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  #1  
Old 10-04-2008, 02:45 AM
ChrisBooth12 ChrisBooth12 is offline
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How fast can a Nimitz class aircraft carrier go?

I know the official speed is "30+ knots" and all my sources online say "30+ knots" Ok i know it can do AT LEAST 30 knots but how fast could one go really? I mean we know how much it weighs right? We know its displacement and engine output right? Is this enough information to figureo ut how fast one could go in theory.
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  #2  
Old 10-04-2008, 03:05 AM
astro astro is offline
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http://www.answerbag.com/q_view/5575
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The practical speed limit in knots for a displacement-type hull is approximately equal to the square-root of the hull length at the waterline (LWL) times 1.34

The Enterprise is the longest warship ever built. You'll find some variation among different sources, but most of them list her length overall (LOA) as 1,123 feet, whereas ALL of the Nimitz class are usually listed as 1,092 feet. The Enterprise and the Nimitz class have the same length at the waterline (LWL), 1,040 feet.

If the hulls may be considered displacement hulls, this puts the limit of both the Enterprise and Nimitz class warships at the square root of 1,040 (32.249) times 1.34= 43.2 knots.

The "threshold speed" is generally considered to occur at a speed of about 1.2 times the square root of the ship's LWL, which would mean that the Enterprise and Nimitz class ships are not likely to exceed a speed of 38.7 knots.

Last edited by astro; 10-04-2008 at 03:07 AM..
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  #3  
Old 10-04-2008, 05:26 AM
Mk VII Mk VII is offline
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You would also need to factor in how much fouling the hull has accumulated since the last time in dock.
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  #4  
Old 10-04-2008, 07:13 AM
sailor sailor is offline
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Originally Posted by astro View Post
That is a *really* bad answer because it is using so many assumptions and simplifications.

The idea is that a displacement hull produces a wave at the bow which is followed by a through. The bow wave and following through get longer as the speed increases and it gets to the point where the boat is sitting between a wave at the bow and a wave at the stern. At this point if the ship is to increase speed it will effectively have to climb over the bow wave and begin to surf (so it is no loger displacing) or break through the bow wave both of which require exponentially more power (sort of like breaking the sound barrier for an airplane).

For small sailboats the formula given is a rule-of-thumb simplification but the simplified formula is not universally applicable nor does it give a clear point but rather a general idea of where power requirements begin to grow or are already growing fast.

A general formula which would give power vs speed for any hull is practically impossible. Formulas have been developed for different types of hulls which take into account many measurements of a hull, including, obviously, the beam. These formulas can be further simplified by making (big) assumptions like beam in relation to length, power available from the sails, etc. and then the very simplified formula is arrived at. But this formula is obviously not applicable to all hulls.

It is clearly invalid for hulls which are very narrow or very wide. A catamaran will sail faster than its calculated (by that formula) hull speed.

Big ships with bulb bows do not have the same bow wave dynamics (the bulb is there precisely for this purpose and I have been surprised at how little of a wave a huge ship can make).

Sea conditions also make a difference so I suppose the reference would be in calm waters. Waves increase the need for power.

And, again, to really measure the top speed of a carrier we would really need to know the power which can be delivered to the props and the power/speed curve for that particular hull. Applying simplified rules of thumb meant for small vessels makes no sense.

On the other hand, if you tell me how far up in the air you can lift it and then drop it I can tell you with great confidence how fast it will be going when it hits the water.

Last edited by sailor; 10-04-2008 at 07:14 AM..
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  #5  
Old 10-04-2008, 09:29 PM
sailor sailor is offline
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http://en.wikipedia.org/wiki/Hull_speed
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Hull speed, sometimes referred to as displacement speed, is a rule of thumb used to provide an approximate maximum efficient speed for a hull. It is only ever an approximation and only applies where the hull is a fairly traditional displacement design. It is usually described as a speed corresponding to a speed-length ratio of between 1.34 and 1.51 depending on which of the limited sources one refers to.

