One of the prevailing myths of the modern fleet is that the US Navy's nuclear-powered aircraft carriers are capable of extraordinary maximum speeds. As is quite common with Urban Myths, these keep growing with each retelling of the basic story. It started with speeds of "over 40 knots" being alleged. This has risen by stages to 45 knots then to its current level of 50 knots. The story invariably ends with an officer turning off the speed readout for "security reasons." Sadly the truth is much more prosaic.
The official listing of the carrier speeds is "in excess of 30 knots". The actual speed of the CVNs is classified; much as the maximum speed of the SSNs in the 1960s through 1980s was restricted information. However, the design speed of the Forrestal, Kitty Hawk and JFK class carriers is public domain. The JFK was designed for 33.5 knots, the Kitty Hawks 33.6, the Forrestal 32.0 and the other CVs of that class were designed for 33.0. All had powertrain installations designed to provide 280,000 shp except Forrestal which had 260,000 shp. In all cases, the power was delivered via four shafts.
So, the question is, how does the performance of the CVNs compare with that of the CVs? To determine this we have to look at the power train itself. The nuclear powerplant does not drive the ship directly; it generates steam which powers turbines which drive the screws. The power rating of the ship is the output of her turbines, not the steam generating capacity of the reactor. The turbines installed on the CVNs are identical to those on the CVs; they generate 280,000 shp over four shafts. Even if the nuclear reactor component did generate huge amounts of additional steam, there would be nowhere to put it. On these grounds alone, it seems extremely unlikely that a CVN would be any faster than a CV.
Unofficial figures for the Enterprise confirm this; they suggest the ship was designed for 33.0 knots and it has been unofficially suggested that she reached 33.6 knots while running machinery trials after her latest refit. It has been suggested that this figure was "leaked" in order to counter suggestions that she was worn out. In passing, although Enterprise has an eight-reactor power train, only six of the reactors are on line at any one time (the reactors being rotated so that all are used regularly). The reason is quite simple; after recoring, only six reactors are needed to provide all the steam the turbines can handle. [Editor's note: This is not correct. All eight reactors are continually on-line. The Navy originally published a note saying that only six were on line at any one time, but later corrected it.]
The Nimitz class carriers were originally designed to have 260,000 shp, the reduction being due to a steam deficiency caused by a shift to the use of a pair of large reactors. In fact, they have now all been recored and are rated at the same 280,000 shp as the other carriers. It might be expected that they would, therefore, have speeds in the same 33 knot range as the other carriers.
In fact, this is not correct. The dimensions of the Nimitz class were set by building dock and other industrial and infrastructure considerations. Their hull is the largest practical design without massive investment in base and construction infrastructure. This placed grave pressure on internal volume and forced the adoption of some unusual designs solutions and the use of a significantly fuller hull form. This translated directly into loss of speed. Although the official figures are classified, it is unofficially reported that the design speed of the Nimitz was 31.5 knots on 260,000 shp. This would fit the reduced power and less advantageous hull form. Quite independently, the US Navy has suggested that the "Nimitz Class" have achieved trials speeds of 31.5 knots - this seems to be intended as an average for all the ships in this class rather than specific to any representative ship of that class.
Later ships of the Nimitz class are substantially larger than the earlier members but do have the uprated, 280,000 shp plants. Its unlikely that the extra power fully compensates for the extra size and it has been rumored that the latest ship, CVN-75 USS Harry S Truman, was hard put to reach 31.0 knots on trials.
There is a caveat here. The CVNs effectively have no concerns about running out of fuel. They can be optimized for running at high speed continuously (that is, their hull form can be selected for maximum efficiency at maximum speed). In contrast, a conventionally-powered carrier has to be optimized for optimum performance at cruising speed - 20 knots. Their hulls become progressively less efficient as the ship speed increases. This means that the sustained speed of a CVN over long duration is close to the ships maximum speed (say 30 knots) while the sustained speed of a CV over long duration is the ship's cruising speed (20 knots). So, while there is no significant difference in maximum speed of the two ships, the CVN will have a much higher transit speed. It is quite possible that it is that difference in transit speed that gets misapplied to maximum speed and is the core of the "40 Knot Myth."
I must stress that all the figures in this note are unclassified and are obtained from public domain sources (even if somewhat obscure ones!)
After this essay was originally published in April 1999, the USN publicly released the speed of the nuclear carriers in June 1999:
|Enterprise||33.6||After last refit|
|Harry S Truman||30.9|
- 29 April 1999