Are you sure they make them out of titanium to save weight? 
I have two anecdotal pieces of evidence:
1. I have a friend who does welding for the navy and was called in to replace part of the hull of a nuclear sub due to a very hilarious design flaw. It was &gt;6&quot; thick and took days to fill in... with steel.
2. I&#x27;ve read &quot;Concepts of Submarine Design&quot; which goes into great detail on the principles and systems of military sub design. The authors make the case that getting <i>more</i> weight is often a problem. This makes sense because every foot or air space needs to be displaced by more weight in the steel of the sub. So if they had a problem with subs weighing too little, they could just make them more spacious. And they&#x27;re not very spacious.