The weight of a crew can make a huge differance into the stability of a ship.
HMS Bristol had a serious machine space fire(caused by an idiot removing a temp gauge probe pocket in a pressurised fuel injector feed line - instead of simply removing the thermometer!!)
The resulting fire caused huge amount of heat to be conducted through steel bulkheads, and so some fire crews were ordered to play water over these bulkheads to keep them cool enough to retain structural intergrity, and prevent spread of the fire.
Unfortunately no-one thought to turn off the hose nozzles and only provide just enough water to do this, and the result was a build-up of water above the machinery space (this is called free surface water).
This water actually was boiling too, but most importantly, it was relatively high up in the hull, which caused HMS Bristol to loll over, the ship was nearly up for capsizing and to save her it was necessary to get as much of the crew as possible to station themselves on the upper deck on the high side of the ship.
This bought enough time to pump the excess water away, but it was a damend close run thing, if she had not been in Milford Haven where the supertanker firefighter tenders were nearby then she would almost certainly have been lost.
Bingo! The Cg remains in place with respect to the sessel. The CB moves as the waterline moves. Depending on the position of the CB with respect to the CG the righting force can be positive or negative.
In general you want the lowest CG as that gives the best stability. Anything that will lower the CG can act as ballast even if it is water or if it is lighter than water. It does not matter but the heavier, the more it will lower the CG if placed low.
In the case of completely immersed vessels like submarines, the CG can be (and usually is) below the CB, because there aren’t as many external heeling moments. With surface ships, especially those with a superstructure, getting the CG below the CB just isn’t possible or practical. So the righting moment increases to a point of diminishing stability, then as you said, the moment works to capsize the ship. The idea is to design the ship to maximize this angle of maximum stability.
I think it’s possible the CG can be aboce the CB and have a righting force within limits. Sailor’s example of a floating tray (a barge) say with a bottle standing up in the middle has a high CG but is reasonably stable.
av8rmike, i did not mean to sound snotty but come on, you set yourself up for it. There is a common misconception among sailors that the CG needs to be lower than the CB. This stems from the fact that a low CG and high CB is a good thing. We all know that the lower the CG the better stability. And then you finish by saying “I’m an engineer”, like that settles it. Well, you set yourself up and I am allowed to respond to that statement you made. But it is all done in fun and good spirit and not in any way intended to offend.
One further clarification: Ballast must be secured or it will not work. Ballast, whether it is solid or water or cargo must be kept in place relative to the hull. Water compartments must be separate so the water stays in place relative to the hull. Just filling an open hull with water does not work. In fact, the sloshing of liquid ballast in half empty tanks can make matters worse. Also, a shift in cargo can make things pretty bad. The whole principle is that the CG stays in place and the movement of the CB is what provides the righting force.
I remember reading somewhere that in the pre-WWII years, a number of Japanese destroyers were re-designed after the keels were laid in order to incorporate larger guns and more torpedos. This ruined their trim and made them quite susceptible to capsizing in heavy seas.
Ok, let us try to get back to the OP which no one has really addressed yet. We have seen that a vessel with its CG below its CB will be very stable but a vessel with CG above the CB needs to get the stability from its shape (width, beam). The greater the vertical distance between the CG and the CB, the greater the width needed to gain stability.
At first sight a carrier appears to have a high CG, a low CB and a narrow beam at the waterline. This does not add up so the question is, which assumption is wrong? Let us go step by step. The CB has to be below water level (duh!). The CG seems to be quite high and surely it is above the water line. There is nothing wrong with this so long as we have enough beam at the waterline that will generate the necessary righting force. This is what, at least at first sight, seems to be missing.
Again, can someone provide links to plans showing sections of a carrier and ballast? That’s the only way to find out.
Also, some have said a carrier uses water for ballast but that is meaningless. It could have some movable ballast which was only a small fraction of the total. Numbers, we need numbers. And plans.
Unfortunately, sailor, that’s easier said than done. As an engineer (yes, again…) working for government contractors, shipyards don’t like to share that kind of information, and they especially don’t like it shared with third parties.
That said, I do have something to contribute: A notional inboard profile of LHD-3, the U.S.S. Kearsarge, in which, there no “ballast” listed. However, I quote from SNAME’s Principles of Naval Architecture, Vol. 1, Sect 12:
So a ship the size of an amphibious ship or aircraft carrier may or may not even have permanent ballast added. According to the Naval Vessel Registry, the John C. Stennis (CVN-74) at full load, displaces over 3.6 million cubic feet of water. Any way you slice it, that gives you a lot of buoyancy.
