ships falling over

Factual Questions
1

I am aware that sailing ships and boats sometimes capsized in extreme conditions. Also, modern powered craft are susceptible to extreme weather.

What I want to know is this :- modern powered ships, some of them huge vessels eg, aircraft carriers, cruise liners, warships have colossal top hampers in relation to their below-water profile.

Aircraft carriers in particular are very slim at the water line, and very broad at the top, and are obviously top heavy.

Why do they not fall over in general operation? What keeps them upright?

2

Ballast.

3

You gotta love landlubbers. “Fall over”? I gotta admit it’s pretty descriptive.

I have to admit if you look at a carrier head-on it seems like an impossible thing that it would have a positive righting force. And the bow section by itself probably doesn’t. It is the middle section which is much wider and provides it. This and ballast. Both things.

Pleasure sailboats use lead, iron, concrete and such things for ballast. I wonder what a plane carrier uses. It would be really interesting for me to see the diagrams showing the sections around the center of the ship and how ballast is stowed.

4

Isa.Gottanow, welcome to the Boards, I just saw it is your first post.

For more photos of carriers go to http://www.chinfo.navy.mil/navpalib/images/image-cv17.html

5

I don’t know for certain but I’d wager the ballast is in the form of water. The ship pumps it in and out and moves it around to different tanks to maintain an ideal trim. For instance, if the carrier has all of it’s planes removed for some reason it’s going to ride higher in the water…they probably pump more ballast in to compensate. When the planes return out goes the water.

If you ever see an oil tanker that is empty it is amazing how high it rides out of the water. Once filled it’s amazing how low it is.

6

The weight of the planes is probably tiny compared to the ship. I will leave it to the reader to search for actual numbers.

Water ballast in a plane carrier is probably not very useful. Racing sailboats use water ballast but it only works because the boat is heeled over (“tipped over and looking like it might fall over” for landlubbers). Tankers are designed to float at a certain waterline and need to take in ballast water when they are not carrying cargo. But in a ship with zero heel water ballast provides no righting force. Water ballast needs some serious heel to provide some useful righting force. I doubt carriers would use water. My guess is they would use something much heavier and probably their shape provides more righting force than may seem at first sight.

7

Nope, Carriers use good ol’ water, just like submarines do.

I remember one year the USS Geo. Washington (CVN-73) was tied up to the “wet piers” at Newport News Shipyards. That March we had a Nor’easter that was stronger than any in a while. The winds were in excess of 55+ knots sustained, and the ship was broadside to the wind. Since we weren’t fully operational (being in the shipyards), we couldn’t put out to sea. The winds were strong enough to blow the ship to a 4 or 5 degree list - that was only being limited by the mooring lines, and the mooring lines were being strained to their breaking point. In order to keep the lines from snapping, they had to fill all the port ballast tanks and empty all the starboard ballast tanks just to keep the ship at that 4 degree list.

Once the storm subsided, they blew down all the ballast tanks to regain a proper attitude.

Also, as noted above, the below waterline profile is quite different than is usually portrayed in cartoons/casual drawings.
The keel of a carrier kinda looks like it’s had a submarine welded to the bottom. That is, it is a cylinder-shape with rounded end caps. This promotes (doesn’t guarantee it, just makes it more likely to happen), for lack of better technical knowledge, the “weebles wobble but they don’t fall down” set of conditions.
critter42

8

I don’t know that they use water but carriers certainly do heel. They are able to turn hard enough to cause a significant tilt in the deck. Obviously they try to avoid that since people and equipment can roll off. In fact, I think the ship has a maximum heel that the drivers need to be aware of as the ship could actually heel even further if they really cranked the wheel and accelerated through a turn.

You may be right that the planes alone may not be enough by themselves to cause a significant rise in the ship…certainly not during normal operations. Still, if you removed all planes, ordinance and jet fuel I would guess the carrier might rise noticeably out of the water.

I have no doubt the carrier probably has some permanent ballast built in but I would think they would still want the ability to add or subtract ballast which would likely be done by pumping seawater in and out. Also, aside from tipping I would think they might also want to be able to move ballast forward and back in the ship to maintain an ideal trim. Just a WAG though.

9

The main righting influence in large ships is not ballast weight but the shape of the hull’s cross section.

Put simply, for small angles of heel the hull volume pressed below the waterline on the “down” side is larger than the hull volume lifted above the waterline on the “up side”. This produces a righting moment that counteracts the heel.

10

:smack:

After I posted this, I figured out what I was trying to convey in the last paragraph…The keel is designed to be naturally-self righting, and the shape makes it act like a pendulum in the water.

And, as usual, if there’s a marine engineer that can provide corrections/amplifications to my admittedly dim knowledge, please post :slight_smile:

critter42

11

Yes, the scar on my back will support this statement quite well.

One of the maneuvers performed during a carrier’s shakedown cruise is a high-speed turn. The carrier gets up to near it’s maximum speed (which, as all the books say, is in excess of 30 knots…WAY in excess of 30 knots), then turn the ship hard to port. This is an emergency procedure, but everyone on the ship is given plenty of notice during testing so everything not nailed down can be stowed/locked/strapped/closed. One of the copier machines didn’t have its strap attached properly and during the turn came loose and slid down the deck and damn near ran me over, slicing my back open in the process…

The rooster tail created by a turn like this can (and does) go higher than the flight deck.

critter42

12

Aircraft carriers also have the supply of jet fuel to use as ballast in addition to using water as ballast.

13

p.s. Ships don’t have drivers, they have helmsmen.

