Standard Configuration of Counter-Rotating Propellers

In the vast majority of twin-engine marine vessels, the Left-Hand (LH) propeller is located on the Port side, while the Right-Hand (RH) propeller is located on the Starboard side.[1] [2] This specific arrangement is known as "outturning" or "turning outboard," because the tops of the propeller blades move away from the vessel's centerline when the engines are in forward gear.[3] [4]

The Mechanics of Outturning Propellers

The primary reason for placing the LH propeller on the port side and the RH propeller on the starboard side is to improve vessel handling, particularly during low-speed maneuvers and docking.[5] When propellers are configured to turn outboard:

• Propeller Walk: In forward gear, the transverse thrust (propeller walk) of each propeller cancels the other out, allowing the boat to track straight without constant steering corrections.[6] [7]
• Maneuverability: When one engine is in forward and the other is in reverse (a maneuver used to "spin" a boat in its own length), the outturning configuration provides a wider "lever arm" effect.[8] The propeller walk of the reversing engine pulls the stern in the direction that assists the turn, making the vessel more responsive to differential thrust.[2] [9]
• Stability: Outturning propellers generally provide better directional stability at cruising speeds compared to "inturning" configurations (where the LH prop is on the starboard side).[10]

Identifying Propeller Rotation

To determine which propeller is which, maritime experts use several physical checks. A Right-Hand (RH) propeller rotates clockwise when viewed from astern (behind the boat) looking forward.[11] Conversely, a Left-Hand (LH) propeller rotates counter-clockwise from the same perspective.[1] [12]

Physically, if you lay a propeller flat on a table, a LH propeller's blades will slant from the top-left toward the bottom-right.[1] Another common method is the "hand rule": if you place the hub in your palm and your left thumb naturally follows the slant of the blade, it is a left-hand propeller.[1]

Engine vs. Transmission Rotation

In modern marine engineering, the distinction between "engine rotation" and "propeller rotation" is critical. Historically, manufacturers like Detroit Diesel produced "mirror-image" engines where the crankshafts actually turned in opposite directions.[13] In those setups, the port engine was typically a standard rotation (LH/counter-clockwise when viewed from the flywheel) and the starboard was a reverse rotation (RH/clockwise).[2] [14]

However, most contemporary marine diesel and gasoline engines are "Standard Rotation" (Left-Hand).[2] The counter-rotation required for the propellers is now almost exclusively achieved through the marine transmission (gearbox) or the lower unit of an outboard motor.[1] [15] One transmission is shifted into forward gear using an idler gear or a specific clutch pack that reverses the output shaft's direction relative to the engine's crankshaft, allowing both engines to be identical for easier maintenance and parts sourcing.[2] [13]

Exceptions and Specialized Applications

While the LH-Port/RH-Starboard setup is the industry standard for over 99% of recreational and commercial twin-screw vessels, exceptions do exist:[13]

1. High-Speed Racing: Some ultra-high-performance racing boats utilize "inturning" propellers (LH on Starboard, RH on Port) to help "lift" the stern or adjust the boat's attitude at extreme speeds, though this makes docking significantly more difficult.[10]
2. Tugboats and Workboats: In specialized towing vessels with very large propellers and rudders, the rotation may be customized based on the specific hydrodynamic interaction between the hull and the propeller wash to maximize "bollard pull" or flanking capability.[9]