Cargo owners often assume that a larger vessel must provide the better freight solution. For breakbulk cargo, this is not always correct.
A smaller tween-deck vessel can sometimes use its available space more efficiently than a larger single-decker, particularly when the shipment contains bagged commodities, crates, pallets, machinery or mixed cargo units that cannot be safely stacked to the full height of a deep cargo hold.
The commercial comparison should therefore consider usable cargo space and handling requirements, not only deadweight and total cubic capacity.
The problem with deep single-deck holds
A single-deck vessel may offer a large open hold and attractive overall capacity. This can work well for homogeneous cargo that can be stacked safely and efficiently.
Breakbulk cargo often behaves differently.
The cargo may include:
- units with limited stacking strength;
- different package heights;
- machinery that cannot carry other cargo above it;
- crates requiring access during discharge;
- bagged cargo that can deform under excessive pressure;
- several parcels intended for different ports.
When cargo cannot be stacked through the full depth of the hold, part of the vessel’s theoretical cubic capacity becomes unusable.
What tween decks provide
Tween decks divide the cargo space into separate vertical levels. Depending on their design, they may allow cargo to be arranged on the tank top and on one or more intermediate decks.
This can provide several advantages:
- reduced stacking pressure;
- separation of different cargo types;
- improved use of vertical hold space;
- easier segregation between parcels;
- better access during multiport discharge;
- additional surfaces for positioning machinery and packaged cargo.
A smaller tween-decker may therefore accommodate a breakbulk parcel more efficiently than a larger vessel with one deep cargo space.
Floor strength must be checked
Tween decks are not automatically suitable for every cargo. Their permissible deck loading may be lower than the tank-top strength.
Before placing heavy machinery or dense cargo on a tween deck, the parties must verify:
- maximum permissible load per square metre;
- concentrated load limits;
- footprint of each cargo unit;
- required load-spreading arrangements;
- position of deck supports;
- structural restrictions imposed by the vessel.
Heavy cargo may still have to be stowed on the tank top, while lighter crates, bags or pallets are placed on the tween decks.
Hatch dimensions and access
The vessel must also provide practical access to every intended cargo level.
Relevant considerations include:
- hatch opening dimensions;
- tween-deck hatch dimensions;
- crane outreach;
- hook height;
- obstructions inside the hold;
- removable deck panels;
- sequence for opening and closing the tween decks;
- ability to position cargo in the required stowage location.
A vessel may have sufficient total cubic capacity but be unable to pass a large unit through the tween-deck opening.
Cargo sequence affects voyage performance
Tween decks can be particularly valuable for shipments involving several parcels or discharge ports.
Cargo can be separated by destination, reducing the need to shift unrelated units during discharge. However, the loading plan must follow the intended discharge sequence.
Poor sequencing can create additional handling, restowage costs and delay. The theoretical advantage of the vessel is lost if cargo for the first discharge port is blocked beneath cargo intended for a later destination.
The complete stowage concept should therefore be considered before the final fixture.
Comparing freight correctly
The freight rate of the smaller tween-decker may appear higher per deadweight tonne. That comparison can be misleading.
The proper comparison should include:
- actual quantity the vessel can load;
- usable cubic capacity;
- port and canal costs;
- lifting and handling requirements;
- expected loading and discharging time;
- need for shifting or restowage;
- risk of cargo damage;
- cost of unused space;
- possible part-cargo combinations.
The cheapest vessel is not necessarily the vessel with the lowest headline freight rate. It is the vessel that carries the required cargo safely and produces the lowest realistic total transportation cost.
The commercial conclusion
Vessel size should not be assessed separately from hold configuration. For breakbulk cargo, a well-configured tween-decker can sometimes outperform a larger single-decker by using vertical space more effectively and providing better cargo separation.
Best Break Bulk evaluates hold geometry, tween-deck strength, hatch access, lifting arrangements and cargo sequence as part of breakbulk cargo chartering. The choice should be based on the cargo’s actual stowage requirements, not simply the vessel’s deadweight or total cubic capacity.