About a year ago we had a thread asking Why don’t ships fall over where you can see that I do understand the physics and the chemistry of the matter.
But I was looking at some Container Ships and they have cargo containers stacked up on deck pretty high up. My question is: what is below decks? It seems all the containers have to go on deck but there would be a lot of wasted space below and, in any case, I doubt the ship could sail with such a high center of gravity. What do they put belowdecks? Ballast? Other types of cargo? (I doubt this)
That page does not say container ships fill their holds belowdecks with water. It just talks about ballast water generally used when ships sail in ballast, not when fully loaded.
>> I looked at the pictures, and one thing I’m not sure of is whether there is a deck at all. Maybe they just stack the containers all the way down
They have decks which they need for several reasons not the least of which is to keep water out.
I would think they have some form of stowing containers below decks like the one linked by don’t ask. Anything else seems terribly wasteful.
I sailed on several container ships for American President Lines. They carry containers in the holds also. The ones on deck are stacked on top of the hatch covers and between the hatches and rail. Ballast tanks are mainly used to trim ship plus the engine room and fuel oil storage tanks add ballast. The containers get loaded and unloaded so quickly we were constantly trimming ship, either filling or emptying the ballast tanks. We rarely had to take on ballast for a voyage.
Based on CBEscapee’s post they are containers “all the way down.” However, considered as a box beam, a hull might not be strong enough to resist the loads resulting from ocean waves without an almost complete deck structure.
I can’t guarantee that this is typical, but on the 1200TEU Container Ship, they give the loading for 20’ containers as 470 containers in the holds and 704 on deck and, for 40’ containers, 221 in holds and 351 on deck. This means that more than 1/3 of the containers are below deck. Adding the weight of the hull, the machinery (propulsion, bow thruster, steering gear), the fuel and stores, it is quite possible that the center of gravity is much lower than it appears.
The TEU (20’ unit) generally has a maximum weight of around 26 tons. The Deadweight is rated at 17,250 tons. Given a maximum capacity of 1,174 TEUs, (based on the limit to how high they can stack the containers before they interfere with navigational visibility), they wind up with almost 31,000 tons, so it is pretty clear that they generally operate with a number of empty or lightly loaded containers. It is also clear that we are not going to find a simple extrapolation to the amount of weight that is typically carried above or below decks.
Larger ships are inherently heavier and generally have heavier machinery. The 12000 TEU ship to which I linked also mentions having a weather-reinforced hull which would add weight.
So they have big hatches which allow the containers down below. But how can the move the containers once below decks? They would need to be moved, and may be stacked.
Below the hatch covers, there are walls or bulkheads which extend across the width or beam of the vessel every forty feet. That is because a standard intermodal container is forty feet long. A TEU is a twenty foot container so a forty contains two TEU’s. Imagine now that you have what is essentially a series of boxes the entire length of the hull. There are guides or “cells”, basically rails attached to the bulkheads and which extend vertically from the keel to the top of the hatch. These are placed every eight feet or so, the width of a container, the entire width of the ship. Modern vessels are at least 14 cells wide, tapering towards the keel.
Harbor cranes have a large framework, called a bridle, which has the dimensions of the top of a container. It has locks on each corner to attach to the container. This is used to lower the containers into their cells like you would load an ammunition clip. They are unloaded the same way.I think that containers can be stacked a maximum of nine deep in the hold of modern vessels. This bridle is also used to move the hatch covers.
So when you see a cargo ship with containers five high on deck, you know that the cargo extends far below the waterline.
galen, that makes a lot of sense. The way I understand it though the hatches are not hatches in the traditional sense but really cover most of the deck so, in reality, you are removing sections of the deck. It must be pretty difficult to keep the deck water tight.
I got to take a class once on (very basic) container ship construction, loading and layout, from a HAZMAT point of view. Loading HAZMAT on these ships seems to be an incredibly complicated ordeal - all done with computers nowadays. Anyway, when searching for some pics, I came across some good images:
Wonderful! Thanks! That is exactly the information I was looking for. I see only a fraction of containers is carried above decks and it makes sense.
It seems the hatch/deck is less than watertight but I imagine with containerized cargo some water getting below is not a big concern and they just have large bilge pumps. The containers have to be tight anyway as they could go on deck.
Such a large ship would probably not get much water on deck from waves anyway. I am reminded of the shourt dociumentary of the huge Sailship Pekin rounding Cape Horn about 1930 and in spite of its size the huge waves come over the decks even taking a seaman away. The ship rolls and shakes the water off only to have another huge wave wash the decks. These ships had to be watertight as they had no powered pumps and the bilges had to be pumped by hand. Still, the crews had to pump almost continually.
It seems to me the only way to carry substantial ballast would be in containers as I doubt they would flood the compartments where the containers are carried. Or do they?