14

Thank you folks. I am not too sure about the whole balast thing though. Certainly, cargo ships run ‘in ballast’ when not actually carrying cargo. I guess all ships must have some ballast,but surely the whole purpose of a warship is to deliver munitions of some sort.

On a carrier, space taken up by tons of water or tons of lead would deny that space to storage of munitions etc.

I had alway fancied the idea that ships do not fall over ( or heel/capsize) because of some hyraulic effort exerted by the hull and keel in the water.

But then, I am a mere landlubber, but I have been in some hairy situations where I was praying that some force would continue to keep the boat upright - despite the actions of the skipper.

15

[technical answer]
What keeps them upright is positive intact stability, meaning that the buoyancy of the ship will return it to its equilibrium position after a displacement. Physically, this means that the center of gravity (the point at which all the weights act downwards) is above the center of buoyancy (the point at which all the forces of the displaced water act upwards). So when designing a ship, you want it to be partially top-heavy to maximize what is called the “righting moment,” that forces that returns the ship to equilibrium. However, if the VCG (vertical CoG) is too high, the positive moment becomes a negative one that tends to capsize the ship rather than return it upwards.
[/technical answer]
Depending on ship type and classification, some can heel 60 degrees or more before capsizing. Some can also automatically self-right, even when completely inverted.

16

In fact it is possible for a ship to be top heavy and still not fall over, it takes a certain set of circumstances to tip a ship.

If a ship is vertical, then it can be as top heavy as it likes and stay upright, but the more it is allowed to heel over, the more likely it will turn turtle.

Ships are designed to take a certain amount of heel, the greater allowed heel, then the less top heavy the ship can be made.

There are tilt tests carried out on all ships, where the centre of mass along a line axis donw the ship is compared to the point around which the ship revolves along a line down the ship when heeling.
Usually there is a vertical differance in height between them and the rotation point is higher than the centre of mass.The greater this distance then the more stable the vessel is, but it can change under all sorts of conditions.

Things that would change this are, amounts of fuel, and freshwater on board, cargo loading, and on a warship even the weight of the crew can be significant, since warships are verymuch more heavily manned than cargo vessels.

It is not just the weight itself that is important, its the weight distribution, for example, a trawler may well be very stable in port, or at sea in normal conditions, but when the booms are out, and the superstructure ices up, such as might happen off Greenland, things can become extremely dangerous.

On some ships there is a measuring device, which looks a bit like a manometer, or sometimes its just a moving pointer attatched to stuff and this displays the the stablity of that ship at at any given moment. I don’t know exactly how it works, but it is based upon the way the ship moves about an axis along the length.

All ships are supposed to be inspected for stablilty issues, a ship that has been designed to be stable may unintentionally become unstable through changes in use, where differant equipment is fitted. This happened to Mary Rose, Henry VIII flagship.

She was stable when built, but the addition of extra guns, plus the extra crew complement to man them, and the extra crew that was put on as a boarding party(which was on the main deck at the time - ie relatively high up) and where she turned water flowed into the lower gun ports which were already closer to the water line due to overloading, and over she went, in just a few seconds, taking virtually all her crew with her.

17

Just a quick mental calculation but all the aircaft on a modern carrier might weigh in the ballpark of 3,000 tons at full gross takeoff weight. That’s a very rough estimate but a Nimitz class carrier displaces about 95,000 tons with the ships I was on, the Constellation and Ranger, weiging in a little less. Unloading all the planes isn’t on the same scale as emptying an oil tanker but if all the planes are parked on one side of the flight deck will require a little trim to keep the deck level. The widest part of the flight deck can be more than twice the beam of the hull.

Looking at the ouside it’s hard to imagine a carrier being stable but that’s only because you don’t see the heavy stuff in the hull. IIRC the flight deck on the Connie was 65 feet above the waterline and she drafted 38 or 40 feet in the water. I wouldn’t say the hull is cylindrical or looks like any sub I’ve seen. There is a very wide flat bottom with a reverse teardrop shape. The point is at the vertical secion of the bow and the stern section is soped where the screws and rudders are.

Sounds like a fun ride Critter. We often did greater than 30 knots on the Connie, though without heavery maneuvering, which makes me wonder what the fastest carriers can do. Never saw a roostertail though. I suppose you’d only get that with the engines at flank and the rudders hard over.

18

Come on now. Carriers are the most crew heavy ship there are with 5,000-6,000 swingin’ ****s (they were just starting to put women on carriers when I was in the navy :D) onboard. Even allowing 300lb each for 6,000 cremembers including their personal gear that’s 900 tons, less than 1% of the the displacement of a Nimitz class carrier. Carriers have vastly larger crews than othe warship types so I’d bet that percentage of total weight is the extreme high.

FWIW my estimate of aircraft weight was for the typical 65,000lb gross takeoff weight of an F-14 multiplied by 100 aircraft.

19

Very roughly…

Nimitz class carries 85 aircraft. Assume they are the heaviest fighters (F-16s). Each one weighs 9.5 tons empty, for a total of 807 tons. (Using gross takeoff weight isn’t quite right, since they aren’t sitting around fueled, right?)

The carriers displace 97,000 tons with a full load of planes. So the planes make up less than one percent of the weight.

20

Also, too stable and they don’t like to go with the flow so to speak and fight the big seas too much. You may assume fighting the sea too much is not good. Sea is bigger.
Also, usually excessive stability = slow or incredibly difficult to make and maneuver. Imagine a catamaran aircraft carrier.

Hummmmmmmmm maybe I got a good idea here?