Mercury - Rudder Angle Gauge Black Face 2-1/8" (52 mm) 79-8M0052851. Overview. The Mercury Rudder Angle Gauge (79-8M0052851) shows your rudder position at a glance, giving you better steering awareness and control. Whether you're docking, running narrow channels, or handling a large inboard, this gauge lets you know exactly where your rudder is pointed. The black face, bold white markings, and glass lens offer clear visibility in any lighting, and the standard 2-1/8" (52 mm) fit makes it easy to