The concept of hull speed is not used in modern naval architecture, where considerations of speed-length ratio and Froude number are considered more helpful. It is still used by amateurs in relation to traditional displacement hulls.
...
This very sharp rise in resistance at around a speed-length ratio of 1.3 to 1.5 probably seemed insurmountable in early sailing ships and so became an apparent barrier. On the other hand, these values change dramatically as the general proportions and shape of the hull are changed. Modern displacement designs that can easily exceed their 'hull speed' without planing include hulls with very fine ends, long hulls with relatively narrow beam and wave-piercing designs. These benefits are commonly realised by some canoes, competitive rowing boats, catamarans, fast ferries and other commercial, fishing and military vessels based on such concepts.
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  #6  
Old 10-04-2008, 09:38 PM
panamajack panamajack is offline
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How fast it can go in theory might depend on how big of a rocket we can build.
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  #7  
Old 10-05-2008, 05:46 AM
Boozahol Squid, P.I. Boozahol Squid, P.I. is offline
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Originally Posted by panamajack View Post
How fast it can go in theory might depend on how big of a rocket we can build.
Read sailor's answer again. A boat sitting in water has a maximum speed, no matter how snazzy a rocket you strap onto it. Now, if you had a treadmill...
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  #8  
Old 10-05-2008, 12:45 PM
sailor sailor is offline
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Originally Posted by Boozahol Squid, P.I. View Post
Read sailor's answer again. A boat sitting in water has a maximum speed, no matter how snazzy a rocket you strap onto it. Now, if you had a treadmill...
Sorry but my link says the exact opposite. Given enough power you can raise speed until the boat planes or disintegrates or whatever. Still, a rocket seems a bad way to propel a carrier unless you want to propel it upwards in which case you can get it to go pretty fast by just dropping it from a certain altitude.
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  #9  
Old 10-05-2008, 06:26 PM
Lemur866 Lemur866 is offline
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Oh, and just in case anyone didn't understand why the reference doesn't give the exact top speed of the carrier, the exact top speed is classified information. I'm sure there are lots of navy guys and a few Russian spies who could tell you the exact top speed, but they won't.
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  #10  
Old 10-05-2008, 06:38 PM
OtakuLoki OtakuLoki is offline
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Originally Posted by Lemur866 View Post
Oh, and just in case anyone didn't understand why the reference doesn't give the exact top speed of the carrier, the exact top speed is classified information. I'm sure there are lots of navy guys and a few Russian spies who could tell you the exact top speed, but they won't.
You might be surprised.

I was in the engineroom of my ship (not a carrier) for a couple of speed runs.

I know exactly how many shaft RPM we were running, what percent reactor power were pulling down, and how hot everything was, and how loud the main turbine was.

I don't know how fast we were going. Just the same in excess of 30 knots that you'll see on Wikipedia and other cites.

Some people knew. The OOD, I imagine, the QMOW, and a few others on the bridge. But it was never widely disseminated. Respecting the "need to know" is a lot better at protecting secrets than telling someone that they can't mention what they know.
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  #11  
Old 10-05-2008, 09:17 PM
medstar medstar is online now
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Was it fast enough to let the captain go water surfing behind the ship?
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  #12  
Old 10-06-2008, 12:36 AM
panamajack panamajack is offline
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Originally Posted by sailor View Post
Sorry but my link says the exact opposite. Given enough power you can raise speed until the boat planes or disintegrates or whatever. Still, a rocket seems a bad way to propel a carrier unless you want to propel it upwards in which case you can get it to go pretty fast by just dropping it from a certain altitude.
Yeah, I was thinking of the ridiculous "up in the air" or "out in space" ideas, in addition to the "forced disintegration in the water" one.

That needs a moderately large rocket, though ...
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  #13  
Old 10-06-2008, 12:43 AM
Cisco Cisco is offline
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I knew a Navy E-6 who was on the USS Ronald Reagan (which is Nimitz class) and he told me he knew for sure it could go 55 knots. I have no idea how he knew that, but I suppose you pick things up after being on a ship long enough.
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  #14  
Old 10-06-2008, 01:15 AM
Danalan Danalan is offline
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A co-worker was on the Nimitz when it responded to some emergent situation somewhere (it was a while ago, so I don't remember what situation). I remember he said that they had just passed an island in the Sulu sea when they cranked up, took off, and left all their escorts behind.

They arrived on station at some other place, in the Indian ocean I think, in a specific amount of time. My co-worker found the distance between the two points, did the math given the time involved, and came up with a speed of over 60 mph.

They're plenty fast, all right. BTW, you can water ski behind most Navy ships, the problem is in 'getting up'. The acceleration isn't quite the same as a speedboat.
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  #15  
Old 10-06-2008, 02:29 AM
Princhester Princhester is offline
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Originally Posted by medstar View Post
Was it fast enough to let the captain go water surfing behind the ship?
30 knots? Hell yes.
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  #16  
Old 10-06-2008, 03:56 AM
sailor sailor is offline
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[QUOTE=Danalan;10279947]A co-worker was on the Nimitz when it responded to some emergent situation somewhere (it was a while ago, so I don't remember what situation). I remember he said that they had just passed an island in the Sulu sea when they cranked up, took off, and left all their escorts behind.

They arrived on station at some other place, in the Indian ocean I think, in a specific amount of time. My co-worker found the distance between the two points, did the math given the time involved, and came up with a speed of over 60 mph. [QUOTE] I doubt it. But I can assure you the Russians can also divide and know full well what kind of speed American Navy ships have done.