For what was, to me, a simple question calling for a simple answer, some of the answers have given me much confusing thoughts to ponder.
I understand a little now , but the the answer, in any simple concise form, has not yet “clicked” in my tiny brain.
However, I thank you all very kindly for your efforts.
I think the best thing to do is to see if I can get hold of “Janes”, no disrespect intended, but, like so many things, what was aintended in all honesty as a simple question, has turned out to be an immensly complex one. To think, these principles have evolved over several thousand years - and they must be there somewhere, written in simple terms for dum dums like me to find.
And I believe you are right. The keel of most ships is a structural element and usually doesn’t project out of the hull. The keel on a sailboat is both a structural element and also projects down into the water to reduce leeway and to provide a fulcrum for the rudder to act with in turning. The keel isn’t a factor in stability although there is usually a weight on a sailboats keel to keep the wind force on the sails from blowing it over.
As to stability. The hull shape is designed so that the center of buoyancy is high in the hull. The ship proportions are such that the center of gravity is below the center of buoyancy. The buoyancy exerts an up force on the center of buoyancy and gravity exerts a down force on the center of gravity. The result is a righting moment (or force) that prevents the ship from capsising.
Ships are loaded very carefully with attention paid to trim, or the way a ship sits in the water. There are marks painted on the hull, Plimsol Marks, that are used to determine how much a ship is down by the bow, or stern as the case may be, and how much heel there is. As fuel and other stores are used the trim changes and ballast water is pumped around various places in order to maintain the proper trim. If the ship is running empty then ballast has to be provided to maintain stability. Without the ballast the center of gravity rises and the distance between it and the center of buoyancy decreases causing less righting moment and therefore possible instability. In storms that would be hazardous as would be expected.
The truth is no one has really provided an answer. To recap: A carrier must surely have its CG above its CB. A vessel like that can only be stable if the beam at the waterline is wide and will result in the CB being displaced in the same direction as the direction of list. This looks to not be the case in a carrier which has a beam on deck probably greater than twice the beam at the waterline. So no one has really given any satisfactory explanation.
If you’re looking for a book, try Naval Architecture for Non-Naval Architects. It’s been a while since I read it for work, but I think it uses enough non-technical terms. The Janes books are simply factual information, not theories and design methods.
sailor can you please explain why you think this is the case? I’m not seeing why, as long as the transverse CG is along the centerline, the beam above the waterline has any bearing on stability. The BOA of the Stennis is 252 ft, while the BWL is 134 ft, at a draft of 41 ft, if you need numbers.
>. the beam above the waterline has any bearing on stability
Well, assuming all that stuff up there has mass (a safe assumption I should add) then you are going to have a pretty high CG. Again, with a high CG the only way to achgieve stability is with a wide hull underneath which will move the CB to the side of the inclination.
This is the crux of this thread: A carrier seems to have a high CG and a narrow beam which we know are conditions which lead to instability. Why is a carrier stable in apparent contradiction to the premise? Either the beam at the water is greater than we are assuming or the CG is much lower than we are assuming or both. But we do not have an answer or anything close to an answer. Maybe this is one for Cecil himself.
I don’t believe this is possible. Fig 1 and Fig 2 explain why. Although a carrier might look topheavy it can’t be and still stay upright, the physics won’t allow it.
If the CB starts below the CG the ship will roll irrespective of the waterline width of the hull. It will roll until the CB is above the CG and stay in that stable position.
A very accessible reference is the Encyclopaedia Britannica which is available at most libraries.
Oh David, please. The least you can do is read the thread. I have taken several hours to do some drawings and explain the basic principles. The least you can do is read the thread and study what has already been said and posted. But now you arrive here and want us to discuss all over what we have already done? No thanks. Read the freaking thread. Your post is wrong and the explanation is right in this thread.
Originally posted by David Simmons: If the CB starts below the CG the ship will roll irrespective of the waterline width of the hull. It will roll until the CB is above the CG and stay in that stable position.
av8rmike, was I right or was I right? Is this a widespread misconception or not?
David Simmons, you’re not an engineer as well are you? Just wondering.
well, sailor I don’t know if I’d call David Simmons a “widespread misconception”… but even I, a self-vaunted engineer, can tell those diagrams are drawn wrong. Maybe when I get back to work on Monday I’ll post similar figures from the PNA.
