Containers overboard

Do shipping containers sink?

Here are a series of three photos from our archives, showing containers falling from a containership in a storm:

The question of whether shipping containers sink or float is often debated in maritime circles. Another question is often posed - is this happening more or less frequently today?

WHAT are the chances of a ship hitting a container lost on the high seas? Despite widespread fears about the danger to both small and large vessels from lost boxes, the answer appears to be: negligible. The number of containers lost overboard is said to be a tiny percentage of those transported, and of those that do go adrift, most inevitably sink within a short time. So say experts at the Through Transport Club, who insure 15 of the top 20 container lines for their container losses, in addition to many other carriers.

At any given time, between 5 million and 6 million boxes are in transit. The TT Club calculates that the total number lost over the side is probably less than 2,000 per year. This means that less than 0.005% of the containers shipped each year end afloat in the ocean. Most dry cargo containers are steel boxes weighing between two tonnes and four tonnes, and are constructed to be weather-proof, rather than watertight. If empty they sink as a result of water ingress. If full, they may float for a while: air trapped in the cargo may hold a box on the surface until the cargo becomes waterlogged.

Sink or Swim?

This question was posed some time ago in another TT Club claims publication, and various correspondents replied with their theories.

The deadweight of conventional ships used to be calculated as the number of tons (of 2240lb) a vessel can transport of cargo, stores and bunker fuel. It is the difference between the number of tons of water a vessel displaces ‘light’ and the number of tons of water a vessel displaces when submerged to her waterline.

A similar calculation can be done for a shipping container. One correspondent provided an answer as follows. By multiplying the container length by its breadth and depth, and dividing by 80, the deadweight of the box would be established. Thus a 20’ box would have to exceed 16 tons before it sank, and a 40’ box would have to exceed 32 tons. If containers are watertight (which in our experience is rarely true!) then most 40’ containers would float.

In metric terms, a 20’ box has a volume of just over 38 cubic metres, and a 40’ box a volume of 77 cubic metres. The density of seawater is 1.025 which increases the volumes (or displacement values) to just over 39 and 79 cubic metres respectively. The forces required to push the box under the water, or to sink it, must therefore exceed the volume of water to be displaced. A 20’ box is allowed a maximum gross weight of 24,000 kg and a 40’ box a maximum of 30,480 kilos. It therefore seems that if either size of container is watertight and not overloaded (another brave assumption!) then it will float.

Containers are rarely watertight. Most have small openings and distortions. However, if 11 kg of seawater per hour entered a 20’ container, it would take some 57 days it to sink; and some 183 days for a 40’ container! These times may be considerably shortened by the in-water deterioration of seals, but this does indicate that floating containers can remain a hazard to shipping for some time.

Our information is that most containers do in fact sink. This may be due to the effects of poor maintenance, the fact that a container is a fragile object not intended to fend off a boarding sea, the initial distortions as the container breaks free, the subsequent impact with the ocean and the battering of loose cargo. Nevertheless, on this last point, one of NZ’s largest imports by volume is empty containers to bear away our primary exports.

In rough weather, boxes may be smashed up by the waves. With up to 20 tonnes of cargo moving inside, the containers soon tend to lose their structural integrity. Refrigerated boxes and tank containers create the greatest threat, because of their inherent buoyancy, but because of their high value, from $25,000 to $70,000 each it makes it worthwhile for companies to build in tracking devices. Such containers may in any case be easier to spot with the naked eye or by radar.

The Through Transport Club suspects that the humble container is being unfairly blamed for many accidents. There are many hazards floating around in the oceans, from fallen trees to other material carried to the sea by rivers, and even sleeping whales, as mariners have testified for generations.

The TT Club's Claims Department is sceptical of allegations that relatively minor damage can be caused by floating containers. Any collision with a container that was lying dead in the water was likely to do serious damage to a lightly built vessel. The potential to improve lashing and securing systems was limited. They believe that there has been constant improvement for 30 years, and there is only so much further the technology can advance. The Club is working with tracking specialist Tri-Mex International on the feasibility of monitoring systems, but at this stage it is seen as uneconomic and too big a task to tag 8m containers. Changes in ship design may help, but the latest vessels are being built with conventional stowage.

Is it happening more frequently?

Early container ships were built with their deckhouse at the forward end of the ship, with containers often in stacks two or three high. The early concept was to protect the containers from boarding seas and to aid navigational control. Whilst both these aspects remain current, as time passed, competition became keener, ships became faster, bigger and more costly. Due to arrangement and cost considerations, a design with a forward deckhouse was replaced with one utilising a substantial breakwater or just omitted. During the 1970s cargo container vessels were built to accommodate stacks six high, which is close to the level of weight compression of the lower units. Current designs are seeing vessels with up to three-quarters of their containers on deck.

The effects of such high deck loadings are numerous. Ship stability is compromised in difficult sea conditions. Bridge visibility is lessened, increasing the likelihood of collisions. Ships are difficult to manoeuvre at low speeds, again heightening collision risks. Container stacks are exposed to storm weather and seas. Lashing arrangements have reduced effectiveness. Low freeboards allow deck edge immersion at low levels of vessel heel.

Unfortunately, when a container or containers are lost overboard, there is rarely a news release and the fact is seldom publicised by the shipping company. The loss is only revealed to those in a need to know situation i.e. the shipowner, the exporter and importer, and the insurer.

The position in NZ

The Marine Committee of the NZ Insurance Council ( has been researching issues surrounding the dangers of lost shipping containers in New Zealand waters, particularly to smaller craft and modern fast passenger ferries. It is known that a significant number of lost containers in New Zealand waters are not reported. Some containers remain afloat, often below the surface, long enough to be a real hazard to shipping.

The Committee has raised this issue in 2000 with the International Marine Organisation (IMO) and with New Zealand’s Ministry of Transport, and is calling for some form of mandatory reporting.

Vero Marine acknowledges these sources:
“Door to Door” magazine of the Through Transport Club
“The Adjuster” magazine
Address of Capt. James J MacNamara, National cargo Bureau, to IUMI 2000
Lloyd's List, 25th May 2001.

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