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Subject: Most Efficient Carrier Size
Roman    3/2/2004 12:57:47 PM
It is often said that the bigger the Aircraft Carrier the more efficient it is, but I suspect that only works up to a point and in any case it may be preferable to have more smaller hulls to be able to deploy them in more places simultaneously. So what is the most efficient size of an Aircraft Carrier? Do you think Aircraft Carriers larger than the Nimitz would see further gains in efficiency?
 
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USN-MID    RE:Area of improvement   2/1/2005 1:19:21 AM
Why does a CVN need to go through Panama in the first place? It'd be nice, but it's not really essential. Our primary current concern is the Gulf, which is roughly equidistant from the Atlantic and Pacific. The big threat on the horizon is East Asia, with the Taiwan situation. A threat there would require an immmediate response from one or more CSFs. Atlantic CSFs are not going to make it in time due to the endurance of the ESCORTS. Two carrier groups have been considered the optimal force to take into a hot zone for a long time, mixing flexibility, capability, and threat minimization...why do we suddenly need a concentration of more than 4-5 CSFs? Even with nuclear reactors, unless the Navy as a WHOLE goes all nuclear, the CSF escort's radius is a liability. You may be able to design a CVN that steams at 40+ knots(the Enterprise reportedly did this), but assuming a hull to take this was feasible, your escorts simply can't keep up. The DDX is reportedly acoustically quieted significantly...the same features could be applied to a carrier. Why is a smaller carrier a better idea? I'm pretty sure the Hornets, if loaded fully, need more or less the entire cat launch length. Second, Hawkeyes and Prowlers were designed for Nimitz sized carriers, and the Prowlers STILL are dicey to take out. Do you think ski jumps would be enough of a compensation?
 
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CoastieKen    RE: and then there's the Panama canal...   2/1/2005 5:04:09 PM
The current carriers are far too large to pass through the Panama Canal. I THINK the assault ships like Tarawa and Wasp can do so but given that the Chinese now control the canal this is of limited utility except in peacetime. I do seem to remember reading in a late 80s version of Brasseys or Janes that the USN was contemplating a 200,000-250,000 ton carrier, this having been mulled since the 50s but never politically feasable (the first supercarrier United States was cancelled because it was seen as too big and it was a tad smaller than Nimitz). The end of the Cold War put an end to that IIRC.
 
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Librarian    RE:Area of improvement   2/2/2005 2:52:40 PM
New propellors would be unlikely to increase top speed much as speed is largely a function on the length of a ship. It is possible to gain a bit, but given that nuclear carriers already have something close the maximum possible top speed (rumours run 35 to 45 knots), there is speed to be gained. There are other advantages. Potential advantages of improved propellor design: -Reduced vibration (which improves reliability not only of propulsion systems but also of other nearby unconnected systems.) -Reduced noise (less important, but still useful) -Improved effiency (while nuclear carriers have enormous range, eventually you do have to pay for any wasted energy. In addition, I am under the impression there is a certain trade off of steam for the catapults (which is generated at some level by the nuclear plant) and power for the propellors. If you can reduce the power you need for a given speed, you can increase the amount of oomph in each catapult shot.) -Increased manuverability. (With variable pitch propellors, modern frigates can stop very fast (a few ship lengths) Scaling up to carrier size, the effect would be less, but still useful. In addition, being able to quickly alter the amount of thrust any given propellor is putting out, would help during delicate manuvers such as docking. Finally, improved propellor design would allow a carrier to reach its top speed more quickly.)
 
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elcid    RE:Area of improvement   2/10/2005 8:25:33 AM
Carriers are indeed limited by the hull length. All displacement hulls have a characteristic number which defines efficient maximum speed. Above that speed the power required for each increase goes up at an unbelievable rate. For that reason, tales of super carrier speeds (I remember rumors of 54 knots for USS Enterprise) are nonsense. It does FEEL fast if you are outside with the wind in your hair though! The limit of economic propulsion is indicated by a formula: the velocity in knots = square root of length in feet. Merchant ships use a maximum value of 0.8 of this value. Warships can use a larger fraction - not less than 0.9 and not more than 1.9 times this value. Regretfully, a very large ship cannot use more than about 1.0 - so try working out the length of a ship able to do 45 or 50 knots!
 
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Roman    RE:Area of improvement   2/10/2005 8:49:37 AM
Why is it that the length of ship determines the speed of the ship? I am sure there is a good physics reason behind it, but I cannot see it. Also, does the same formula apply to submarines as well, or is it limited to surface ships?
 
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Roman    RE:Area of improvement   2/10/2005 8:52:27 AM
Ha, according to the formula, to get a ship able to go (cruise or max speed?) 54 knots (knots are nautical miles per hour, right?) you would need a ship 888.8 meters long. That is quiet insanely large. I am wondering, if the length of the ship determines speed, why not just build longer, leaner ships? You could increase the length to width ration - there must be a reason why this is not done - why not?
 
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EW3    RE:Area of improvement - hull speed   2/10/2005 12:06:20 PM
The max speed for a diosplacement hull is 1.34*square root of length at water line. It has to due with the reduction of wetted surface. (I do a lot of sailboat racing). When I was in I heard stories about the Enterprise, but they suggested she started toplane. When that happens the equation goes out the window. When racing sailboats, going downwind in moderate seas, you can get them to plane which really reduces the wetted surface. I know someone who got a J-24 over 12 kts!
 
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EW3    RE:Area of improvement - hull speed   2/10/2005 12:32:05 PM
Just a bit more on hull speed. If you lookk at the pic at this link http://www.destroyersonline.com/images/f1038001.jpg you'll notice to the left of the 1038 where the water goes below the waterline. This is caused by the bow wave. As you go faster and faster the point at which the water returns to the waterline of the ship moves back. If you think of the waterline as the circumference of the ship in the water, the wetted surface (the part of the ship in the water) is the area inside the circumference. Hull speed is where the bow wave goes all the way to the stern. And the surface of the ship in the water is at it's least. You can actually see fairly far under a ship or boat at that time.
 
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Roman    RE:Area of improvement - hull speed   2/10/2005 8:37:10 PM
Great explanation EW3! Just three clarification, though: 1) Is the length in your 1.34 times the square root of length formula measured in meters or feet? I assume it's feet, right? 2) Does the formula still apply with very large ships, or does it break down after a certain size? 3) Why don't navies simply build ships with greater length to width ratios to take advantage of this?
 
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EW3    RE:Area of improvement - hull speed   2/10/2005 9:02:36 PM
Thank you. The length is in feet and speed is in knots, so the 1.34 constant fits only those units. Also remember the length is at the water line. A carrier at the waterline is not 1000+ feet. To my knowledge there is no breakdown of the formula, as it has to do with the "wavelength" of the water. Think of the high points as holding up the bow and stern. http://www.sailnet.com/images/content/authors/colgate/061100sc_figure4.gif (losey pic but it will give you the idea) Bigger does not mean more efficient. Hull speed is a function of length, the power to get to hull speed or any other speed for that matter is due to surface friction of the water to hull. That is why at hull speed you have minimum contact of water to ship. One last thing, I'm sure the formula does not apply to subs. I'll sniff around and post what I find. My major was physics and I love to sail, so it's a passion :) BTW - I spent 2 years in the north atlantic on that ship I posted. It was very narrow. After I got off, I never went near the ocean again for 15 years!!! Nuff said?
 
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