Given these two choices and the numbers that I’ve posted, I’d say our flawed assumption is the height of the CG. There is a lot of immersed volume and a lot of crap installed below decks. Therefore the VCG is probably much lower than we think. A SWAG, I know, but that’s the best we can do with the available information.
Sailor suggested this may be worthy of Cecil’s attention - I thought so too, but it was declined. It seems that there are so many subjects crying out for the wisdom of the great man, and so little space available, that he is unable to tackle this one ( actually, I suspect it may be because he don’t know).
It was suggested that a question posted here may well elucidate the answer - but despite all the work and thought put into it by all you good people, I suspect the answer is still out there, striving to remain out of reach.
If you are wondering why I do not just go out and dig up the answers - I am presently suffering from sickness and trauma, and simply do not now have the capacity and concentration I used to have when I was fit and younger. Hence the earlier reference to simplicity.
Isa.Gottanow, don’t give up hope just yet. We’ll keep trying. At least console yourself with the thought that the question is not only perperxing to you but to the whole bunch of us. You brought up a very good question.
av8rmike, it seems very difficult to keep a low CG when you have that huge upper part up there, If the waterline beam is about half of the max beam and continues to decrease below the waterline, it seems pretty darned impossible to have a low CG unless you have tons of mass way down and a draft of 40 ft does not suggest that. Maybe the US has found a way around the most basic laws of physics._
QUOTE]*Originally posted by sailor *
**Oh David, please. The least you can do is read the thread. **
[/QUOTE]
Well, I did read the cites about kayak and sailboad stability and most of the rest of the thread. I really am not too keen on comparing the characteristics of a kayak with those of an aircraft carrier. The CG of a kayak is under pretty direct control by the operator and he has a really sophisticated computer in the brain to manage things. Kayaks and canoes are fidgety things the proper manipulation of which is highly dependent on the skill of the boatsman. Shifting weight around to control the CG of an aircraft carrier is another matter entirely.
Now, I think I was a little careless in saying that the CB has to be below the CG in order to provide a righting moment. That isn’t true because of the shift of the CB toward the low side during the heel over. What is crucial is to have the metacenter always above the CG. The metacenter is the intersection of the centerline of the ship with a vertical line drawn through the CB as shown in Fig. 1.
Just the same, having the CG above the CB, it seems to me, would lead to a nervous ship that did a lot of rolling in response to relatively small disturbances. I would be glad to have a critique of the argument inFig 2.
Y’know, “Sticks and Stones” and all that. One way to learn is to put out a statement and let people take potshots at it. I’ve been corrected numerous times by real experts in different areas.
Well, you obviously missed the crucial points made in the thread and links provided.
The laws of physics apply the same to all boats. I do not think the captain of the Ike left Annapolis saying he didn’t want to bother with small boats as he was intending to only command carriers. Midshipmen learn handling small boats for a reason. It gives the feel and intuitive understanding which landklubbers lack. In any case, there were several links and I specifically made one page explaining the issue with graphics. It did not refer to any particular kind of vessel, only to the general laws which apply to all vessels.
You can say that again.
That’s a nice way of saying what I already explained and what the rest of us had understood several posts back. At least we are on the same page now.
I am pretty sure most ships have their CG above their CB. Have you seen those cargo ships with containers stacked high on deck? Naval vessels too. The only case where it might be reversed is in tankers and even there I’m not sure. Only sailboats like mine with a deep external keel may have the CG below the CB and for that they need to be very narrow beamed.
I do not know what you mean by “nervous” but the period of oscilation or roll is determined by two things (as I have already also explaind before): the righting torque and the moment of inertia around the CG. Whether the CG is above or below the CB is not relevant. A lower CG means the vessel can heel farther and still have a righting force. A higher CG means the righting force becomes negative sooner and then the vesel tends to capsize.
I disagree. Obviously, we are all putting forth thoughts and theories and submitting them to the scrutiny of others so they me be criticised and corrected. There’s nothing wrong with that. But putting out statements wildly, without giving them any thought, just to see if they fly, when they obviously show you have not bothered to understand what has been said so far is a disruption to the thread and inconsiderate to those who are putting effort into following what’s going on.
Well, at least I’m glad you’ve caught up with the rest of us and now we are all on the same page.
