Single Hull Vs Double Hull Submarine, The Debate Continues

  

 

Submarines can be divided into 2 classes by construction, these are single hulls and double hulls. The single-hull submarines have only one layer of steel between the crew and open ocean while the double Hull has an external outer hull that completely encases the inner hull. The relative merits and demerits of these two submarine construction approaches are still hotly debated. The German XXI SSK submarine during the World War 2 was the first submarine that had its entire hull encased inside.

Some countries (mostly Western) build single-hull submarines while others (mainly Russia and China) build double-hull submarines. Planners and Naval Architects have their own reasons, and may not agree on the relative trade-offs. Double-hulled submarines are a lot larger on the outside than they are on the inside, so it is not possible to estimate displacement without factoring in this variable. From there we can start to deduce the internal arrangement and calculate things like the weapon fraction to assess the submarine’s combat capabilities.

In general, double-hull submarines are ideally used in combat situations, where they are less likely to be sunk by enemy torpedoes. It allows for a more streamlined hull form as the outer hull can be smooth even if the inner hull is of varying diameter. But they are typically slower, noisy, and more expensive to build and maintain. Also, they are larger and heavier in size, which makes them much more difficult to maneuver than single-hull submarines.

It is argued that double-hulled submarines are built stronger and can dive deeper. This is not logical since it is the strength of the inner watertight pressure hull, where the crew and equipment are stationed, that withstands the crush depth pressure. The outer hull is the structural hull, which gives the submarine its shape and protects the inner hull from damage. It is true that some double-hull submarines have strong ‘armored’ outer hulls, but generally, the outer hull does not make a submarine safe at greater depths.

Another misconception is that double-hull submarines are quieter and more streamlined due to extra space between machinery and the outer hull. Though double hulls are much more resistant to damage than single hull submarines, it has no real bearing on the actual noise signature of a submarine as quietening can be done in many other forms. Many Soviet-era double-hulled submarines of the 1960s and 70s have been known to be incredibly noisy (it may be due to the fact that the Soviet Navy emphasized primarily combat survivability) and so are most current Chinese submarines that are built on Soviet-era design.

Double-hulled submarines have been built since the early 20th century with French and Swedish coming out with such submarines. However, after World War II Soviet submarines have been traditionally built as double-hulled except possibly for the Lada class and Yassen class submarines, whereas nearly all western submarines were single-hulled; a practice abandoned by the US in the mid-1960s. Before and after the end of the Cold War, the Soviet Union and now Russia is still the most active submarine-producer nation in the world, churning out maximum number of submarines.

A submarine pressure hull needs to be strong enough to withstand the enormous pressure of the ocean depths. Calculation of the pressure the submarine hull can withstand is done using the formula P = F/A. P is the pressure in pounds per square inch, F is the force in pounds, and A is the area in square inches. As such double-hulled submarines have an increased chance of survival, yet there is a notion that the Soviet submarines were “technologically inferior” to the US submarine design.

Survivability has 2 aspects, first is “immune to attack”; second is “incapable of being damaged.” Though built to be more survivable, a fact remains that the most significant number of Soviet submarines though based on the second aspect, have been lost at sea after World War 2 due to some kind of problem. On the other hand, the USA believes in the first aspect, for an invulnerable submarine; is the one that is undetectable thanks to its stealth, safety, and reliability has lost a far smaller number of subs. As such the USA gave up on double-hull submarine design in the early 1960s as it would have resulted in a larger hull with increased drag requiring a larger power plant to maintain the same military capability.

Soviet/Russian double-hulled submarines have in turn focused on increased survivability, increased useful volume, reduced costs, and signature reduction. A fact is that a double hull design allows for the use of external stiffeners for framing, then internal stiffeners in a single-hull submarine, leading to space savings within the inner pressure hull allowing for air bottles, auxiliary propulsors, heat exchangers, and extra tanks being stowed between the inner and outer hulls.

Double-hulled submarines also help reduce costs. The exterior hull, which is not a pressure hull, may be constructed of much thinner plates with the hydrodynamic shape being only important, whereas the inner pressure hull can then be formed in a structurally desirable shape better adapted to pressure deformation due to increased depths. Also, it allows for retrofitting of future advances something that may not be possible with single-hull submarines where the thick pressure hull sections themselves would have to be shaped as per requirement.

The volume (V) or displacement of a submarine is given by Π r2 L. It means a US single-hull submarine with 3000 tons displacement will have a wetted surface area of 1711 meters if the length-to-beam ratio is 11. On the same scale, a double-hulled submarine with the same wetted surface area will have a 28% increase in submerged displacement over a single-hulled submarine if the /D ratio is reduced to 7.

Learning from the past in recent years, the Soviets/Russians have begun to field an impressive “new generation” of nuclear-powered submarines with improved combat survivability, quieting, submerged speed, dive depths, and combat support systems; which have virtually eroded the technical advantage single hulled US submarines otherwise had. The Soviets/Russians achieved their submarine design goals by using off-the-shelf and advanced technologies, and have rarely required Western technologies. They have, however, benefited immeasurably from war-winning information provided to them by spies about Western submarine programs.

Overall advantages and disadvantages of double-hulled submarines can be summarized as follows: -

Advantages

1. Better outer hull fairing possible; decreased drag.

2. Pressure hull protection; ASW weapon effect sur­vivability; frames tear from pressure-hull, allowing shell membrane strength to develop.

3. Increased hull protection during torpedo exer­cises, grounding, or surfaced collisions.

4. Superior main ballast tank survivability.

5. Easier and less costly fabrication costs of exter­nally framed pressure hulls.

6. Use of inner-hull standoff distance for degaus­sing coils, remotely operated vehicles, communi­cation buoys, air bottles, ASW weapons and countermeasures, and hull coatings (anechoic and weapons protection).

7. Increased flexibility during the design stage and later modifications.

8. Better use of pressure-hull volume if the pressure-hull is externally framed.

   Disadvantages
 
1. Increased weight for a given pressure hull volume, unless high-strength and/or lightweight materials are used.

2. More corrosion and maintenance problems with steel double-hull submarines.

3. Equipment stowed outside the pressure hull must be designed with high-pressure casings.

4. Improperly designed MBT vent shutters can cause outer-hull flow-induced resonances and vibration.

5. Increased maintenance and possibly overall higher construction costs.

6. Poor crew habitability in smaller hulls.

Although the USA is expected to continue to maintain a comfortable lead in the number of high-quality submarines for another 15 years over the Russians; the truth is that US and Russian submarine designs are today nearly at par in several important areas. Therefore, US and Russian submarine developments in the future will depend on their respective countries’ longer-term strategies to rule over the ocean waters.