I find the concept that "the top speed is classified information a bit silly. IMHO it is not so much classified information as much as irrelevant and non-existant information. Just like asking how fast can a certain car go or how much weight can a certain airplane carry. The answer is "it depends". The reason they say "30 knots+" is not because they want to keep it a secret but because that is the best definition which can be givern. A ship is not going to have a hard figure which can be said is its "top speed" because the speed it can achieve depends on so many factors, including, the state of the sea, the state of the hull, how loaded the ship is, for how long you can maintain the powerplant power output etc. A ship might be able to maintain a certain powerplant output for say, 15 minutes, somewhat less for 30 minutes, somewhat less for 1 hour, somewhat less for 4 hours and somewhat less indefinitely. Even salinity and the wind and state of the atmosphere will have an effect. There are just too many variables. Asking for a top speed figure with any more precission than "30+ knots" is kind of meaningless and open to speculation in the same league as "can a Boeing 747 do backflips".
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  #17  
Old 10-06-2008, 04:12 AM
astro astro is offline
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[QUOTE=sailor;10280139]
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Originally Posted by Danalan View Post
There are just too many variables. Asking for a top speed figure with any more precission than "30+ knots" is kind of meaningless and open to speculation in the same league as "can a Boeing 747 do backflips".
That's hardly the case. While a request for "top speed" might not be answerable with absolute precision, and is surely dependent on a number of factors, a reasonable assumption could be made that the questioner is asking for a general sense of how fast it go under under near ideal conditions. If the top end speed is apparently up into the 50-60 mph range under ideal conditions this is a perfectly acceptable answer.
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  #18  
Old 10-06-2008, 04:38 AM
sailor sailor is offline
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Originally Posted by astro View Post
That's hardly the case. While a request for "top speed" might not be answerable with absolute precision, and is surely dependent on a number of factors, a reasonable assumption could be made that the questioner is asking for a general sense of how fast it go under under near ideal conditions. If the top end speed is apparently up into the 50-60 mph range under ideal conditions this is a perfectly acceptable answer.
Well, 50-60 mph I very much doubt. 60 mph is 52 knots and I very much doubt it. Any suggestion that it can do 52 knots under *any* circumstances requires very serious support. I think when they say 30+ knots that is exactly what they mean and I do not think they are intending to conceal the fact that it can do 52 knots.
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  #19  
Old 10-06-2008, 06:44 AM
minor7flat5 minor7flat5 is offline
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Originally Posted by OtakuLoki View Post
I know exactly how many shaft RPM we were running, what percent reactor power were pulling down, and how hot everything was, and how loud the main turbine was.

I don't know how fast we were going. Just the same in excess of 30 knots that you'll see on Wikipedia and other cites.

Some people knew. The OOD, I imagine, the QMOW, and a few others on the bridge. But it was never widely disseminated. Respecting the "need to know" is a lot better at protecting secrets than telling someone that they can't mention what they know.
Indeed.

I was in the engine room on the Nimitz and knew exactly the numbers you describe about the power plant, but never had a clue about what it meant in actual speed.

Kind of like asking how deep subs can go, I guess. I remember asking one of our teachers in nuke school the sub-depth question (he was a sub sailor) and his answer led me to understand that this was the one question that was never to be asked of a submariner. I imagine he knew the answer since he was a lieutenant commander -- an officer and not the greenest one at that.
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  #20  
Old 10-06-2008, 06:50 AM
Mr. Slant Mr.  Slant is offline
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Originally Posted by Danalan View Post
A co-worker was on the Nimitz when it responded to some emergent situation somewhere (it was a while ago, so I don't remember what situation). I remember he said that they had just passed an island in the Sulu sea when they cranked up, took off, and left all their escorts behind.

They arrived on station at some other place, in the Indian ocean I think, in a specific amount of time. My co-worker found the distance between the two points, did the math given the time involved, and came up with a speed of over 60 mph.

They're plenty fast, all right. BTW, you can water ski behind most Navy ships, the problem is in 'getting up'. The acceleration isn't quite the same as a speedboat.
A US aircraft carrier left its escort behind?
Wouldn't that make it rather vulnerable to the host of threats which its escorts are expected to nullify?
Is your friend the type you'd trust with your life to be honest?
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  #21  
Old 10-06-2008, 07:58 AM
Rick Rick is offline
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Originally Posted by Cisco View Post
I knew a Navy E-6 who was on the USS Ronald Reagan (which is Nimitz class) and he told me he knew for sure it could go 55 knots. I have no idea how he knew that, but I suppose you pick things up after being on a ship long enough.
I have also heard that number from an ex Navy man. Keep in mind the difference between a fairy tale and a sea story is how they start.
A fairy tale starts "Once upon a time"
A sea story starts "Now this is no bullshit"
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  #22  
Old 10-06-2008, 08:04 AM
sailor sailor is offline
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Originally Posted by minor7flat5 View Post
Kind of like asking how deep subs can go, I guess. I remember asking one of our teachers in nuke school the sub-depth question (he was a sub sailor) and his answer led me to understand that this was the one question that was never to be asked of a submariner. I imagine he knew the answer since he was a lieutenant commander -- an officer and not the greenest one at that.
Again, this is probably question that does not have a single definite answer and is a judgment call depending on circumstances.

