How Sackville's triple-expansion engine reused steam

The engine admits steam alternately above and below each piston, making every stroke productive. Steam then expands through high-, medium-, and low-pressure cylinders of increasing volume so the plant extracts energy repeatedly before exhaust. A shared crankshaft converts the pistons' motion into rotation for the ship's single shaft and screw. The arrangement favored efficient long-distance steaming at convoy speed over the greater high-speed power of a turbine. Its roughly 3,500-nautical-mile design range at 12 knots suited an escort that had to remain with merchant ships across long passages. Two boilers fed that single engine, shaft, and screw, so there was no second propulsion line if the engine or shaft failed. Grease lubricated the moving parts and helped protect the machinery in a saltwater environment.