tomndebb, thanks for your links which are extremely interesting and show in great detail the structure of the ship and the construction process. I was particularly interested in the bulb keel. I am always marveled when one of these giants goes by my small boat and makes less waves than a small speedboat. Their lines are very efficient but I really do not understand why the bulb helps with that. Can you explain it?
The propeller, bow thruster and general structure are also interesting to see.
Not a container ship, but a cargo ship, the Stellamare recently tipped over around here. Scroll down for pictures.
They haven’t determined the cause, though it did have a 500-ton turbine aboard. It was docked at the time and weather wasn’t a factor.
So ships can tip over
I am not fluent in the theory of the bulbous bow, but I recall that it has to do with the dynamics of the bow wave. There is a lot of friction and drag associated with the bow wave, and extending the hull beneath the water ahead of the stem causes the sub-surface waves to actually interfere with the surface waves generated at the stem, leading to less drag. I do know that it was an almost accidental discovery. The earliest steam warships were built with a “ram” bow that extended beneath the water. When they recognized that no ship with a 10" gun was actually going to ram its opponent and changed back to a raked stem as found on sailing ships, they discovered that the ships’ speed decreased. (I do not know that they actually built any “slow” raked stem ships. I believe they discovered the advantages on the extended sub-surface hull in test tanks.) Some time in the 1950s, they began experimenting with designs that really extended the sub-surface hull and, together with information garnered from submarines similar to the USS Skipjack they came to the bulb design.
As to actual hull design, I get way out of my league pretty fast. The ships I sailed (lake freighters) had specific constraints regarding getting as much ship into as short a lock at the Soo as possible, in floating as much cargo in a river dredged to 28 feet as possible, and in bulldozing its way through pack ice in the Spring, that caused them to deviate from ocean hulls quite a bit.
If the ship is riding completely empty, the ballast tanks should be more than adequate to keep it stable. (I don’t know that any container ships actually go anywhere completely empty.) The containers, themselves, weigh one to two tons, empty. The ship to which I linked earlier has over 5,700 cu meters of ballast tank which is about 1/3 of the rated cargo capacity (17,500 tons). That is a significant amount of mass to be carrying around below the waterline (and should reduce the freeboard, substantially. If it is also carrying 1,174 containers (which it would need to do in order to get any significant freeboard interacting with the wind) the empty containers plus ballast can provide a weight of almost 7,000 tons. To the extent that you reduce the number of empty containers (lightening the ship) you also reduce the freeboard.
Some pretty impressive photos in that thread RealityChuck. It would be interesting to know what they determine to be the cause of that. Just by looking atthem for the front and seeing the immense weight up high and the narrow waterline you would get the impression all ships should do that.
When they launched the replica of Columbus’ flagship Santa Maria for the 5th centenary commemoration in 1992 the tug pulled the bow sideways, instead of straight ahed, and the thing fell over and sank just like that. Quite an embarrassing moment.
This page seems to say the containers are stacked from the bottom of the hold up with no deck.
I sailed on several American President Lines C6 container ships on 28 day turnaround voyages between Seattle, Taiwan and Japan.
The normal route is up through the Gulf of Alaska and depending on the time of year as far north as allowed. Summer time meant passing thru the Unamak Pass in the Aleutians, winter would be several degrees farther south.
A normal trip would have us in Seattle for 2+ days, 11 days to Keelung, Taiwan. Maybe 36 hours there and then another day’s run up to Kobe. We usually had an overnight in Kobe and then sailed to Yokohama. Sometimes the stay there was only for 4-6 hours depending on cargo. Then 9 days back to Seattle.
We used ballast mainly for trim, there was always a full load of containers going in both directions so we drew plenty of water. In winter it was very common to run into heavy seas. In fact there were days at a time when no one was allowed on deck. You went fore or aft thru tunnels that ran inside the hull. We used to lose containers off the deck all the time.
One memorable trip in January we got the crap knocked out of us. The skipper was so adamant on keeping on the 28 day schedule (a big selling point for freight customers) that he refused to slow down even in very heavy seas. Of course a ship traveling at 20+ knots ramming thru 30’ seas really starts to take its toll on both the vessel and the crew. First we lost some vans that were stacked just forward of the house. Next a couple of vans aft went over the side and one stack collapsed into the space they created. These banged around for a couple of hours until they finally went over the side. Of course the noise of them banging around didn’t allow anyone off watch to get any sleep which was hard enough as it was.
Finally after a huge wave tore off the starboard gangway, the Chief Engineer convinced the skipper to slow down a few knots. We weren’t in any danger but the ship was really getting beat up.
When we came into Seattle they found a 20’ container wedged between stacks back aft and they had a helluva time removing it so they could work those hatches. All in all it cost the company big time, the gangway alone was about $60,000, plus the missing containers.