It is like asking how much load a rope can bear. The answer is "certainly more than X and certainly not more than Y". Between those two extremes you have to make many assumptions and qualifications. At what tension will a particular piece of rope break? It is a guess in a particular range. It depends on many things. How old is the rope? How long will the tension last? Should you use a piece of rope to lower a person away from danger when there is a certain risk that the rope might break or should you assume the risk of facing the danger which is also uncertain? All these are judgment calls.

I suppose a submarine is guaranteed up to depth X and from there the risk increases. In a crisis it is going to be a judgment call whether the risk from enemy action is greater than the risk from going to a greater depth. There are just no hard and fast numbers.

How high can an airplane fly? How much weight can it carry? How far can it go?

How much weight can a bridge bear? It is marked as safe up to X but that does not mean it cannot bear more. Should we take the risk of passing with slightly more in order to save lives? Certainly.

How fast can your wife run? Well, certainly faster than X and certainly slower than Y. then you have to take into account many circumstances. For how long does she need to run? Is she running to get a piece of cake? How hungry is she? Or is she being chased by a bear? How hungry is the bear? What have they each eaten in the last six hours?

Demanding hard figures as answers to these questions is just too simplistic.

Even questions like "how long is my penis?" do not have clear-cut answers.
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  #23  
Old 10-06-2008, 08:13 AM
slaphead slaphead is offline
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You might find these two articles from the Naval Technical Board interesting, even if they are old.

Speed Thrills III - Nuclear aircraft carrier speeds
Speed Thrills IV (nuclear ships in general)
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  #24  
Old 10-06-2008, 09:16 AM
sailor sailor is offline
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Originally Posted by slaphead View Post
You might find these two articles from the Naval Technical Board interesting, even if they are old.

Speed Thrills III - Nuclear aircraft carrier speeds
Speed Thrills IV (nuclear ships in general)
Good articles!
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  #25  
Old 10-06-2008, 09:26 AM
sailor sailor is offline
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Originally Posted by sailor View Post
Good articles!

Quote:
Enterprise 33.6 knots after last refit
Nimitz 31.5 knots
Theodore Roosevelt 31.3 knots
Harry S Truman 30.9 knots
Is probably the closest we can get to a definite answer.
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  #26  
Old 10-06-2008, 09:35 AM
smiling bandit smiling bandit is offline
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The number is easily calculated, and you can get it much more accurate than better than what was listed. A friend of mine nearly gave the Navy security people a heart attack when he was in the nuclear power program. What happened was that he showed them how to calc it, and that the information was in 300 year-old books. He also panicked them because he brought in perfectly accurate Elemental Charts from the Atomic Energy Comission. See, in the Navy, that was classified since 1943 or so. Of course, you can get them for a buck each by mail order these days.

You don't need to be a Russian spy to get a pretty darn good knowledge of people's capabilities. On the other hand, if you want the exact information, well, that's not only pointlessly precise but will vary depending on a lot of factors, including how long it's been since they cleaned it off.
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  #27  
Old 10-06-2008, 09:39 AM
What Exit? What Exit? is offline
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Originally Posted by Mr. Slant View Post
A US aircraft carrier left its escort behind?
Wouldn't that make it rather vulnerable to the host of threats which its escorts are expected to nullify?
Is your friend the type you'd trust with your life to be honest?
That also sent up a large red flag for me. This violates what I know about carrier ops. I can't imagine a carrier leaving its escort behind.

I know that the conventional and now retired USS Ranger (Forrestal class listed to top out at 33.6) got up to about 34 knots in the late 80s when she was already 30+ years old. I was on board for that speed run.

I find these lower number very suspect. I would guess there are unofficial, unpublished speeds significantly higher than those in the article. What exactly does design speed mean in this case?
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  #28  
Old 10-06-2008, 10:01 AM
OtakuLoki OtakuLoki is offline
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smiling bandit, I don't know why the people your friend talk about were shocked. There's a lot of stuff in the Navy nuc field that is classified for people within the field, but still available in various publicly available sources. And this fact is well known to everyone I'd ever met while I was in the nuc field.

I don't keep quiet about the numbers I know because of National Security. Like you said - most of them can be determined with a little knowledge and basic calculatons. I keep quiet because I like looking at pretty girls too much to want to spend time in Leavenworth.
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  #29  
Old 10-06-2008, 10:16 AM
sailor sailor is offline
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Originally Posted by smiling bandit View Post
The number is easily calculated, and you can get it much more accurate than better than what was listed. A friend of mine nearly gave the Navy security people a heart attack when he was in the nuclear power program. What happened was that he showed them how to calc it, and that the information was in 300 year-old books. He also panicked them because he brought in perfectly accurate Elemental Charts from the Atomic Energy Comission. See, in the Navy, that was classified since 1943 or so. Of course, you can get them for a buck each by mail order these days.
Look, I know we're most just bullshitting here but let us try to keep things at least moderately plausible. There is no way a 300 year old book has anything which can usefully be used to calculate anything reagarding the speed of a carrier. Nothing. Not even 200 year old. Even 100 year old is going to be very severely lacking.

