They were well-streamlined, having no external ballast tanks, casing, or deck gun, and a streamlined spindle-shaped hull of circular cross-section[2] (not reproduced until the American USS Albacore) which tapered sharply towards the stern and allowed only for a single screw.

A single 8-cylinder 480 hp (360 kW) diesel engine was installed for surface propulsion, while high underwater speed was given by two large electric motors arranged one behind the other to drive the single propeller shaft, and powered by a 200-cell battery of the same type fitted to J-class submarines.

[3] Despite being designed for maximum underwater performance, the R-class submarines were extremely difficult to control submerged, especially at high speeds.

[4] As designed, one spare torpedo was allowed for, but in operation six reloads were carried in place of the senior ratings' accommodation.

It was originally intended to fit a 4-inch (102 mm) gun on the foredeck, but this was dropped due to the adverse effect it would have had on submerged speed.