Prop walk, also known as paddlewheel effect or asymmetric blade thrust, is the tendency of a propeller to push a boat’s stern sideways. It’s most noticeable in reverse on single-screw vessels, and it can make close-quarters maneuvering, including docking, a daunting challenge.
Or, prop walk can become your ally, giving you absolute control when it matters most. The trick is to understand it, plan for it and make it work for you.
Prop walk is a result of the angle of the propeller to the water’s surface. If the prop is not perfectly perpendicular—if the shaft is angled slightly downward, as most are—then the cylindrical distance that the propeller blades travel on their upstroke is greater than on their downstroke. As a result, the upstroke pushes more water, generating greater thrust on that side. Coupled with lateral deflection off the hull, this effect pushes the boat’s stern sideways, pivoting the vessel around a point about one-third abaft the bow.
Slow speeds and high rpm amplify prop walk, as do greater propeller diameter or pitch. Shallow water, on the other hand, reduces upward water flow from beneath the vessel, and can reduce or even cancel out prop walk.
Likewise, the amount of clearance between the prop and the hull has an impact, as does the hull’s shape. Two-blade propellers generally produce less walk than three-blade props; folding and feathering props, less still. The yaw rate—the side-to-side movement of the bow and stern of the ship underway—can either negate or augment prop walk, as can relative wind direction and strength. An offset shaft may also alter the effect.
However, if a prop shaft is perfectly parallel to the water’s surface, as in saildrives, there is no prop walk at all. Clearly, the phenomenon can differ widely from boat to boat, and from one occurrence to the next on the same boat.
Asymmetrical blade thrust occurs in forward and reverse, in opposite directions. However, in forward gear, the rudder easily compensates for it, so you hardly notice it. In reverse gear, the rudder is much less effective, so the blade thrust is more obvious and harder to control.
Anticipating this sideways shove and its direction is a prerequisite to knowing how to compensate for it or, in some situations, to turn it to your advantage.
First, you need to know which way your boat’s going to walk. It’s going to be to port or to starboard, depending on whether the propeller is right-handed or left-handed (the direction it spins). Twin-screw boats typically neutralize prop walk by having one right-handed propeller and one left-handed prop; they walk in opposite directions, canceling out the effect of each.
The initials RH (for right-handed) or LH (for left-handed) are usually stamped on the prop, along with its diameter and pitch. That’s how you can tell which type you’re dealing with. If the initials aren’t there, it’s easy to determine a prop’s direction in the water. Simply watch which way the prop turns when the transmission is put in forward gear. If it’s clockwise when viewed from astern, then the prop is right-handed. Counterclockwise means left-handed.
If you cannot see the propeller, then you can deduce its rotation by observing the direction of the prop wash on the water’s surface. While dockside with the engine idling, shift into reverse. Wait a moment to let the dock lines take the strain, and then accelerate to about one-quarter throttle. Then, look over the stern quarters, port and starboard, and see which side has turbulent water. If the prop wash is to starboard in reverse, then the propeller is right-handed and it will walk the stern to port. If the turbulence is to port in reverse, then the propeller is left-handed and will kick the stern to starboard.
You can conduct the same test underway by bringing the boat to a full stop in calm conditions or dead downwind. Shift to reverse, give her a few seconds’ burst of throttle, and observe whether the stern moves to port or starboard. Whichever it is, it will always be the same direction on that boat.
Now that you know what prop walk is, what it does, and which way it’s going to turn your boat, you’re ready to put the effect to work for you.
In a crowded marina or narrow channel, it is sometimes necessary to turn 90 degrees or more with little room. Using prop walk, you can pivot a boat in place, or nearly so.
Always plan to pivot a boat with a right-handed propeller clockwise, to starboard. That way (and only that way), you’re using the prop’s starboard thrust in reverse to push the stern to port, aiding the clockwise pivot.
Turn the wheel hard over to starboard and hold it there. Then, give the throttle a sharp one- or two-second burst of power in forward. Since the idea is to remain in one spot while turning, don’t stay in forward gear for long. Throttle down to idle, shift to neutral for a moment, and then shift to reverse and give her another strong burst of power. This stops the boat’s forward motion and kicks the stern to port, enhancing the lateral rotation.
