Direct-coupled propulsion

Direct-coupled propulsion connects a low-speed two-stroke crosshead engine straight to the propeller shaft with no reduction gear, so the engine and the fixed-pitch propeller turn at the same speed, typically 70 to 120 rpm. The large bore-and-stroke two-stroke is built to run at propeller speed and to reverse for astern by restarting in the opposite direction, removing the gearbox, its losses, and a reversing mechanism. This is the standard merchant-ship arrangement for bulk carriers, tankers, and container ships, prized for high propulsive efficiency from a slow large-diameter propeller and high thermal efficiency from the two-stroke. The trade-off is engine height and weight, and a barred speed range to avoid torsional resonance.