Single & Double-hulled submarines demystified
Submarines can be divided into those with a Single Hulls and those with Double Hulls. The former has only one layer of steel between the crew and the open ocean while the latter has an external outer hull which completely encases the occupied inner hull. The relative merits of these two approaches is hotly debated on the Internet, almost as much as Air Independent Power (AIP). This article hopes to demystify the topic and provide a useful reference guide. It is written from an Analyst’s perspective and intended for a general audience.

From a Defense Analysis standpoint, working out whether a new class of submarine has Single or Double Hull construction can help with calculations. Double Hulled submarines look 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, thus we can start to asses the submarine’s combat capabilities.

When I first see a new class of submarine I can generally tell whether it is Single-Hull or Double-Hull. Here are some of the conscious variable I use. Comments and corrections welcome.

Pros & Cons
The sides of the debate mostly form along national lines. Some countries (mostly Western) build Single-Hull submarines while others (mainly Russia and China) build Double-Hull submarines. Planners and Naval Architects on both sides have their reasons, and may not agree on the relative trade-offs.

In general, Double-Hull submarines allow for a more streamlined hull form because the outer hull can be smooth even if the inner hull is of varying diameter. On the flip side, they are typically slower and more expensive to build.

Myths
It is sometimes argued that Double Hulled submarines are stronger and can dive deeper. This is not logical single it is the strength of the inner watertight pressure hull, where the crew is, which defines the crush depth. It is true that some Double Hull submarines have strong ‘armored’ outer hulls, but generally the outer hull does not make a submarine more survivable either.

Another misconception can be that Double Hull submarines are inherently quieter due to the extra spacing between the machinery and the outer hull. This may appear to make sense on some levels but has no real bearing on the actual noise signature of a submarine since quietening takes many forms. Many Soviet-era double-hulled submarines were incredibly noisy.

The basics

A section of USS Minnesota (SSN-783) showing the Single-Hull construction. Note the reinforcing rings ('frames') on the inside of the hull.

Double-Hull construction has the Frames on the outside of the pressure hull, encased in a smooth outer hull. The Inner hull is typically about 80% of the diameter of the Outer hull.

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Double Hull construction of a Russian Project 636 KILO Class SSK:

Some older Single-Hull submarines have saddle tanks grafted onto the outside. At first glancce these can appear like Double-Hull construction but critically the saddle tanks do not fully enclose the occupied pressure hull:

Notes by Country

Australia
Swedish (Collins) and French (Shortfin-Barracuda) designs. Single-Hulled.

China
Based on Russian and French influences. Double-Hulled. Possible exception is 40m midget submarine.

Yuan Class SSK, showing double-hull features.

France
Single-Hulled.

Germany
Single-Hulled.

Iran
Single-Hulled.

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Italy
Single-Hulled.

India
Mixed fleet. German, French and Russian designs. Indigenous SSBN/SSN program is Double-Hulled.

Japan
Single-Hulled. Some Double-Hulled designs postwar.

Netherlands
Double-Hulled.

North Korea
Single-Hulled. Some legacy Soviet Double-Hulled designs still in service (ROMEO/MING)

Pakistan
Single-Hulled. Italian designs (Cos.Mo.S).

Russia (Post-USSR)
Double-Hulled. Exceptions include LADA Class SSK and possibly YANSEN Class SSGN.

LADA Class SSK. Single Hull construction, note the raised casing running along the top.

South Korea
Single-Hulled.

Spain
Single-Hulled.

Sweden
Single-Hulled.

UK
Single-Hulled.

USA

USS Albacore, the first teardrop hulled submarine which set the scene for modern types. Generally regarded as a Double-Hull submarine but actually with a section of Single-Hull construction towards the stern.

USS Skate, a 1950s first-generation nuclear powered attack submarine. Single-Hull construction but with extensive casing along top.

USS Skipjack, the first modern US Navy attack submarine combining the teardrop hull of Albacore with nuclear power. Unlike the albacore however the hull is Single-Hull.

USS Los Angeles - Single Hull.

USSR

First generation NOVEMBER Class SSN: Double-Hull.

ALFA Class SSN: Double Hull.

CHARLIE Class SSGN. Mostly double hull, but unusually for a Soviet boat the central sections are Single Hull.

Yugoslavia
Single-Hulled.

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