How do you design something as complex as a submarine

Small nitpick–there is rarely a “sheath” separate from the fuel tanks. The fuel tank is the skin of the vehicle. There may or may not be a cylindrical part that joins the rounded ends of the tanks; sometimes they are joined in a way that there is continuous propellant behind most of the length of each stage (called a “common bulkhead”).

This is a very good question Wesley one that I have often wondered about also. I have served on submarines back at the young age of 19 on diesel submarines and later on to nuclear submarines till I was 28.

My thoughts even as I stood top side watch was how do they build the submarine to be exactly at the level it is while in the water with the sail in the perfect position of port and starboard and not leaning to one side or the other.

In qualifications we learn that the boat has tanks that fill with water to submerge the boat and to surface you blow the water out with high pressure air and then close the tank valve and she surfaces. They call them ballast tanks all carefully designed to balance the whole ship under and above the water. The diesel boat was even harder to understand due to the fuel tanks had to have salt water pumped into them as you use the diesel fuel to balance the sub and keep her on an even keel. They would then separate the diesel fuel from the salt water using fuel water separator’s.

This is how BAE in the UK builds their submarines:


The engineers at BAE use PTC CAD products and turned to Gold PTC Partner Advantage Program member, Virtalis (Manchester, UK) to create, install and integrate Virtual Reality (VR) systems for product development and manufacturing.

Tasked with building three Astute Class nuclear submarines, the engineers decided to accelerate the process using a VR system instead of creating physical prototypes. The VR system had to be easily accessible both to the engineers, as well as to welders and pipe fitters, who would physically build the submarine.

For design, the engineers used the PTC CADDS 5 shipbuilding program, a tool that enables groups of engineers to work simultaneously on design, validation, and machining of the same assemblies. For the VR system, they commissioned Virtalis to integrate the PTC Division MockUp program with Virtalis’ VR tools.

In a large 3D stereoscopic theater, this program exposes potential clashes between submarine components and assemblies. If needed, a redesign can be performed prior to the build.


Designing this into the sub (or any other vehicle) requires accounting for all of the fixed mass. With 3-D solid-modeling programs, this is much easier than it used to be. I’m most familiar with SolidWorks, in which you can design a 3-dimensional part of arbitrarily complex geometry, and as soon as you specify the material type, it will tell you the total mass of the part and the 3-D location of its center of mass. When you put various parts together in an assembly, the program keeps track of where all that mass is, so you’ll know in the end whether you’ve got a nicely balance submarine or whether you need to move some mass around. The software can account for the mass of every nut, bolt, washer, pipe, hose, electrical wire, connector, and other hardware.

This of course does nothing to account for how the movements of the submariners or any stores or munitions within the sub changes the center of mass (google “submarine trim party” for fun). Presumably this is where ballast tanks and/or control trim comes into play. Design software makes it easy to determine how big and where the tanks and/or control surfaces should be to account for various worst-case mass distributions - or conversely, what kinds of mass distributions can be tolerated by a given vessel design.

Sometimes the answer is “they don’t.

Allegedly, when the Germans took possession of some French submarines after the fall of France in World War II, they were suspicious they’d been duped by the French authorities because some of the bolts in the French submarines tightened clockwise and some counter-clockwise. The orderly German engineers couldn’t imagine anyone designing a ship with different directions of rotation for its fasteners.

I don’t know the reasoning behind the French design, but my guess would be it made economic sense at the time somehow (or was “not worth the cost of fixing”).

Could it be that vibrations tended to loosen the bolts and those vibrations turned the bolts in opposite directions in different parts of the ship?

Could be. Or it could be normally-open and normally-closed bolts. Or that the construction process preferentially allowed tightening in one direction.
But The whole thread-cutting business was once much less standardised than it is now, and only gradually changed. Even in the 1930’s.