I was given a tour thru a Sea-Land container ship being built in Sturgeon Bay, Wisconsin, about 15 years ago. As we toured, they were pumping high-density concrete into the bottom of the hold to solidify for permanent ballast.
Interestingly, Sailor, not only can container vessels maintain stability, they sometimes have the opposite problem, namely that of being “too stiff” ie too stable, resulting in a righting motion in a seaway that is too jerky.
This (and not the big seas in themselves) is often what results in the collapse of container stows, with multiple containers over the side.
If the vessel is too stiff, the accelerations at the top of the stack as the vessel rolls can be huge. So the trick is to manipulate container weights and ballast to get a nice slow roll without compromising stability to the extent that the nice slow roll goes right over!
When there is container damage or damage within containers, and the shipowners are denying liability on the basis of a “heavy weather” defence, one of the first things a savvy cargo interests’ lawyer will demand is stability records, to see if he/she can find evidence that the vessel was too stiff.
As others have said, no. Containers are weatherproof but not waterproof. Occasionally someone does something silly or a line bursts or something rusts through and a container vessel’s hold gets flooded. When this occurs, there is mucho damage, the accusations fly and with any luck, Princhester gets some work! 
There are hatchless container vessels, but I have never entirely understood how they work, why they don’t sink in storms or how they get a Load Line Certificate. I think basically their “deck” for LL purposes is at the bottom of the container stack, and the walls that come up and keep the seas out (one hopes) are effectively coamings. Or something. Must ask Stuart Ballantyne about this next I see him.
and
They pretty much are hatches as you think of hatches, they are just huge. And it is difficult to keep them watertight, but they pretty much are. Much maintenance etc goes into achieving watertightness.
And a gutter just inside that seal, leading to the bilges, to catch anything that does get through.
They will keep out even a little green water, depending on just how much, how old the ship is, whether the hatch collapses entirely (it happens), whether the ship is still straight (some develop slight twists and bends with age) etc.
It’s hard to get an idea of the scale until you see a ship up close, but the coamings are about maybe 6-8 feet high, the hatches often similar. So they are pretty damn strong. And they are dogged down hydraulically, usually.
and
As you say, Sailor, the principle of General Average still applies.
I think you will find, though, KPM that the principle and the word go back much further than the French. There is evidence that the Pheonicans practiced this principle, and that the word derives from an Egyptian word meaning damage to cargo, which then was conflated with a French word for transportation of goods.
These days, jettisoning cargo is essentially unheard of. However, General Average is still declared by shipowners where there is a casualty. The cost of steps taken to save the ship and cargo are shared between all those with a financial interest. Those costs could include the cost of putting in to a port of refuge or of hiring salvors.
The concept is pretty outdated of course, with shipping being nowadays a pretty routine enterprise and not the risky undertaking it was when the principle was first devised.
Yes to both. The tanks under the cargo holds are called the double bottom tanks and lie between the outer hull and the floor of the holds on which the containers are stacked. Indeed that floor is not called the floor of the hold, it is called the tank top.
(I’m sure you’re familiar with all this, Chandeleur, since the terminology is just the same on bulkers)
There is software that crunches the possibilities to work out how best to stack the containers so that the number of container movements is kept to a minimum. The software has to take into account not only container movement efficiency but also:
[ul]- hazardous materials (which have to be stowed on deck),
weight (trying to keep heavy containers to the lower stacks),
trim (keeping approximate side to side balance so as to minimise ballast usage)
temperature sensitive cargo (it’s cooler below deck)[/ul]
and probably a whole load of other factors I’m not aware of.
Freecell, anyone?
Yes they are completely emptied, but very very rarely. Usually only on the way to/from a shipyard for special survey (every 5 years) or when they have suffered damage. Shipowners loathe and resent every single minute that their ships are not moving cargo, but sometimes they have to grit their teeth…
As you say, Tomndebb, the double bottoms do have sufficient capacity to maintain stability even when the vessel is empty, but if a vessel is in that condition and is facing a storm, they can sometimes fill one or more specially strengthened holds for additional ballast. This is exceedingly rare, though.
Finally, while you have all done a great job of answering Sailor’s question, no one has yet posed or answered the interesting but at least to me obvious corollary question, namely "why the heck do they build ridiculous ships that put as many or more containers on deck as in the holds, instead of just building a ship with higher sides, thereby avoiding container collapses etc?
The reason is not, as you might think, anything to do with naval architecture or safety or whathaveyou. The reason is taxes.
Most ports charge dues and taxes on the size of the vessel, based on its GRT (Gross Registered Tonnage), which is a measurement of enclosed volume. So smart shipowners figured out that if they built ships that had comparatively low below decks capacity, but high capacity on decks, they would pay smaller dues and taxes.
This situation is clearly a totally ridiculous tradition but the problems is that getting all the countries around the world to change the basis on which they tax is rather tricky.
Some pretty impressive posts in this thread. Wow.
Hey, great info Princhester, thanks mate!