Submarines from early days to modern (5)

It's amazing how, in the world of submarines, the basics haven't changed. In World War I days, the most advanced submarines, and their predecessors, had many common attributes: a cylindrical shape, ballast tanks to allow the boat to dive, and periscopes. Fifty years later, I served in a Royal Navy A-boat.

Here is a photo of HMS Andrew, typical of her class. The crewmen visible are standing on the free-flooding casing, a shell that allowed safe if often precarious footing when surfaced. The blister-like long shape at the waterline outlines the port ballast tank, and the periscopes are housed in the fin, ready to be raised for use when dived. (The radar mast is raised in the photo.) Her basic design, though, is a sixteen-foot diameter steel pipe. The casing and ballast tanks are external to the steel pressure hull.
Andrew boasted an atypical feature: she was the last RN boat with a deck gun.
The A-boat featured in my upcoming novel set  during the Cold War will also carry a 4" gun.