For what it’s worth, I spoke to a friend of mine who used to “drive” an aircraft carrier (and Monty, they do call it that, with their tongue firmly in their cheek). He says this question is asked often of those in his role, and the answer (as I suppose it has to be really) is that the CG is much much lower than you’d think. The upper decks are very much less dense than the lower.
Imagine you are holding a dumbbell by the handle by just a thumb and forefinger so that it verticle, rather than horizontal. Below your fingers could be considered ballast. It balances out what is above your fingers. Now, change the center of gravity by moving your fingers farther down. The dumbbell will start tipping because there is more weight above your fingers (or sea level) than below.
Just last month a freighter here in Seattle heeled over dangerously. The ship’s computer suddenly started shifting its water ballast over to the starboard tanks and nearly everyone had to be evacuated. The only thing that kept it from going completely over was that it was at dock and ship started resting against the huge offloading cranes onshore.
It took about 6 hours for the officials to figure out what happened. The ship had run aground while traversing the Panama Canal several weeks earlier. It was inspected and deemed seaworthy. However, when it got here and the off loading process started, the shifting weight made the computer think the ship was listing and it started shifting ballast to compensate for it. Apparently, this had something to do it having run aground.
If you would like to find more about it, do a search at The Seattle Times or The seattle P-I.
::sigh:: Tikki please read and understand the thread. The fact that in ships the center of gravity is almost always above the center of buoyancy and that there’s nothing wrong with that has been discussed ad nauseam. I do not know what else I can say. A ship does not resemble any long vertical object suspended from one point, it much more resembles your refrigerator or your kitchen table. Your refrigerator is not hanging from the ceiling, it is standing on the floor. It is supported by the floor and the CG is quite above the floor. You can move things around inside it and you are shifting how much weight each leg will bear but, as long as the force acting from the CG goeos through the polygon defined by the legs, the refrigerator will stand, notwithstanding the fact that the support is below the CG. If you now open the freezer door and hang yourself from it you may well tip the refrigerator over because you have moved the CG outside the perimeter.
A ship works the same way. Look at a catamaran. There is no way in the world that its CG is below water. If you move weight to one side , that side sinks, thereby displacing more water and providing more buoyancy (support) on that side while the other side rises and provides leverage. It has nothing to do with the CG being below the CB. Nothing.
It seems I am spending most of my time here refuting this widespread misconception. The next person to pop into the thread and repeat the old canard gets a slap upside the head. Geez Louise.
Clearly if the CG is below the CB the vessel will be stable.
(I hope eveyone can agree with this)
In my mind, if the CG is above the CB, the vessel will be stable if when the CG shifts to one side, the CB shifts MORE to that side.
Brian
what keeps anything stable is its stability
for a ship it is the centre of gravity being below the centre of buoyancy
when the cg gets above there are no restoring horizontal reactions to right the vessel in sway.
pitch is another area for geometrical and dynamic resolution
i worked with naval architects and picked up the jargon but not the maths
the metacentric height seemed to figure in their calcs
the plimsoll requirements are to give freeboard in differing parts of the world in differing waters and conditions of passage to avoid swamping the decks
water on the deck is very heavy and can sink a boat regardless of its design.
especially if it gets below decks.
>> what keeps anything stable is its stability
No shit Sherlock. And where can I get me some?
>>> for a ship it is the centre of gravity being below the centre of buoyancy
ARGGGGGGHHHHHHHH!!! I give up! It is hopeless. Ignorance is winning and there is nothing we can do.
>> i worked with naval architects and picked up the jargon but not the maths
You can say that again.
>> water on the deck is very heavy and can sink a boat regardless of its design.
Um no, but never mind.
>> especially if it gets below decks.
Then is is no longer “on the deck” is it?
Anyway, don’t mind me. I’ll be in the corner weeping sadly and considering suicide. The situation is hopeless. The fight is lost. I give up.
Go look at how the interior of an aircraft carrier is layed out. The upper decks are very light and mostly hollow. They contain hangar decks, elevator channels, crew spaces, etc.
The lower decks below the waterline are more dense. The very lowest part of the ship are the heaviest. This is where the ammunition is stored, where the engines are, etc. The ship’s structure and cargo IS ballast.
sorry sailor
but i would be happy if you could correct me so i can post clearly in future and be a welcome member on the forum
from your comments i deduce you are able to do this concisely
cheers:)
Slight hijack . A couple of days ago a cargo ship ended up on its side after a collision with another ship in the English Channel. Its cargo consists of brand new BMWs , Volvos and Saabs worth 30 million pounds (45 million dollers ). Anyone got a diving suit ?
3 recent points show up now.
“Sam Stone” mentions ammunition being stored below the waterline. I mentioned this earlier, and asked what happened when this is expended.
“Mr.Crow” quotes the Plimsoll line asbeing there to give adequate clearance above the water line. Tut tut, Sir - your own history lessons ( or readings) will tell you that Mr.Plimsoll campaigned violently in the 19th century aroung the docks of britain to ensure that cargo ships were not overloaded. The Plimsoll line becaome a part of ships, in order that they be made safe.
“Rayne Man” refers to the recent “falling over” in the English Channel. This happened after a collision, when the ship obviously became suddenly unstable. This happened, I believe, because this ship is built as a car ferry, therefore has open decks, bow to stern, exactly like the ferries which sank after driving open bows into the waves.
Add a new slant to the old saw " Darling, the car won’t go, I think I have got water in the carburrator !!"
I thought sailor’s explanations and his diagrams were clear, but in case it wasn’t: the stability of a hull depends on he shape of the hull. If it is cylindrical, like a wine bottle floating on its side, the center of buoyancy (CB) is always at the center of the bottle. When you rotate the bottle a little bit, the CB is still at the center. In this case, the only way to make it stable is to add weight below the CB to lower the center of gravity (CG).
But most boats are more angular in shape. Imagine a rectangular box floating on water. When you tilt it, one edge digs deep into water while the opposite edge lifts out of the water. So most of the buoyancy comes from the lower edge of the box. This means there is a restoring force created by the shape of the box. With such a shape, you can put the CG above CB (but not too high) and it can still be stable. The extreme examples are catamarans and pontoon boats - the CG is obviously above CB, but the double-hull shape creates a huge amount of restoring force if you try to tilt it.
(I don’t claim to be better at explaining this than sailor is, but I thought it might help to hear someone else explain it. Besides I’m bored…)
i said this:
the plimsoll requirements are to give freeboard in differing parts of the world in differing waters and conditions of passage to avoid swamping the decks
when i should have said this:
The following year Samuel Plimsoll managed to persuade Parliament to amend the 1871 Merchant Shipping Act. This provided for the marking of a line on a ship’s sides which would disappear below the water line if the ship was overloaded.
winter north altlantic?