case of expansion nearby cargoes will not be damaged and the hull structure will not be subjected to any additional stresses (adequate side and top spacing).

Container Ships

As far as it concerns container ships there is a general rule determining the position of cargo according to its weight: heavy containers are to placed on the bottom and the lower 'tween decks, while lighter containers are to be placed on the weather and generally upper decks and in the middle (along the centerline). As far as it concerns transverse stability, containers must be stowed in such a way as to produce even moments about the centerline (port/starboard).

Other factors that have to be borne in mind while loading a container ship are:

-the sequence of calling ports (position of cargo according to its destination),
-requirements for reefer containers,
-requirements for ventilation,
-open-top containers.

General Practice

According to the nature and density of the cargo, it is frequently the practice in large carriers to distribute cargo in alternate hatches. Careful consideration must be given to get the longitudinal stresses within accepted limits, to this end Masters of large carriers should be provided with loading diagrams setting out distribution of cargoes for varying conditions.

If, as sometimes happens, the cargo is delivered from only one tip, the vessel should be moved frequently so as to bring all hatches, in fairly quick alternate rotation, under the tip, it being an exceedingly dangerous practice to receive all the cargo intended for one hold while other holds are empty.

Loading a ship is a very complex function but the final result is to be always achieved by producing a mean draught with the load-line marks above the water line. Other factors such as the crossing of different weather zones, the consumption of fuel oil and fresh water, etc. must also be considered.

While it is considered most dangerous, in some cases heavy cargo is stowed at the end (forward and after) hulls so that while the ship is subjected to sagging, the load-line marks remain above the water line. This is done because under normal circumstances, loading the same weight in mid-hulls would bring the ship down to her load-line marks and space could remain idle.


Where delivery is to be made to consignees' lighters, trucks, etc., port speed will be greatly accelerated by stowing small or medium sized, or a substantial part of large, consignments together, and avoid mixing them with other goods intended for the same port. Much shifting of lighters and loss of time will be thus avoided.

When loading heavy cargo the vessel must be upright and the ballast tanks must be either dry or pressed up.

Bulky unit loads have to be allocated to compartments of adequate dimensions.

Stowing must be done as compactly as possible and the intervening space must be tightly pack with suitable packages. Where pillars or stanchions are nearby, lashing must be done by using wire lashing rove slack and Spanish windlass fashion must be set up.

Other Factors

Apart from weight and density, there are a few other factors which require careful consideration when preparing a stowage plan for a ship and are worth mentioning:

-fire precautions (flammable),
-radio active goods,
-compatibility of goods,
-dangerous goods, poisonous goods and their properties,
-reefer goods,
-goods requiring ventilation,
-goods subjected to heating (spontaneous heating and deterioration),
-mixture of cargo (claims , loss of time, expense for labor),
-order of calling ports,
-turnaround times,
-economy of cargo space: between and around packages,
-dunnage: at the sides, ends and top of cargo, next to pillars, bulkhead stiffeners, etc.,
-loss of space:

-shape of the compartment,
-type of cargo,
-compactness of stowage
-provision for damages: leakage, drainage, moisture, sweat, contamination, due to cargo pressure, chafing, dust through ventilation or penetration through leaks, oil stains from the double bottom, rats and mice, etc.

As we have seen cargoes, according to their weight and density, must be distributed and placed within a ship's hulls in such a way as to:

-avoid any excessive stresses on the ship's structure,
-produce safe stability,
-be easy and safe to handle,
-protect them from being damaged and damaging other cargoes,
-efficiently exploit the given space.

This is not an easy task an requires special skills, knowledge of many rules, regulations and characteristics of the goods and most of all experience.

Copyright ©, 1997 Marinet