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notation and snapped his personal logbook closed. A gift from the crew of his old boat, the last Lafayette-class boomer to be in service, the waterproof box, slightly larger than a clipboard and an inch thick, had the name of his present command stenciled boldly across its front. He sneered at the reminder. The Pennsylvania , despite being the most advanced inhabitant of the aquatic world, had been nothing but a pain in the ass this cruise.
    “Still the shaft gears?”
    “Aye, sir.”
    The Pennsylvania , like all submarines in the U.S. inventory, was powered by a nuclear reactor, a General Electric S8G natural-circulation model in this case. But unlike other submarines, the propeller shaft of the Ohio class had no direct mechanical contact with the steam turbines. These were turned by steam produced when water in the “cold” loop passed through heat exchangers, which transferred thermal energy from the superhot reactor by way of the circulation of coolant through the “hot” loop. This two-loop system isolated the “hot” radioactive coolant. The turbines, instead of transferring their circular motion directly to the shaft by way of lifter rods or gears, turned an electric generator, which powered a quiet electric drive system that drove the propeller shaft. The process, known as turboelectric drive, was highly efficient and exceedingly quiet, a must for survival in the life of a boomer, whose mission was to go to sea and cease to exist.
    But it was that search for silence that had begun a series of events the result of which would soon manifest itself.
    During the Pennsylvania’s last refit, completed two months earlier, the gear assembly that transferred power from the electric motor to the shaft had been replaced with newly designed ones that had proved less prone to harmonic transmission—the propagation of even the slight machinery sounds from the electric motor through the shaft and into the water. The new gears used a unique sound-dampening system of assembly to cut the sub’s acoustic signature a further 10 percent. The twin cylinders of high-stress alloy into which grooved channels had been machined were basically reengineered into twelve “slices” of gears that fit together over a main connecting core made of a layered combination of titanium, ceramic laminates, and more titanium. Each slice was fit over the core in sequence, and in between each was a gasket made from a carbon-fiber laminate, which was the true trick of the new assembly. Quite literally, they cut the transfer and reflection of sound from gear to gear, and thus helped remove a source of harmonics that had plagued all subs for decades.
    Key to the successful use of the new system was its proper installation and maintenance, and the crew of the Pennsylvania had done well following every safety and operations check the assembly required. But there was one thing they could not check, as it was an installation item, designed to be configured during refit and left alone until the sub’s next stay in port.
    Holding the twin gear assemblies—one lay on each side of the similarly geared propeller shaft—tightly together as one unit was a single twelve-inch nut at the aft end of each cylinder. This nut was set and torqued during refit according to a very tight tolerance, as even the barest variance could negate the positive effects of the system and actually add to the problem of harmonic transmission. But the nut on the starboard assembly had not been torqued properly—it was recessed inside the aftmost gear by almost six inches. The technician responsible for its installation had added four-and-a-half foot-pounds too much torque when mechanically tightening the fifty-six pound nut. That alone was enough to cause an excess of harmonics because it compressed the composite gaskets too tightly and degraded their sound-dampening qualities. But in combination with another phenomenon, one the crew of the Pennsylvania had no control over, the error