A terribly small nitpick as we’re both making the same point that aircraft weight is not a big portion of a carrier’s load but the F-16 is very light as combat aircaft go so may not be a good example. A typical bomb load on an A-6 intruder could be up to 7.5 tons, not far from the dry weight of an F-16. An F-16 at max takeoff weight plus an empty F-16 is about the same as an F-18 at max weight. Carrier aircraft are consted much more heavily than land based planes becaus of the abuse they take. Look at the main landing gear struts on an F-14 or A-6 compared to an F-16 or F15 and you’ll see tree trunks and tinker toys.
Aircraft do sit around fueled as that’s often the first thing that happens when they get spotted after landing operations. Doesn’t effect the net weight of the ship but does put more weight up on the flight deck than down in the hull.
Um… F-16’s don’t go on carriers. The D model are 19,421 pounds empty. An F-14 is longer and wider than the Falcon, with an empty weight of 40,100 pounds.
Good point UB. Actually I looked at some other specs, for the newer F-18E/F models that change my earlier post. Take a fully loaded F-16, stack an empty F-16 on top, add ten thousand pounds of bombs and you’ll have the max weight of an F-18E/F
(releved sigh) Nobody seems to have noticed that this statement violates Archimedes’ principle. I will try to come up with an illustration somewhere that shows what I really meant to say.
I think it’s just a difficult thing to state but it doesn’t violate out pal Archimedes and his principle. The displaced volume is the same as with the hull neutral but the buoyant force of the down side is a farther average distance from the center of mass as the volume on the down side. There is no net change in upward buoyant force but the angular force on each side is different.
It’s easier to visualize it with a dual outrigger design. Heel the boat over with one outrigger rising out of the water and the other going deeper and the net displacement stays the same but the bouyant force on the low side outrigger acts as a torque to right the boat.
Understanding why and how ships do or do not fall over is much more complex than what first meets the eye. Righting forces on ships and boats are a huge area of study and every design is a compromise among many factors. As has been said, some of the righting force comes just from the shape of the hull. A flat tray will float on water and have good stability because of its width.
Then some of the stability comes from having the center of gravity as low as possible and this is done by adding ballast below and building light above. A ship that is wide has lots of initial stability. This means it is difficult to start heeling. But if the ship continues to heel, very soon the righting force becomes negative and tends to capsize rather than right the ship. A good example of this is a catamaran. Difficult to start heeling but once it goes over a certain point it will roll over an stay that way. The joke among sailors is that the good thing about catamarans is that, when they capsize, the mast always points to the nearest land.
OTOH, a ship which is narrow beamed and has ballast has much less initial stability but much more reserve stability when heeled. A sailboat with a narrow beam and deep keel can have a positive righting force up to 145 degrees of heel. Of course, if you get your mast in the water you got other problems because the boat may right itself but you’ve lost your mast.
Also righting forces and weight distribution Affect the way the ship rolls and this is vital also. Very strong righting forces and weight concentrated near the center of gravity make for fast rolling ships. Weaker righting forces and mass located furter away from the CG make for slower rolls.
Of course, we have all heard about the Mary Rose. There is another ship which also capsized due to bad design. HMS Captain capsized in a gale off the coast of Finisterre in the late 19th century (near to Coast of Death, so called due to all the shipwrecks, where the Oil tanker Prestige broke up recently) with great loss of life. The investigation which followed determined the faulty design of the ship was responsible. But this cannot be proven with a single number and I have an entire table of numbers comparing characteristics of HMS Captain with the same characteristics of HMS Monarch. While some numbers are in the same range, others are totally different. For instance: reserve of dynamical stability at an angle of 14 in dynamical foot-tons: 6500/410.
But getting back to the OP, I would definitely be very curious to see some cross-sectional plans of a carrier and some explanation about its ballast and a study of its stability.
I am not an “Engineer”, but recently completed a navy career in an engineering rating. I can’t give specifics about Carrier ballast systems but can give some info on LHD class amphibious assault ships.
Water is the primary ballast agent for every ship I have been assigned to, including a carrier, two frigates, a destroyer tender and the above mentioned LHD. Fuel, fresh water and other liquids are taken into account when determining a ships “liquid load”, but sea water is the primary agent.
