The cambered skeg featured on select Mercury Racing® gearcases is a solution to problem rooted in physics and hydrodynamics.
As the propeller rotates in the water it creates forward thrust to propel the boat but it also creates a pulling force in the direction of its rotation. In the case of a standard-rotation gearcase (right-hand, or clockwise when viewed from the rear of the boat) this force pulls the outboard or outdrive right and causes the boat to turn to the right. To maintain course the operator must steer to the left. This pull to the direction of prop rotation is often referred to as “fighting prop torque” or “torque steer.” It may be especially pronounced on bass boats and smaller performance craft running at very high trim angles.
This is a phenomenon that afflicts all single-propeller drives, even small outboards. We are often warned of the danger of being thrown overboard without wearing an ECOS (Engine Cut Off Switch) lanyard – as the boat will immediately begin to circle to the right and may come around and strike the person in the water.
Torque steer is not an issue for dual-propeller drives such as the Mercury® MerCruiser® Bravo III because the counter-rotating propellers create torque in both directions and cancel each other. Multi-engine boats have counter-rotating props for the same reason and are often rigged with tie bars which stabilize the motors or drives.
A skeg that is either cambered or offset or port or starboard (depending on prop rotation) is an option for Mercury Racing Sport Master gearcases and is a standard feature on the Mercury Racing 5.44" HD for 450R and 300R outboard models. This creates high pressure on that side of the skeg, which counteracts the pulling force of the propeller in a single-engine installation.
Boats rigged with Mercury Racing Joystick Piloting for Outboards (JPO) require a gearcase with a cambered skeg on the side of prop rotation on each outboard because propeller torque will cause the hydro-electric power steering system to constantly work to correct for that pull. This ampere draw increases the boat electrical demand. Because JPO requires the motors to steer independently when JPO is active, the motors cannot be rigged with tie bars.