A doorway interrupts the pressure hull
A submarine’s pressure hull is designed to carry the immense, even squeeze of seawater around it. Cut a hatch into that shell and the load can no longer flow smoothly through the metal: it has to bend and divert around the opening. The edges of the cut become places where stress can concentrate, much as a small notch in a sheet can make it easier to tear.
Engineers reinforce the area around a hatch so the hull can carry that redirected load. But reinforcement takes space and adds weight, and the hatch itself must close securely against a pressure-tight seat. A larger opening therefore means more than a bigger lid; it can require a heavier, more complex section of hull.
The tradeoff continues inside
Crew members need to move through hatches, sometimes in bulky equipment, and the boat needs room for fittings, pipes, and systems around them. Designers balance those practical needs against the structural cost of making each opening wider. The result is not simply the smallest hole possible, but an opening sized for its job without unnecessarily weakening or burdening the hull.
That is why passing through can feel awkward: the hatch is a carefully managed break in a structure whose strength depends on staying continuous. Each opening has to serve the people aboard while still allowing the surrounding shell to withstand the sea’s pressure.
