Engine-driven boats with a propeller generally suck water into the propeller (“suction screw current”) and eject water from the propeller (“discharge screw current”), but there is third effect due to the propeller’s rotation known as “unequal blade thrust,” and this has an effect on boat handling commonly known as “prop walk.”
All boats have their own unique handling characteristics based partly on how much draft is exposed to current; how much topsides area and other above-water features are exposed to the wind; by water flowing over the rudder; by rudder size and placement, as well as by engine configuration: single screw inboard, twin screw inboards, or outboard(s) and sterndrive boats. Using your own boat’s particular characteristics to your advantage takes some understanding and practice. One thing all boat owners ought to know is what direction their prop turns when the boat is going ahead. If it turns clockwise when looking at it from astern, you have a “right-hand prop” which is the case for most boats. If it turns counter-clockwise it’s a “left-hand prop”.
The reason you’ll want to know if you have a right- or left-handed prop is to understand your boat’s inherent handling characteristics, particularly when performing slow speed maneuvering, such as docking and undocking. All boats with right-hand props tend to kick the stern slightly to starboard when going ahead and to port when going astern. Again, this is usuallyreferred to as prop walk.
According to Chapman’s Piloting Seamanship and Small Boat Handling , a contributing factor for “unequal blade thrust” is the angle of the propeller. For this reason prop walk is generally more pronounced on single-screw inboard boards where the angle of the prop and shaft contribute to prop walk. Some outboard boats with relatively level props may not see the effect as much, particularly since you may also unconsciously compensate by turning the engine slightly to assist steering. Regardless of the engine setup on your boat, learning to take advantage of prop walk will help your maneuvering abilities.
My summer job as a schooner captain requires that I back down a long fairway between docks (nearly 300 feet) with boats on either side, using the boat’s single screw engine—so I have my own experience with prop walk. Without the aid of a bow-thruster I would be forced to back and fill, back and fill, to back down the fairway without incident. My own experience shows that gunning the engine to get moving creates more walk, and that if I take it slower, the prop is actually more efficient and kicks the stern less. Try varying speed while reversing your own boat and you’ll see what I mean. Of course, a maneuver like coming alongside, such as in a launch, can take advantage of prop walk by purposely gunning it in reverse to kick the stern to port (or to starboard if you have a left-hand prop). So knowing your own boat’s characteristics and the existence of prop walk can improve your boat handling.
One final note: on most twin-screw boats (even outboards) the ideal arrangement is to have the tops of the blades move outward when going ahead, meaning the port engine will have a left-hand prop and the starboard engine a right-hand prop, but it is still worth checking your own boat’s prop rotation so you understand its prop walk characteristics and effect on boat handling.