Walking The Prop
Prop walk, the tendency of a turning propeller to push a boat’s stern sideways, can be a real nuisance when maneuvering under power. Or it can be your biggest ally. The trick is to understand it, anticipate it, and make it work for you.
Also known as “asymmetric blade thrust,” prop walk affects most single-engine vessels. It occurs when a prop and its shaft are not perfectly parallel to the water’s surface. When the shaft is angled downward, as most are, the cylindrical distance traveled by the propeller blades on their upstroke is greater than on their downstroke. The upstroke pushes more water, generating more thrust on that side. This, coupled with lateral deflection off the hull, pushes the boat’s stern sideways, pivoting the vessel around a point located about one-third of the boat’s length aft from the bow.
Prop walk is much more noticeable in reverse and is more pronounced at slow speeds and at high RPMs. It is less pronounced when a prop is in shallow water (i.e., on a boat with shoal draft), as this reduces upward water flow from beneath the vessel. The amount of clearance between the propeller and hull has an impact, as does the hull’s shape. Increased wheel diameter or blade pitch increases prop walk. Two-blade props walk less than 3-blade props; folding and feathering props walk less still. A moving vessel’s yaw rate can negate or augment prop walk, as can the wind, depending on its direction and strength. An offset shaft may alter the effect, and when a shaft is parallel to the water’s surface, as in a saildrive, its prop doesn’t walk at all.
When driving forward you can easily compensate for prop walk with your rudder, so you hardly notice it. In reverse your rudder is much less effective, so prop walk is more obvious and more difficult to control. To compensate, or to take advantage of it, you must anticipate its force and direction.
First you need to know whether your boat’s propeller is right- or left-handed. The letters RH (for right-hand) or LH (for left-hand) are usually stamped on a prop, along with its diameter and pitch. If you have not noted this information, it’s easy to determine in the water. Simply watch which way your prop and/or shaft turns in forward gear. If they turn clockwise (viewed from astern), your prop is right-handed; if counter-clockwise, it’s left-handed.
If you cannot see your prop or shaft, observe the direction of the prop’s wash on the water’s surface. Put your engine in reverse at one-quarter throttle while tied to a dock and look over the stern quarters. If you see prop wash on the starboard side, then your propeller is right-handed and your stern will walk to port. If the wash is on the port side, your prop is left-handed and will kick the stern to starboard. You can conduct the same test underway by bringing your boat to a full stop dead downwind or in calm conditions. Shift to reverse, give her a few seconds burst of throttle, and observe whether the stern moves to port or starboard.
Pivoting a Boat
A common situation where prop walk can work for or against you is when maneuvering a single-screw vessel in tight quarters. Using prop walk, you can actually pivot a boat in place, or nearly so.
Always turn a boat with a right-hand propeller to starboard, rotating her clockwise. Put your wheel hard over to starboard (or push your tiller hard to port), then give the throttle a sharp 1- or 2-second burst of power in forward. The prop wash hits the cocked rudder and begins turning the boat. Since the idea is to turn in place, don’t stay in forward gear for long. Throttle down to idle, shift to neutral for a moment, 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, increasing the lateral rotation. Idle down, shift back to neutral, and then into forward again.
Keep repeating this sequence, alternating throttle 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 should have no effect in reverse, since the boat’s not making sternway—and always spend a moment in neutral between each shift to spare the transmission any sudden jolts.
This technique will spin most boats in their own length, plus a little—a bit less for fin-keel boats, more for longer keels. It only works pivoting to starboard with a right-handed prop or to port with a left-handed prop. If you try to pivot the wrong way, prop walk will work against you each time you power up in reverse, slowing or even thwarting the turn. If you must turn a right-hand prop boat 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 feel of it.
Docking With Prop Walk
It is easier to dock a boat with a right-hand prop port side to (or starboard side to with a left-hand prop). Aim for a point on the dock about one-third of a boat length aft of 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 hard away from the dock and simultaneously shift to reverse and throttle up sharply, enough to stop the boat’s 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, then the prop walk kicks the boat’s stern in towards the dock and 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 they snub up hard on that line, the boat will stop short, the bow will smack into the dock and the stern will swing out, ruining the approach. Instruct your crew beforehand to keep dock lines (other than a midship aft spring line, perhaps) on board, or at least slack, until the boat has come to a complete stop alongside.
If you must come to a dock wrong side to (i.e., starboard side to with a right-hand prop or port side to with a left-hand prop), you should approach at a shallower angle, say 10 to 15 degrees. When the bow is about a quarter of a boat length off, turn hard away from the dock, and this time give the throttle a quick burst in forward gear. Then shift to reverse and give her just enough throttle to stop the boat. The forward burst will thrust water on to 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.
Follow the same principles when backing into a slip. Pivot your boat until it lays at a 20 to 40 degree angle just outside of the slip mouth. Next engage reverse 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 decrease 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.
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—cocked to starboard if the prop is right-handed, to port if left-handed. The angle will quickly by corrected by prop walk as you begin backing down. By the time the boat straightens out she is making sternway and the rudder is biting. Steer against the prop walk to compensate for it. Once the boat is moving along nicely, lower the engine RPM. This reduces prop walk, keeps the boat moving, and lets the rudder steer.
Making the most of prop walk requires timing, feel and finesse. As you practice these techniques, you must of course adjust for wind, current and any obstacles. Practice in open water until you’re comfortable and confident. In the end, the more you use prop walk, the more you will come to appreciate rather than dread it.
Tor Pinney has logged 150,000 miles under sail and is the author of
Ready for Sea: How to Outfit the Modern Cruising Sailboat
You can check out his website at tor.cc