The submarine was long and abeam. "Unrivalled" had a single hull with internal ballast tanks and had a draft of when surfaced. She displaced while submerged, but only on the surface. The submarine was equipped with two diesel engines and twin General Electric electric motors—for surfaced and submerged running, respectively. They were coupled together with a diesel-electric transmission. "Unrivalled" had a surface speed of up to and could go as fast as while underwater. The boat could carry up to of diesel fuel, giving her a range of at . Her electric motors and batteries provided a range of at while submerged.