Cutaway diagram of a Soryu submarine which uses NS110 pressure hull steel. The diagram displays the cylindrical pressure hull shape, which is rounded at the ends. The diagram by "wispywood2344" was in the article of October 26, 2015. A larger version is at http://blog.livedoor.jp/wispywood2344/others/Soryu_cutaway.
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This article is about pressure hull materials, with the costs and benefits of using Titanium (Ti) alloy or Steel alloys. My friend has provided the following comments comparing Japanese submarine steel NS110 for Soryus with the Titanium alloy used for the deep diving Japanese manned research submersible Shinkai 6500.
For Shinkai 6500 Ti alloy was selected due to its light weight. Light weight is important because Shinkai needs to regulate its buoyancy and if it were excessively heavy its range/endurance would be too limited. It relies on limited battery power.
In contrast retired US deep sea submersible NR-1 avoided battery limitations by being nuclear powered and may have been able to dive to 1,000+ meters.
To achieve Shinkai's maximum diving depth of 6,500m [hence "Shinkai 6500"] the pressure resisting configuration of two pressure hull spheres is used.
If fighting naval submarines also used two pressure hull spheres they could also achive 1,000+ meter depth, but they would have severe fighting limitations. For example they could not move at a useful fighting speed of 4 to 20 knots. A fighting submarine uses at least one pressure hull which is shaped like a cylinder (with rounded ends) reinforced by inner or outer frames.
The weight of NS110 hull may be around 30% heavier that of Ti alloy.
So NS110 hull may be able to achieve nearly the same strength as that of Ti alloy. NS110 is no doubt expensive, but Ti alloy is much more expensive.
PETE ADDED BACKGROUND
Titanium alloy is lighter than steel alloy in at least two respects:
1. Ti (per cubic centimeter) is lighter than steel, and
2. Ti alloy can be built thinner than steel alloy to provide the same pressure hull strength.
The Soviets/Russians on a large scale developed Ti alloy for submarine pressure hulls from the 1960s-80s, but there were many downsides. Ti proved expensive to mine and process. Ti alloy was difficult to form/work or roll for pressure hulls. It was difficult to weld. All this made it more difficult to maintain and repair subs.
Most notably the Soviet/Russia's Alfa class SSN used hundreds of tonnes of Ti alloy and may have test dived to 1,300 meters. This was a highly risky glory test which destroyed many fittings (like sonars) that were outside the pressure hull.
Later the huge (48,000 tonnes (submerged)) Typhoon class SSBNs used thousands of tonnes of Ti alloy for their multiple pressure hulls. This Ti alloy formed a major cost component of the 6 Typhoons. But these 6 alone represented one of the Soviets' major defence projects that bankrupted and demorilised the Soviet Union from the late 1980s (more see http://gentleseas.blogspot.com.au/2015/07/previous-use-of-titanium-in-russian.html).
Ti alloy, for submarine pressure hulls, can thus be thanked for helping break up the Soviet Empire. The more practical alternative of high yield steel alloy NS110 (and less known foreign equivalents) exists today. I calculate NS110 may permit an operating depth of 600m (redded further down in this Submarine Mattersarticle of January 20, 2015).
Pete and friends