You need to calculate the behavior of the hull and of the screws. Fluid mechanics has developed greatly in the 20th century and is still developing and even then models are used for testing. Developing screw designs is very high tech and complex and there were no screw-powered boats 300 years ago so let us not be silly here.

Not to mention thermodynamics and the power a steam turbine could yield. The assertion is just preposterous on its face.
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Old 10-06-2008, 10:21 AM
Winsling Winsling is offline
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In February 1975, Typhoon Gervaise struck the island nation of Mauritius. The USS Enterprise responded to calls for disaster relief from Mauritius, arriving at Port Louis the carrier personnel spent more than 10,000 man-hours rendering such assistance as restoring water, power and telephone systems, clearing roads and debris, and providing helicopter, medical, food and drinkable water support to the stricken area.
From Wikipedia. I think this is the root of the "a CVN left her escorts behind" story. A bit of digging didn't turn up anything conclusive either way.
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Old 10-06-2008, 10:27 AM
What Exit? What Exit? is offline
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Is design speed the same as the maximum speed in the usage seen in this thread and the articles linked to?
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  #32  
Old 10-06-2008, 10:40 AM
sailor sailor is offline
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Originally Posted by What Exit? View Post
Is design speed the same as the maximum speed in the usage seen in this thread and the articles linked to?
I think "design speed" is the normal cruising speed which minimises fuel consumption while maximum speed is what the boat could do, without caring about fuel consumption, if it was in a real hurry.
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  #33  
Old 10-06-2008, 10:41 AM
OtakuLoki OtakuLoki is offline
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What Exit?, I wouldn't think so - anymore than crush depth for a sub would be the same as its test depth or design depth.
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  #34  
Old 10-06-2008, 10:41 AM
rhythmonly rhythmonly is offline
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Not being all that familiar with speed calcs (I was but a lowly Gunner's Mate at the time), I was stationed aboard the Carl Vinson (CVN 70) back in the late 80's. We stopped in San Diego after a WESPAC, to drop off the Air Wing. IIRC, liberty for the crew was secured around 1200 hours.

We got ourselves haze gray and underway, at which point the Skipper came on over the 1MC:

"This is the Captain speaking. We will be dropping anchor in Alameda tomorrow morning."

And we did just that. Not sure what the top speed figured out to be; I was just impressed that a Big Gray Thing could go that fast in open water.
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Old 10-06-2008, 11:19 AM
Rick Rick is offline
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Originally Posted by What Exit? View Post
That also sent up a large red flag for me. This violates what I know about carrier ops. I can't imagine a carrier leaving its escort behind.

I know that the conventional and now retired USS Ranger (Forrestal class listed to top out at 33.6) got up to about 34 knots in the late 80s when she was already 30+ years old. I was on board for that speed run.

I find these lower number very suspect. I would guess there are unofficial, unpublished speeds significantly higher than those in the article. What exactly does design speed mean in this case?
I think you will find the answer in slaphead's second link
Quote:
Speaking of the Enterprise, she left Bainbridge behind just as she did to many ships during the Vietnam War. When she was launching planes, she accelerated very quickly and kept at high speed for hours on end. Those techniques made her look much faster than she actually was. To really understand this, you have to be along side Enterprise (or a Nimitz) when they accelerate. It is impressive. The Bainbridge could out accelerate the Big E easily, but no conventional steam-powered ship has a chance. You see, you just can't wing the throttles open in a tin can like you can in a "nuke." Heat input is too low. Steam pressure falls off, you lose critical heat, the boilers depressurize and cool down, and the steam bubble collapses… nastily. You have to increase speed slowly on a conventional critical steam plant. You have to build up heat (actually heat flow), and maintain temperature and pressure as you slowly accelerate in a tin can.