The ballast procedure for an LHD and other amphibious ships is somewhat different than others do to the nature of their mission. They are required to rapidly ballast and deballast the ship in order to launch landing craft. The LHD class has a light displacement of around 28,000 tons but can rapidly take on or remove 15,000 tons of seawater to change the ships draft. IIRC the draft can be changed by 15ft with this procedure. The ballast/deballast system is also used to correct list and trim.
To see the console used to monitor and control the ballast/deballast system see this link.
Here is a more simplified illustration of the righting momentum caused by the hull’s form (see the link given above by sailor for a much more detailed explanation.)
Note that, as is the case in that link, the center of gravity can be above the center of buoyancy and still have a positive righting force. There seems to be the misconception that the CG needs to be below the CB.
A misconception among whom? The CG has to be above the CB, otherwise there wouldn’t be a restoring moment. Look at the diagram tschild posted, but imagine the positions reversed. The resulting moment would continue to heel the ship to the right instead of back to equilibrium.
And, I am an “Engineer”, FWIW…
This reminds me of what happened to a British ship during the Falklands War. Over the years, the superstructures (the top part of the ship) of ships have gotten heavier with the addition of all kinds of high-tech gear. The British Navy decided to compensate for this by having the superstructure of the ship made out of aluminum. Things went well until the ship got hit by an Exocet missile. Aluminum has the nasty habit of burning if it gets hot enough. The ship burned down to the waterline.
OK, somebody needs to explain to me how water can be used as ballast. Water is water, right? So the water-filled areas of the hull are going to have the same density as the outside water. So they don’t displace water and don’t keep the ship up, but they aren’t heavier than water either, so why would they sink? Can anyone understand the question I’m trying to ask? How can you have ballast that’s the same density as water?
The point of using water and ballast tanks is to manipulate Archimedes’s principle. With more water (which certainly has weight) inside te hull, the ratio of the water displaced by the hull to the overall weight of the vessel changes.
Put a plastic cup in a sink full of water. It will fll over.
Fill the same cup about 1/3 - 1/2 full and place it in the sink, again. It will tip, but it will float.
Since the cup is not porous, the water in the cup adds mass (and hence weight) to the cup. With greater mass for the “hull” of the cup to support, the volume of the water it displaces changes. Add ing water adds stability.
Without this aspect of Archimedes’s principle, submarines could never dive. They operate on a principle of flooding their ballast tanks to change their displacement to the point where their buoyancy fails to keep them above the surface of the water.
Actually, it works better with a beer bottle. A plastic cup is so light that it needs to be filled/ballasted nearly to the brim before it stops capsizing.
The CG has to be above the CB, otherwise there wouldn’t be a restoring moment. ??? You might want to think again and then amend your response.
Everything else being equal if the CG is below the CB then the object is more stable. Remember the CG does not move but the CB moves as the object tilts. Think of a glass cylindrical bottle of the type used for wine full of air. The CG is below the CB and there is no way you can make it float in any other position than upright. Add a bit of solid cement to the bottom and it will become even more stable as you have further lowered the CG. An object with the CG below the CB is inherently stable contrary to what you say. The existance of a positve righting force does NOT depend on the shape of the object. This is so intuitive and obvious to most people that I do not understand how you can make such a blunder.
Now, an object with the CG above the CB will only right itself if the CB moves to the same side the object is tilting to. The CB moves as the object is tilted. If the CG is below the CB then it does not matter in what direction the CB moves, but if the CG is above the CB then the object is only conditionally stable with the condition that the CB move towards the same side the object is being tilted. The existence of a positive righting force depends on the shape of the hull. The simplest of drawings will show this.
An object with CG below the CB can be stable throw the complete 180 degrees of tilt. In an object with CG above the CB the righting force may be positive at first but will eventually become negative and tend to upset rather than right. An object with lower CG is inherently more stable.
>> And, I am an “Engineer”, FWIW…
In this case to show “Engineers” can make the most obvious of blunders which would be obvious to a high schooler.
And to those still saying they do not understand how water can work as ballast, please read the links. There is one very specifically dedicated to explaining that very issue.
I still don’t quite get it. I looked at that diagram and it seems that the restoring torque is created entirely by the horizontal shift of the CB. The direction of the torque would be the same regardless of the height of CG, wouldn’t it?
