Determining Drive Height for Your Boat
Published: Monday, October 25, 2021
By: Professionals of Henry H. Smith Co.
The X dimension is an important stern-drive measurement (from the center of the engine output shaft to the bottom of the hull) that determines the distance of the propeller shaft from the boat's bottom. A more universal measurement for propeller height is the vertical distance between a straight edge (yard stick) projecting back from the bottom of the hull to the anti-ventilation plate (often inaccurately called the anti-cavitation or cavitation plate).
Changing the vertical position of the propeller changes the output force in relation to the boat’s center of gravity. Lowering the drive’s depth in the water increases bow lift but also increases drag. Raising the drive decreases drag, which will increase speed and engine RPM, but will reduce bow-lift and can make getting ‘out-of-the-hole’ more difficult.
Standard mounting height will typically have the anti-ventilation plate level with the bottom of the boat. This provides for the best all-round performance but can be optimized by dialing-in the best height for the way you use your boat. For example, moving the anti-ventilation plate 1” to 2” below the bottom of the boat would be helpful for slower, heavy work applications. Conversely, raising the drive-height—so that the anti-ventilation plate is 1” to 3” above the bottom of the hull—will reduce drag and increase speed, making it appropriate for ski and sport boats. Raising the drive so the anti-ventilation plate is 4” or more higher than standard is reserved for high performance applications with a priority focus on speed.
The process of determining the optimum drive height is very subjective to the specific way that you load and use your boat. Changing the drive height can be difficult, depending on the engine, drive, and mount. Jack plates can provide outboard mounting that allows easy adjustments. Dialing-in your boat’s drive height can require repeated trials to get it right but the reward is a boat running with optimized performance, precisely for you.
About the Author
Founded in Detroit by Henry H. and William H. Smith, the company continues to employ 3rd and 4th generation members of the family, carrying on a legacy established in 1900 to be your Marine Drive Specialists.
A version of this article appeared in the Fall Issue (September/October) 2021 of Great Lakes Scuttlebutt magazine.
tags: Boating 101, Propellers, Stabilization