Nuclear power plants simply don't have those limitations. When Big E had to launch on short order, she just ran away from her escorts. Conventional carriers just couldn't do that! Therein lays the seed of deception and myth. Enterprise looked fast, because fast destroyers couldn't catch her! By the time they got up to speed and began closing distance, Big E was back down to what appeared to be normal speed (though she was still at her maximum speed). Sailors that didn't know better (we can go 34 knots, and Big E just ran away from us… we couldn't catch her until she slowed down!), thought that Big E had to be able to achieve speeds of 36-40 knots to do the things that they all saw with their own eyes. In fact, her throttle-men were not limited by fire rates, fuel pumps, or critical boiler conditions. Steam generator temperature was controlled by a reactor, and it could change heat rate in a heart beat. She was limited by steam generator water levels (more power, more bubbles, higher water levels even though mass is constant, chance of dreaded carry-over, oops! Turbines don't like water!). And water levels were easy to control. And that's how the myth became an assumed fact. She was miles down the course before the destroyers could even get speed up. It took hours to run her down, because they were only three to five knots faster than she was. By the time they caught her, flight ops were over, or she was in transition from launching to receiving planes. She only appeared to be going more slowly at that point, but the reality was that she was cruising along at or near her top speed.
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Old 10-06-2008, 11:55 AM
minor7flat5 minor7flat5 is offline
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Originally Posted by sailor View Post
Again, this is probably question that does not have a single definite answer and is a judgment call depending on circumstances.
Remember that this was a casual discussion in the corner of the room, where I would have expected that your points would have been raised and discussed at length by the instructor, giving me better insight into the capabilities of our submarines. Regardless, for a certain make and model of sub, there should be a pretty good ballpark figure of how deep one can go in time of danger. I got the stonewall answer, though.
Quote:
Originally Posted by smiling bandit View Post
The number is easily calculated, and you can get it much more accurate than better than what was listed. A friend of mine nearly gave the Navy security people a heart attack when he was in the nuclear power program.
...
I gotta call BS on this one like the others did.
As another Nuke, I will second the statement that most of the data taught in that program is classified only because of where it is, not because the information is classified per se. For example, you might find a page in a classified book that has Ohm's law printed on it. That doesn't mean that there is anything secretive about Ohm's law, just that it is shown in a military context. Sounds like a sea story.
Quote:
Originally Posted by slaphead's link
You see, you just can't wing the throttles open in a tin can like you can in a "nuke." Heat input is too low. Steam pressure falls off, you lose critical heat, the boilers depressurize and cool down, and the steam bubble collapses… nastily. You have to increase speed slowly on a conventional critical steam plant. You have to build up heat (actually heat flow), and maintain temperature and pressure as you slowly accelerate in a tin can.

I was never a BT (Boiler Technician), but this doesn't sound right. It sounds as if the author is saying that nuke plants have instant power, while oil-fired plants don't.
In both cases, water is being boiled by a heat source; in the former, it's primary coolant, while in the latter it's boiler flames. Why would boiler flames be less responsive, to the extent that one would need to gently nurse the conventional destroyer up to speed? Not that I know jack about it, but I wouldn't mind a confirmation from someone who is familiar with oil-fired plants.
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Old 10-06-2008, 12:03 PM
OtakuLoki OtakuLoki is offline
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Quote:
Originally Posted by minor7flat5 View Post

I was never a BT (Boiler Technician), but this doesn't sound right. It sounds as if the author is saying that nuke plants have instant power, while oil-fired plants don't.
In both cases, water is being boiled by a heat source; in the former, it's primary coolant, while in the latter it's boiler flames. Why would boiler flames be less responsive, to the extent that one would need to gently nurse the conventional destroyer up to speed? Not that I know jack about it, but I wouldn't mind a confirmation from someone who is familiar with oil-fired plants.
I won't speak to tin cans, but the larger oil fired ships (carriers and battlewagons) I've read up on would not steam at normal cruising speed with all boilers lit. Sometimes even cruising with a full fire room/boiler room set cold. So, in some circumstances, an oil fired steam plant would take a very long time to achieve full power, because some of the boilers weren't at full pressure/temperature. And getting from that condition to putting out steam takes time.

I don't have a cite for this off the top of my head, but it is my impression.

ETA: Steaming with a boiler and fire room set offline was something that would be more for normal peacetime cruising, than anything that would happen in a war zone.

Last edited by OtakuLoki; 10-06-2008 at 12:08 PM..
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  #38  
Old 10-06-2008, 12:30 PM
Bryan Ekers Bryan Ekers is offline
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How fast can a Nimitz class aircraft carrier go?

Fast enough to travel through time, once.
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  #39  
Old 10-06-2008, 01:43 PM
sailor sailor is offline
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Originally Posted by Bryan Ekers View Post
Fast enough to travel through time, once.
For twenty minutes.
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  #40  
Old 10-06-2008, 01:55 PM
slaphead slaphead is offline
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Quote:
Originally Posted by minor7flat5 View Post
Why would boiler flames be less responsive, to the extent that one would need to gently nurse the conventional destroyer up to speed? Not that I know jack about it, but I wouldn't mind a confirmation from someone who is familiar with oil-fired plants.
I wouldn't be able to tell a ships engine from an industrial laundry, but FWIW I've often seen rapid acceleration and general engine responsiveness quoted as one of the key reasons why navies are moving to using gas turbine engines. I've always had the distinct impression that getting an oil-fired steam turbine up to full output is a tricky and somewhattime-consuming task.
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  #41  
Old 10-06-2008, 02:52 PM
smiling bandit smiling bandit is offline
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Quote:
Originally Posted by OtakuLoki View Post
smiling bandit, I don't know why the people your friend talk about were shocked. There's a lot of stuff in the Navy nuc field that is classified for people within the field, but still available in various publicly available sources. And this fact is well known to everyone I'd ever met while I was in the nuc field.

I don't keep quiet about the numbers I know because of National Security. Like you said - most of them can be determined with a little knowledge and basic calculatons. I keep quiet because I like looking at pretty girls too much to want to spend time in Leavenworth.
He was in the Navy a long time ago, so things may be different now.