I find this fascinatingly interesting. It is amazing how containerization of cargo has changed the shipping industry over the last 50 years. Now a ship with a smaller crew can carry more cargo than 20 ships could before and can be loaded and unloaded incredibly fast. Compare this with stories of small coastal schooners carrying loads of coal or stone or other loose cargo. They did not carry more than the equivalent of a couple of modern containers and yet they would take several days of hard labor for a crew of men to load/unload in small baskets raised from the hold with tackle, then walked over a gangplanck over the side. I cannot help thinking how much easier life has become for us and how hard our ancestors had it.
100 or more years ago when ships were more likely to sink the people who had the cargo high up on the ship were more at risk. If the ship ran into trouble they would start throwing the top cargo overboard to help keep the ship afloat.
If the ship didn’t sink the owners of the other cargo would have to chip in and pay the owners of the cargo that was lost for their losses. The system was called general average. The reason they used the word average was it comes from the French for "damaged at sea " This is also one of the ways the insurance industry got started - to insure ship cargo.
kpm, what you are talking about are general principles of admiralty law which are still in force AFAIK allthough they probably don’t come up as often in court due to much greater safety and different practices in shipping and insurance. But AFAIK they are still in force. It makes sense that if part of the cargo was thrown overboard to save the rest, all the shippers would share the cost of the lost cargo.
Today this would be a very unlikely situation to happen but the principle stands. What does happen now with relative frequency is that a container or two or ten will be lost overboard. But this was not done on purpose to save the ship so it is a different situation.
Floating containers are a hazard to navigation and I know of one small boat sailing across the pacific which almost hit one. They then sent a couple of guys in the dinghy to open the container so it would sink and they managed to do it.
I remember a story about some containers filled with tens of thousands of Nike shoes which fell overboard and the shoes were washing up on the pacific beaches for months and allowed some interesting study of ocean currents.
sailor, to my knowledge, there are no provisions that allow for water intrusion into ships. That is to say, the hatches have to be watertight. The hard answers lie in the SOLAS (any) and LOAD LINE 66 conventions, neither of which I have here at the house. (I have them at work, but I’ll be out of town for 2 weeks starting tomorrow) I deal almost exclusively with tankships and dry bulk freighters, but I can’t imagine any special provisions for cellular container ships.
Hatch cover integrity, as I’m sure you know, were very closely scrutinized in the Edmund Fitzgerald and Marine Electric cases. (A great book on the latter, is Until the Sea Shall Free Them, which I’m pretty sure you’ll enjoy a lot, knowing what I know of your interests.)
As for ballast, they’ll have ballast tanks for that, and won’t flood the cargo. The general run of the mill dry bulk container is not water tight. They pretty much get treated like rental cars, and are often damaged. The sides of the container ships most likely have wing ballast tanks, and perhaps even some underneath the holds. CBEscapee, any input?
Yeah - imagine those 1100’ Maersk ships that carry some 6,600 containers! It’s amazing that they can load/offload those things in such a way that allows them smooth port calls where only certain containers come off. Are they ever completely offloaded? Plus, a crew of what? 16? 18? :eek:
I know that some ports (Haiti, for example) still do things the old fashion way.
Actually, while they hardly approach the quality of watertightness that a submarine requires, most of those hatches are watertight against anything short of green water over the deck. The hatch overlaps the coaming by several inches and they usually have a rubber or neoprene seal on the lips.
(Water) ballast is carried in tanks between the deck of the hold and the hull and/or between the bulkheads of the holds and the hull. It needs to be contained in tanks, because it would be lethal to the ship to allow water to slosh back and forth in high seas. The holds are not flooded. (In Chandeleur’s Another view link, the ballast tanks will be located in the grey-shaded regions between the cargo and the sea.)
Seems difficult to do with the types in your link but I suppose It can be done.
The Edmund Fitzgerald was 729 feet long, 75 feet wide, 39 feet deep. She was the largest Great Lakes steamer when launched in 1958. She went down in Lake Superior on November 10, 1975 with 29 men on board. Winds were 40 to 45 knots with waves to 20 ft. Although there is no conclusive evidence pointing to what the cause was, the most common theory is that because she was taking on water, the taconite cargo shifted toward the bow making it heavy at the bows. When the Fitz plunged into the valley between two large waves, she submarined to the bottom, striking the lake’s floor with enough force to break her in two.
I had a mental note that she might have struck the bottom and this lead to structural damage but I have no details now. In any case, yes, I suppose the hatches were suspect as may have contributed to the sinking. Still, I am used to the old type hatches with coamings and find it difficult to imagine a section of deck rolling up like a blind.
I am not familiar with this case. thanks for the lead. I shall look into it.
According to SAIL magazine, the occasional container falls off in a storm. We’re not talking intentional jettisoning of cargo, just stuff that slides off during the pitching around. Anyway, the heavy containers go right to the bottom, but the lighter ones float, sometimes just below the surface. To a sailboat, it’s like a steel iceberg. There are hundreds, perhaps thousands of these things floating around the world’s seas, and no one is tracking them. The odds of hitting one are really, really tiny; but it does happen.
