One of the prevailing myths of the modern fleet is that the US Navy'snuclear-powered aircraft carriers are capable of extraordinary maximumspeeds. As is quite common with Urban Myths, these keep growing with eachretelling 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 currentlevel of 50 knots. The story invariably ends with an officer turning offthe speed readout for "security reasons." Sadly the truth is much moreprosaic.

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 ofthe SSNs in the 1960s through 1980s was restricted information. However,the design speed of the Forrestal, Kitty Hawk and JFK class carriers ispublic 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 exceptForrestal which had 260,000 shp. In all cases, the power was deliveredvia four shafts.

So, the question is, how does the performance of the CVNs compare withthat 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 steamwhich powers turbines which drive the screws. The power rating of the shipis the output of her turbines, not the steam generating capacity of thereactor. The turbines installed on the CVNs are identical to those on theCVs; they generate 280,000 shp over four shafts. Even if the nuclear reactorcomponent did generate huge amounts of additional steam, there would benowhere to put it. On these grounds alone, it seems extremely unlikelythat a CVN would be any faster than a CV.

Unofficial figures for the Enterprise confirm this; they suggest theship was designed for 33.0 knots and it has been unofficially suggestedthat she reached 33.6 knots while running machinery trials after her latestrefit. It has been suggested that this figure was "leaked" in order tocounter suggestions that she was worn out. In passing, although Enterprisehas an eight-reactor power train, only six of the reactors are on lineat any one time (the reactors being rotated so that all are used regularly).The reason is quite simple; after recoring, only six reactors are neededto 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 lineat 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 theuse of a pair of large reactors. In fact, they have now all been recoredand are rated at the same 280,000 shp as the other carriers. It might beexpected that they would, therefore, have speeds in the same 33 knot rangeas the other carriers.

In fact, this is not correct. The dimensions of the Nimitz class wereset by building dock and other industrial and infrastructure considerations.Their hull is the largest practical design without massive investment inbase and construction infrastructure. This placed grave pressure on internalvolume and forced the adoption of some unusual designs solutions and theuse of a significantly fuller hull form. This translated directly intoloss of speed. Although the official figures are classified, it is unofficiallyreported that the design speed of the Nimitz was 31.5 knots on 260,000shp. 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 asan average for all the ships in this class rather than specific to anyrepresentative ship of that class.

Later ships of the Nimitz class are substantially larger than the earliermembers but do have the uprated, 280,000 shp plants. Its unlikely thatthe extra power fully compensates for the extra size and it has been rumoredthat the latest ship, CVN-75 USS Harry S Truman, was hard put to reach31.0 knots on trials.

There is a caveat here. The CVNs effectively have no concerns aboutrunning 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 maximumspeed). In contrast, a conventionally-powered carrier has to be optimizedfor optimum performance at cruising speed - 20 knots. Their hulls becomeprogressively less efficient as the ship speed increases. This means thatthe sustained speed of a CVN over long duration is close to the ships maximumspeed (say 30 knots) while the sustained speed of a CV over long durationis the ship's cruising speed (20 knots). So, while there is no significantdifference in maximum speed of the two ships, the CVN will have a muchhigher transit speed. It is quite possible that it is that difference intransit speed that gets misapplied to maximum speed and is the core ofthe "40 Knot Myth."

I must stress that all the figures in this note are unclassified andare obtained from public domain sources (even if somewhat obscure ones!)