A look around
The American “fleet submarine” of the late 1930s and 1940s had two hulls. The inner pressure hull was circular, a long tube closed at bow and stern, designed to resist water pressure, welded rather than riveted, and divided into compartments. Between the torpedo rooms there were two decks or levels inside of the pressure hull.

Compartments
Starting at the bow inside the pressure hull were:

• forward torpedo room,
• space where officers and chiefs slept and officers ate and on the deck below the forward battery of six-foot high cells,
• control room with radio room at the aft end and below the pump room for machinery and above it the conning tower,
• the galley or kitchen, mess and recreation room, and crew sleeping compartment and below the aft battery,
• two engine rooms each with two Diesels and generators and water distillers in the forward one,
• maneuvering room where electrician mates controlled the flow of electricity to power the submarine and below it four electric motors geared to power two screws,
• the after torpedo room.

The controls for diving, keeping the submarine sealed, and many other functions were in the control room. The conning tower above it was a small pressure hull. In its six-foot high space several men were on duty to steer, read and report sonar, radar, fire control with the TDC, and use the two periscopes.

Above the conning tower was the bridge for control of the submarine on the surface. Aft of the bridge was the opening for drawing air into the hull for the Diesels and for crew’s breathing. Aft of the bridge and “main air induction” was the cigarette deck that after the first months usually included a 20mm gun; a smaller deck in front of the bridge had another 20mm gun. On the main deck was a deck gun, that after the early months was a 4- or-5 inch gun. Nearby was watertight store for shells for ready access.

Outside of the pressure hull was a second hull, with a high clipper bow, streamlined for maximum surface speed. Between these two hulls were tanks for fuel, compressed air, drinking water, and sanitary and other tanks. Between the hulls near the center of gravity of the ship was the main ballast tank and near the bow and stern were trim tanks and tanks flooded to compensate for the weight (a ton) of each torpedo as it was fired to “keep” the submarine “trimmed.”

At the bow and stern diving planes controlled the submarine’s diving and rising by deflecting the sea to raise or lower the bow or stern. The bow planes folded up against the hull when on the surface, while the stern planes remained in place.

Diving
When surfaced, the submarine like any ship floats because it weighs less than the water it displaces, and thus has "positive buoyancy". In World War II often the submarine had almost no positive buoyancy so it could dive more quickly. To dive vents were opened at the top of ballast tanks, releasing air, to admit sea water through openings at the bottom, making the submarine heavier until it had close to "neutral buoyancy" and could be raised or lowered with the diving planes. However as it went deeper, the hull was compressed by sea pressure and had less buoyancy, so the diving officer had to adjust diving planes or how much sea water was in the ballast tanks. When the periscope was raised, that hollow tube changed buoyancy!

To surface compressed air "blows the ballast tanks" clear of sea water so with positive buoyancy she would rise, aided by the diving planes.

The crew dives
To dive crew cleared the bridge, sealing the hatch to the conning tower. The motor machinist mates shut down the Diesels. The electrician mates in maneuvering shifted to battery power for the motors. The main air induction was closed, forward diving planes deployed out, and ballast tanks flooded from the sea. The diving officer in the control room directed the crew controlling diving planes and ballast tanks, making adjustments as needed by flooding or blowing ballast and adjusting diving planes to keep the submarine “trimmed” at the depth required by the skipper in the conning tower above him. There was no room for error or for having to think what to do. Team work and practice enabled all that to happen every time in 35 seconds with no room for failures.

Diving planes were controlled with large steel wheels; movement of the diving planes was power assisted, but when running silent they were manually controlled that required great muscular effort. Planesmen were sometimes relieved frequently so fresh muscles took over.

"Trim dive"
Our submarines at night were usually on the surface to recharge batteries, throw out the day’s garbage in weighted bags, blow the sanitary tank, and woe to the officer of the deck who let the smell be drawn into the submarine. At dawn the submarine normally dove for a “trim dive” so the diving officer knew he had compensated for fuel used and other weight shifts so he could keep the submarine level. As skippers became more aggressive, they surfaced as soon as the diving officer "had his trim" to remain on the surface in daytime to search with radar, sonar, and sight. Always they were ready to dive so as to be unseen.

Today's nuclear submarines do not normally dive and surface frequently. They often dive after leaving port and reaching deeper seas and surface again when necessary to transfer personnel or material onto or off of the submarine and at the end of the patrol weeks or months later.