The Rapier was the first of Napier's H cylinder engines. The rationale for the H is fairly straightforward, in that rather than having an engine with fewer large cylinders, more small cylinders could simply be added. It was believed that an H pattern engine would provide substantially more power and higher RPM's for the same frontal area as a large liquid-cooled V engine. The maximum RPM in a dive was 4,800.
The H-block has a compact layout, as it essentially consists of two vertically opposed inline engines lying one beside another driving side by side crankshafts. Another advantage is that since the cylinders are opposed, the motion in one is balanced by the opposite motion in the one on the opposite side, leading to very smooth running. The Rapier suffered many of the same problems as the later Dagger and Sabre engines. The Fairey Seafox and Short S.20 were both powered by the Napier Rapier.