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Article by Tor Pinney                                                                                                                                        Back to List of Tor's Tips

                  

Prop Walk
2013 Tor Pinney - All Rights Reserved

When the propeller kicks the stern over, use it to your advantage.

 

Prop walk, the tendency of a propeller to push a boat's stern sideways, can be a real nuisance when maneuvering under power. Or it can become your ally. The trick is to understand it, plan for it, and make it work for you.

Also known as propeller or paddle wheel effect, or asymmetric blade thrust, prop walk affects most single screw (single engine) vessels. It is caused by the angle of the propeller to the water's surface. If the prop is not perfectly perpendicular, i.e., if the shaft is angled slightly downward, as most are, then the cylindrical distance traveled by the propeller blades on their upstroke is greater than on their downstroke. The upstroke pushes more water, generating greater thrust on that side. This, coupled with lateral deflection off the hull, pushes the boat's stern sideways, pivoting the vessel around a point about 1/3 aft from the bow.

Other factors can affect prop walk. It is much more noticeable in reverse, and more pronounced at slow speeds and high RPM. Shallow water reduces upward water flow from beneath the vessel, reducing or even canceling the propeller effect. The amount of clearance between the propeller and the hull has an impact, as does the hull's shape. Greater propeller diameter or pitch increase prop walk. Two-blade propellers generally produce less walk than 3-blade; folding and feathering props less still. Yaw rate on a moving vessel can negate or augment prop walk, as can relative wind direction and strength. An offset shaft may likewise alter the effect. However, if a prop shaft is parallel to the water's surface, as in a saildrive, 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.

Asymmetric blade thrust occurs in forward and reverse, in opposite directions. However, in forward it is easily compensated by the rudder so you hardly notice it. In reverse gear the rudder is much less effective so prop walk is more obvious and more difficult to control. Anticipating this sideways thrust and its direction is prerequisite to knowing how to compensate for it and, in some situations, to turn it to your advantage.



First you need to know whether your boat's propeller is right-handed or left-handed, which way it spins. They walk in opposite directions. (Twin screw boats typically neutralize prop walk by having one right-hand propeller and one left-hand.) The initials RH (for right-hand) or LH (for left-hand) are usually stamped on the prop, along with its diameter and pitch. If you have noted this information in dry-dock, then you know which type you're dealing with. If not, it's easy to determine 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 it's right-handed; if counter-clockwise, it's left-handed.

If you cannot see the propeller, you can deduce its rotation by observing the direction of its 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 1/4-throttle. Now look over the stern quarters, port and starboard, and see on which side the water is turbulent, thrown sideways by the prop wash. That is the side towards which the propeller is thrusting in reverse, the blades' upstroke side. This sideways force will push the boat's stern in the opposite direction. If the prop wash is on the starboard side in reverse, then the propeller is right-handed and it will walk the stern to port. If the turbulence is on the port side in reverse, 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 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 to starboard.

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 make use of it.

Pivoting

A common situation in which prop walk can work for or against you is when maneuvering a single-screw vessel in tight quarters, as in a crowded marina or narrow channel. It is sometimes necessary to turn 90 or more with little room. Using prop walk, you can pivot a boat in place, or nearly so. Always turn a boat with a right-hand propeller to starboard, rotating her clockwise. Turn the wheel hard over to starboard (or push the tiller hard to port), then give the throttle a sharp 1-or-2-second burst of power in forward. The full thrust of the prop wash hits the cocked rudder blade and begins turning the boat. 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, 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. Idle down, shift back to neutral, and then into forward again. 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 since 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 & fill," will spin most boats in their own length plus a little; a bit less for fin keel designs, more for longer 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-hand prop vessel 90 to port in tight quarters, you'll probably be better off spinning her 270 to starboard. Practice pivoting your boat in open water until you get the feel of it.


Docking

Similarly, it is easier to dock a right-hand propeller boat port side to (and a left-hand prop starboard side to). Aim for a point on the dock about 1/3 of a boat length aft of where you want the bow to wind up. Approach at 1 to 1 knots, at a 30 to 40 angle. When the bow is about 1/4 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, 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 over zealous line handler will get a bow line to the dock while the boat is still moving forward during this maneuver. If they 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 a mid-ship 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 - a right-hand propeller starboard side to or left-hand port side to - approach at a shallower angle, 10 to 15. When the bow is about 1/4 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 forcefully 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 2/3 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 to 40 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 or power in forward (with the rudder cocked) or reverse.

Backing

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 left-handed. This angle will be quickly corrected by the prop walk as you begin backing the boat in reverse gear. By the time the boat straightens out she is making sternway and the rudder gains steerage. Steer opposite to the prop walk to compensate. Once the boat is moving adequately in reverse, lower the engine RPM. This reduces the prop walk 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 or obstacles. Practice in open water until you're comfortable and confident. The more you use prop walk, the more you will come to appreciate rather than dread it.
 

~ End ~

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