Permit class submarine variants

Permit class boats were the mainstay of the attack submarine fleet throughout the 1960s.  Here is what Wikipedia has to say about them:

Preceded by: Skipjack-class submarine
Succeeded by: Sturgeon-class submarine
Built: 1958–1967
In commission: 1961–1994
Completed: 14
Lost: 1
Retired: 13
General characteristics
Type: Fast attack submarine
Displacement: 3,750 long tons (3,810 t) surfaced, 4,300 long tons (4,369 t) submerged
Length: 278 ft 5 in (84.86 m)
Beam: 31 ft 7 in (9.63 m)
Draft: 25 ft 2 in (7.67 m)
Propulsion: 1 S5W PWR
2 steam turbines, 15,000 shp (11 MW) 1 shaft
Speed: 15 knots surfaced, 28 knots submerged
Range: Unlimited, except by food supplies
Test depth: 1,300 ft (400 m)
Complement: 112
Sensors and processing systems: BQQ-2 sonar (later BQQ-5)
Mark 113 Fire-control system (later Mark 117)
Periscopes Electronic warfare
decoys:
ESM
Armament:
• 4 × 21 in (533 mm) torpedo tubes amidships
• 12-18 × Mark 37 torpedoes, later replaced by Mark 48s
• 4-6 × UUM-44 SUBROC anti-submarine missiles
• 4 × UGM-84 Harpoon anti-ship missiles

Below, a typical Permit Class cutaway. (I have a .pdf of this image that is really clean, but don't know how to post that here)

Those were the Permit class ships.  There were two that were built a little different, and those were the Tullibee (SSN-597) and the Jack (SSN-605)

Jack was about 20 feet longer than a normal Permit Class ship, and incorporated a direct drive propulsion system, connected to dual counter-rotating screws on a concentric shaft.  So, an inner shaft would turn a small screw one direction and an outer shaft would turn a larger screw in the opposite direction.  I have heard that the shaft sealing system on this ship was a nightmare.

The purpose for having a direct drive running a pair of smaller screws is twofold:  Eliminate reduction gears (a source of noise) and reduce or eliminate cavitation at the screw.

Reduction gears are (surprise) a set of gears.  Steam turbines are most efficient at pretty high speeds. The screw of a warship is most efficient at low speeds.  The reduction gears take the high-rpm output of a steam turbine and reduce it to low-rpm speed for driving a ships' propellor.

Cavitation is the creation of vapor bubbles on the low-pressure side (trailing edge) of a propellor blade.  As the bubbles come loose from the moving blade, they collapse and cause a crackling noise that is easily detected by sonar.  When your goal is stealth rather than speed, cavitation is not acceptable.

Below, cavitation on the tips of the screw of a small motorboat.  Water is flowing left to right in the image.  Long-term cavitation will eventually erode the metal of the screw.

Smaller screws have less cavitation for a given RPM than large ones, because the blade tips will not be travelling as fast.  In theory, you can have two smaller screws with the same surface area, and run them at a higher speed than a single large screw, with less likelihood of cavitation.

Cavitation is is typically a problem at shallow depths, where there is not much water pressure to prevent forming vapor bubbles.  It is also a problem when the throttles are suddenly opened.  I liken it to spinning the tires on a car - it happens easier from a standstill than at 50 mph.  Not that I know anything at all about operating the throttles of a nuclear submarine ;)

Although counter-rotating props had previously produced impressive gains in speed on the experimental Albacore (AGSS-569), in this case the results were disappointing, and led to the abandonment of this approach.

Below, the screw arrangement of USS Jack

A typical submarine screw looks more like this.  Note that it has an odd number of blades, which helps to reduce harmonic and beat noise.

The USS Tullibee (SSN-597) was an interesting design.  Launched in 1961, she was the first to have a spherical bow-mounted sonar, and angled mid-ships torpedo tubes.  This layout would become standard on all later US submarines.  Her propulsion system was turbo-electric, meaning that she had main propulsion generators, and wound around her shaft was a large motor.  She was also the smallest nuclear attack submarine in the US fleet.  The overall details of this interesting power plant are not available, sadly.  I would be interested to learn more about her.

Tullibee underway.  She wasn't a fast ship - about 15 knots surfaced AND submerged.  Interesting hump behind the sail.  According to this source, she was terribly under-powered - 13 MW reactor heat output (for about 4 MW of useful power), and 2500 shaft horsepower.  Definitely an experimental ship. 

Profile of the one-of-a-kind USS Tullibee