Quote:
Originally Posted by sailor
Look, I know we're most just bullshitting here but let us try to keep things at least moderately plausible. There is no way a 300 year old book has anything which can usefully be used to calculate anything reagarding the speed of a carrier. Nothing. Not even 200 year old. Even 100 year old is going to be very severely lacking.

You need to calculate the behavior of the hull and of the screws. Fluid mechanics has developed greatly in the 20th century and is still developing and even then models are used for testing. Developing screw designs is very high tech and complex and there were no screw-powered boats 300 years ago so let us not be silly here.
Yeahuh. All that proves is that... you don't know jack about ship design. Sure, you can shove enough power into the screw propellor to make it go a little faster, but on a practical level, the hull is the same bloody shape ships (sailing, not galleys) have been in since Carthage and Rome went at it, and there are some soft but tough limits that elementary physics puts on it. These principles have been known since Newton walked the earth, at least.

All our advances in shipbuilding knowledge have involved engineering the ship's power systems and making them tougher. We changed How we build but not What we build. You can model fluid mechanics all you want, but it ain't going to change the fact that a ship's length and width fundamentally determine what kind of speeds it can attain at any given power output.

Last edited by smiling bandit; 10-06-2008 at 02:54 PM..
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  #42  
Old 10-06-2008, 03:33 PM
sailor sailor is offline
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Quote:
Originally Posted by smiling bandit View Post
Yeahuh. All that proves is that... you don't know jack about ship design. Sure, you can shove enough power into the screw propellor to make it go a little faster, but on a practical level, the hull is the same bloody shape ships (sailing, not galleys) have been in since Carthage and Rome went at it, and there are some soft but tough limits that elementary physics puts on it. These principles have been known since Newton walked the earth, at least.

All our advances in shipbuilding knowledge have involved engineering the ship's power systems and making them tougher. We changed How we build but not What we build. You can model fluid mechanics all you want, but it ain't going to change the fact that a ship's length and width fundamentally determine what kind of speeds it can attain at any given power output.
Really? You really believe this? Really?

I need to count to 100 before I start a pit thread. I'll be over here counting...
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  #43  
Old 10-06-2008, 04:03 PM
Really Not All That Bright Really Not All That Bright is offline
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If a ship's speed is limited by hull length, how on earth would a city-sized nuclear carrier outdistance a fast frigate or destroyer? I'm assuming the bows of all warships are more or less the same shape- is that correct?
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  #44  
Old 10-06-2008, 04:21 PM
mlees mlees is offline
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Quote:
Originally Posted by Really Not All That Bright View Post
If a ship's speed is limited by hull length, how on earth would a city-sized nuclear carrier outdistance a fast frigate or destroyer? I'm assuming the bows of all warships are more or less the same shape- is that correct?
No.

Destroyer and frigate bows tend to be more "raked". (Think "clipper bow".)

http://en.wikipedia.org/wiki/Image:U...-_close_up.jpg

http://en.wikipedia.org/wiki/Image:U..._port_side.jpg


The older Nimitz had much straighter stems.

http://upload.wikimedia.org/wikipedi...son_cvn-70.jpg

The newest of the class, the USS Reagan, has a bulbous bow.

http://en.wikipedia.org/wiki/Image:U...ulbous_Bow.jpg

(Check out this photo: http://en.wikipedia.org/wiki/Image:U...-6074Y-053.jpg )
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  #45  
Old 10-06-2008, 04:24 PM
mlees mlees is offline
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Here's another shot of a "small boy" bow. Note in this case, the "bulge" is a sonar array, not a hull-form aid:

http://www.truxtun.navy.mil/Site%20I...Transition.jpg

Last edited by mlees; 10-06-2008 at 04:24 PM..
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  #46  
Old 10-06-2008, 05:56 PM
sailor sailor is offline
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On the develoment of the bulb bow (more recently than some may think).
Quote:
http://en.wikipedia.org/wiki/Bulbous_bow

The first bulbous bows appeared in the USA being fitted to the USS Delaware which entered service in 1910 and the design is credited to David W. Taylor, naval architect and Chief Constructor of the Navy [USA]. In the 1920s other nations experimented with bulbous bows with the introduction of the Bremen and Europa, two German North Atlantic ocean liners. Bremen, which appeared in 1929, was able to win the coveted Blue Riband of the Atlantic with a speed of 27.9 knots (51.7 km/h).

Smaller passenger liners such as the American President Hoover and President Coolidge of 1931 began to appear with bulbous bows although they were still viewed by many ship owners and builders as experimental.

In 1935 the French superliner Normandie coupled a bulbous bow with a radically redesigned hull shape and was able to achieve speeds in excess of 30 knots (56 km/h). At the time Normandie was famous for (among other things) her clean entry into the water and her greatly reduced bow wave. Normandie's great rival, the British liner Queen Mary achieved equivalent speeds with a non-bulbous traditional stem and hull design. However, the crucial difference lay in the fact that Normandie achieved these speeds with approximately thirty percent less engine horsepower than Queen Mary — and with a corresponding reduction in fuel use.