Next, idle down, shift back to neutral, and then put her into forward again. The pattern is power burst, idle, shift and so on, never letting the boat gain momentum in either direction. Keep repeating this sequence, alternating power bursts in forward and reverse, until the boat has spun to the desired heading.
Throughout this maneuver, keep the helm turned to starboard (the rudder would have no effect in reverse because the boat’s not making sternway) and always allow a moment of idle neutral in between each shift to spare the transmission sudden jolts.
This technique, called back and fill, will spin most boats in their own length plus a little, or a bit more for longer, deeper keels. It only works rotating to starboard with a right-handed prop, or to port with a left-handed prop. If you try to pivot the other way, the propeller effect will work against you each time you power up in reverse, slowing or even preventing the turn.
If you must turn a right-handed-prop vessel 90 degrees to port in tight quarters, you’ll probably be better off spinning her 270 degrees to starboard. Practice pivoting your boat in open water until you get the hang of it.
Similarly, it is easier to dock a boat with a right-handed propeller portside-to (and a left-handed prop starboard side-to).
Aim for a point on the dock about a third of a boat length abaft where you want the bow to wind up. Approach at 1 to 1½ knots, at a 30- to 40-degree angle. When the bow is about a quarter of a boat length away, turn the helm hard away from the dock and simultaneously shift to reverse and throttle up sharply, enough to stop the forward motion. Then, throttle down and put her in neutral.
The cocked rudder begins to swing the bow out while the boat is still moving forward, and then the prop walk kicks the boat’s stern in toward the dock. The vessel comes to a halt neatly alongside.
A word of warning: Sometimes an overzealous line handler will get a bow line to the dock while the boat is still moving forward during this maneuver. If he or someone ashore then snubs up hard on that line, the boat will stop short, the bow will abruptly smack into the dock, and the stern will swing out, ruining the approach. Instruct your crew beforehand to keep dock lines (other than an amidships-aft spring line, perhaps) on board, or at least slack, until the boat has come to a complete stop alongside.
If you must put the side of the boat to the dock that is not favored by reverse-gear prop walk, then approach at a shallower angle, say 10 degrees to 15 degrees. When the bow is about a quarter of a boat length off, turn the helm hard away from the dock, this time giving the throttle a quick burst in forward. Then, shift to reverse and give her just enough throttle to stop the boat.
The forward burst will thrust water onto the cocked rudder, giving it extra turning power, followed by the prop walk in reverse, which cancels the bow’s movement away from the dock by kicking the stern out a little. Since the pivot point is about two-thirds of the boat’s length forward of the stern, the boat will swing parallel to the dock as she stops.
Backing into a slip follows the same principles. By backing and filling, position the boat at a 20-degree to 40-degree angle just outside of the slip’s mouth. Next, engage reverse gear and allow the prop walk to straighten the boat as she starts making sternway. Then, throttle down to idle reverse or shift to neutral to lessen or eliminate the walking, and steer with the rudder as the boat eases back into the slip.
If necessary, you can readjust the angle midway with a quick burst of power in forward (with the rudder cocked) or reverse, effectively pivoting the boat just enough to realign her with the slip.
Naturally, practice makes perfect. So, practice.
To steer a single-screw vessel in reverse, it is usually best to get her moving gradually. When possible, start with the stern at an angle to the desired direction (to starboard if the prop is right-handed, to port if it’s left-handed). The prop walk will quickly correct this angle as you begin backing the boat in reverse gear.
By the time the boat straightens out, she is making sternway and the rudder begins to bite, gaining steerage. Use that force to steer opposite to the prop walk, to compensate. Once the boat is moving adequately in reverse, lower the engine rpm. This reduces the paddlewheel effect, keeps the boat moving and lets the rudder steer.
Making the most of prop walk requires understanding, timing, feel and finesse. As you practice these techniques, expect to adjust for wind, current and obstacles. Practice in open water until you’re comfortable and confident, and then practice some more at wide-open, uncrowded docks with fenders down.
Using prop walk to your advantage will soon become second nature. And the more you use it, the more you will come to appreciate rather than dread it.