Things that go bump in the night

March 8, 2009
ken@kensblog.com
All, Mechanical Problems

There has been an active discussion, over the past few days, on the NordhavnDreamers discussion board, about shipping containers. Many of you already monitor the NordhavnDreamers board, so this is “old news”, however I am reposting the one message below, as it seems to contain some real information, and some of you may have missed it.

For those of you who have no idea what I am talking about, one of the popular boating myths is that there are thousands of shipping containers, that have fallen off boats, and are out there floating, just beneath the surface, waiting to sink ships. According to USA Today, industry experts estimate that anywhere from 2,000 to 10,000 containers fall off ships each year. Some percentage of these are really out there floating. This said, I know a lot of boaters, and have never spoken to anyone who has hit one.

The discussion on the NordhavnDreamers site quickly shifted topics to the more realistic issue of logs and floating debris. Several people had hit logs (including myself) and they do represent serious danger.

To very briefly summarize a large number of postings:

Unless the seas are totally calm, radar is very limited in its ability to spot objects floating on the surface.
Sonar might be effective for spotting objects beneath the surface, including whales, but usually isn’t. Sonar has very limited range, and unless you are moving very slow, you are unlikely to have time to take evasive active.
No one thought night vision cameras were use in avoiding collisions with floating objects, while at normal running speeds.
The most effective defense is a good watch stander.
On Sans souci I added a steel plate, at the water line, which should help if I strike something.

How long do containers float for?

The Container Owners Association was recently asked by a member for information

about the length of time that a container will float.

The dangers of floating shipping containers cast adrift or lost overboard from

ships in the ocean are often discussed in newspapers or on the television.

Instead of coming from the

container shipping community, the comments often come from the environmental

sector, yachting and pleasure-boat enthusiasts, or the fishing industry.

Many myths have grown up relating to containers floating in the sea how long

they float, how they frequently hover below the surface, how they can bring down

fishing boats and how these hazards should be disposed of.

To find out more, the Container Owners Association surveyed its membership for

further information on the subject.

How long?

On the question of the length of time that a container float will float for in

the sea – there is, of course, no single, simple, answer, as there are too many

variables. The length of time depends on the type of cargo, the type of

container and its permeability or durability. Some containers sink immediately,

while there are stories of containers floating across the Atlantic â€” in one

case, taking some 15 months to cross the Atlantic from the Caribbean to Spain.

Empty freight containers are not truly watertight under floating conditions and

will take water through vents, seals flooring and damages and sink will sink

almost straight away. Containers filled with lightweight, low density and

buoyant cargoes can float for years

even when holed and waterlogged. A reefer may float until it is broken up – so

sea and wave action is critical. A cargo of low density may float longer, until

the doors are damaged and opened by the sea action. In the case of the MSC

Napoli, which earlier this year beached on the UK’s south coast and lost a

number of containers overboard, about half of those which went into the sea sank

almost immediately, while the other 50 percent floated the one mile to the

shore.

The myth

One particular issue is that of the “sub-floater” the danger caused by a

submerged container located invisibly a few meters or more below the surface of

the ocean.

This, say COA members, is a myth: it is not physically possible that a container

can be totally immersed and freely floating some distance below the surface. The

reason is that, in accordance with Archimedes principle and fundamentals of mass

and displacement. The container can only be at surface level partly immersed, or

on the bottom. (Or in the process of moving from floating to sinking – which is

normally a rapid event anyway).

The upward thrust or force on any free floating object is equal to the weight of

the liquid it displaces. If the weight of the container and its contents is less

than the amount of water displaced it will therefore float, in a partly immersed

state.

If the weight of the container and its contents – which includes water occupying

inside space – is more than the weight of the fluid displaced, it therefore

sinks. Sinking normally rapidly gains momentum as the increasing pressure

compresses the container and contents (if compressible) thus further affecting

the mass displacement ratio and reducing the buoyancy.

The figures

In other words: like a ship, a container will sink in the water to the point

where the weight of water displaced by the container is equal to the weight of

the container. It follows that a fully loaded (MGW 30 tonne) 20ft or 40ft dry

freight container will float. This is because the volume of a 20ft container is

approximately 1300 cu ft. If it was to be fully immersed in sea water, it would

displace about 83,000 lb of water – which is more than 30 tonne (approx 66,000

lb). A fully loaded 20ft x 8ft 6in high container would therefore float with

about 1ft 6in showing above the water. Once enough water has got through the

floor, the door seals and the ventilators – to increase the total weight to more

than 83,000 lb – the 20ft container will sink, If the floor is solid and well

sealed, and if the door seals are good and there are no ventilators fitted, the

container may float for many hours. Indeed, if the cargo is watertight and it

fills the majority of the space inside the container, leaving little space for

water, the container may float indefinitely.

Disposal

One COA member recalls that it had a “floater” off France a few years ago it

was a controlled atmosphere container, loaded with pineapples, which, by its

very nature was sealed and therefore buoyant. A helicopter or fighter jet was

launched, and the container

was used as target practice, to pepper it full of holes until it sank.

Another recalls how the Navy was called out to shell some containers lost

overboard from a cargo ship in the English Channel, which were floating and

creating a danger to shipping. However, these methods are not allowed any more â€”

due to risk of pollution – and the

containers have to be towed on to the shore, which is not something the shipping

lines particularly like, because they then have the recovery and salvage costs

And, also on the NordhavnDreamers board….

A preliminary plan for a new Nordhavn model, the Nordhavn 63, was posted. I know absolutely nothing about this boat, and whether or not it is something Nordhavn is seriously considering. Rumor has it that it is based on the N55 hull, and could someday replace the N62. Very nice!

Lastly..

I just received our “invitation letters” from Russia, which allow us to apply for visas to get into Siberia. Before a visa can be issued, you must be “invited” by a travel agency to visit the country. As I mentioned a couple of weeks ago, applying for a visa isn’t easy. We can only apply up to 90 days prior to our visit, and we’ll be at sea at that time. Our current plan is to solve this by sending our passports, and the invitation letters, from Ketchikan Alaska to a visa-processing agency, then having them sent back to Kodiak Alaska, where we will be a month later.

-Ken W

Post Views: 1,207

One Response

Adam says:

March 9, 2009 at 1:24 am

Ken:

Thanks for the LED information. Jeff Merrill told us about the N63 last weekend and sent me plans a couple days ago. Apparently it’s 60′ LWL but as you know they already have an N60 and an N62 and so came up with a new name for the aft wheelhouse boat. My impression is that they definitely plan to produce it, but I don’t know the status of the molds for the superstructure (the hull mold is apparently the same as the N55/N60).

Things that go bump in the night

One Response

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