Bulbous bows were further developed and used by the Japanese. Some World War II-era Japanese battleships such as the Yamato were fitted with bulbous bows. However, Japanese research into this area did not spread to the western world, and much of the advances were lost post-war.

It is unclear when bulbous bows were conclusively first examined by western researchers, but scientific papers on the subject were first published in the 1950s. Engineers began experimenting with bulbous bows after discovering that ships fitted with a ram bow were exhibiting substantially lower drag characteristics than predicted, and eventually found that they could reduce drag by about 5%. Experimentation and refinement slowly improved the geometry of bulbous bows, but they were not widely exploited until computer modelling techniques enabled researchers at the University of British Columbia to increase their performance to a practical level in the 1980s.
And a nice photo: http://en.wikipedia.org/wiki/Image:Zadm_drydock.jpg (how does the ship stay uprigh and not fall over?)
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Old 10-06-2008, 06:20 PM
sailor sailor is offline
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This figure http://en.wikipedia.org/wiki/Image:HullSpeed.PNG shows power requirements versus speed and has a mark at the conventinal "hull speed". It can be seen that power requirements grow exponentially with speed but that there is no clear cutoff point.

This page about equations of fluid dynamics shows it is a complex field and still in development. http://en.wikipedia.org/wiki/Categor...fluid_dynamics

Quote:
http://en.wikipedia.org/wiki/Navier-Stokes_equations

Even though turbulence is an everyday experience, it is extremely difficult to find solutions, quantify, or in general characterize. A $1,000,000 prize was offered in May 2000 by the Clay Mathematics Institute to whoever makes preliminary progress toward a mathematical theory which will help in the understanding of this phenomenon.
Quote:
http://en.wikipedia.org/wiki/Computa...fluid_dynamics

Computational fluid dynamics (CFD) is one of the branches of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows. Computers are used to perform the millions of calculations required to simulate the interaction of fluids and gases with the complex surfaces used in engineering. Even with simplified equations and high-speed supercomputers, only approximate solutions can be achieved in many cases. Ongoing research, however, may yield software that improves the accuracy and speed of complex simulation scenarios such as transonic or turbulent flows. Validation of such software is often performed using a wind tunnel.
A clear sign that it is not as simple as performing a few calculations is that computers are used for creating models and that real models are tested in basins.

Quote:
http://en.wikipedia.org/wiki/Ship_model_basin

The ship model basins worldwide are organized in the ITTC (International Towing Tank Conference) to standardize their model test procedures. Some of the most significant ship model basins are the David Taylor Model Basin in the United States, The Institute for Ocean Technology [1] in St. Johns, Canada, SSPA, [2] in Gothenburg, Sweden, the Maritime Research Institute Netherlands (MARIN) [3] in Wageningen, the Netherlands, the INSEAN in Rome, Italy, the HSVA[4] in Hamburg, Germany, the "Bassin d'essai des carènes" in Val de Reuil [5], France and CEHIPAR [6]in Madrid, Spain, CTO S.A. [7] in Gdansk, Poland
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  #48  
Old 10-06-2008, 06:21 PM
Really Not All That Bright Really Not All That Bright is offline
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Quote:
Originally Posted by sailor View Post
And a nice photo: http://en.wikipedia.org/wiki/Image:Zadm_drydock.jpg (how does the ship stay uprigh and not fall over?)
And.... why are there holes in it?

So if this bulbous bow is more... fluiddynamic than a raked bow, why don't smaller ships use the design? Does the ship have to be a certain size to make it practical? IOW, is there a downside?

My WAG is that it gives the ship a much larger turning radius, which is fine for a carrier which isn't going to be making any sharp turns anyway, but not good for a littoral ship or frigate.
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  #49  
Old 10-06-2008, 06:30 PM
sailor sailor is offline
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Quote:
Originally Posted by Really Not All That Bright View Post
And.... why are there holes in it?
Those are bow thrusters. They are propellers which push the bow to one side or the other and help the ship maneouver without the help of tugs.
Quote:
So if this bulbous bow is more... fluiddynamic than a raked bow, why don't smaller ships use the design? Does the ship have to be a certain size to make it practical? IOW, is there a downside?
A smaller boat finds more resistance in the waves while a larger ship finds more resistance in the resistance of the water flowing. They are basically very different.
Quote:
My WAG is that it gives the ship a much larger turning radius, which is fine for a carrier which isn't going to be making any sharp turns anyway, but not good for a littoral ship or frigate.
In small sailboats a longer keel does indeed give the boat more directional stability and makes it harder to turn while a shorter keel makes it more maneouverable. I do not think this is a big concern for an airplane carrier.
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  #50  
Old 10-06-2008, 07:29 PM
user_hostile user_hostile is offline
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Quote:
Originally Posted by sailor View Post
... I do not think this is a big concern for an airplane carrier.
That, sir, is an aircraft carrier!
--Commander Alan Shepard (paraphrasing from the Right Stuff)
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