Recently while reading a book on advanced docking techniques it was observed that the explanation of prop walk was described as the same effect of a rotor tiller moving across the ground. In other words the blades on a rotor tiller spin biting into the earth pulling the device and operator in one specific direction. This description is misleading and inappropriate for an advanced docking book.
Let’s take a look at the real factors that cause prop walk. This explanation will lightly touch the theory; however, references will be listed at the end of this article for those that want to work this out mathematically.
Beginning with the Basic Coastal Cruising class students have been taught that two methods are used to determine prop walk on a vessel. First, we put the boat into reverse gear and observe which direction the stern moves. Second, while in reverse propulsion observe on which side of the vessel is the prop wash observed. Most vessels have a right-handed propeller [the prop turns to the right in forward when observed from the stern] and when in reverse to the left which results in wash being pushed upwards and on the starboard side of the vessel. This action moves the stern of the vessel to port.
Aircraft pilots know the term prop walk by other names such as p-factor, asymmetric thrust and asymmetric disk loading. Okay, why jump from the water to the air? Good question. The aviation world has done a better job of explaining this turning tendency of aircraft and the explanation also applies to our watercraft. When a single engine aircraft starts its take off and all the way to level flight the pilot has to push on the right rudder peddle to overcome the left turning effect of prop walk.
Key word in the above paragraph was “level flight”. If the plane is in level flight there is no p-factor [asymmetric thrust] or prop walk. The same applies to our boats. If the shaft exits the vessel level or perpendicular to the water, resulting in the propeller being perpendicular there is no prop walk. [Please keep in mind that hull shape also helps increase or decrease the effects of prop walk.]
Not all boats produce the same amount of prop walk and engineers can design in or out this effect. If you have operated Lionheart you know this boat has a huge amount of walk to the port; while Galen, Redeemed and Whale Song have very little walk. Prop walk is wonderful if used correctly.
There are two results of prop wash caused by prop walk and these will be discussed later but first and succinctly the cause of prop walk. On our vessels the shaft exits the boat at an angle to the surface of water and as such the propeller is also at an angle. This results in the starboard side propeller blade being at a greater angle to the water than the port side. In aviation terms the starboard side has a greater angle of attack than the port side.
Being that the starboard side has a greater angle of attack [AOA] it travels faster in relations to the relative/apparent water speed as the port side. Okay hang on, one blade does not physically move faster than the other but in relation to the relative/apparent water it does.
[From the FAA website: http://www.faa.gov/library/manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2004.pdf ] Comments in […] are my inserts to make the statement relate to our vessels.
“When an aircraft [boat] is flying [backing] with a high AOA, the “bite” of the downward [upward] moving blade is greater than the “bite” of the upward [downward] moving blade. This moves the center of thrust to the right of the prop disc area, causing a yawing moment toward the left around the vertical axis. To prove this explanation is complex because it would be necessary to work wind [water] vector problems on each blade while considering both the AOA of the aircraft [propeller] and the AOA of each blade.
This asymmetric loading is caused by the resultant velocity, which is generated by the combination of the velocity of the propeller blade in its plane of rotation and the velocity of the air[water] passing horizontally through the propeller disc. With the aircraft being flown at positive AOAs, the right (viewed from the rear) or downswinging [upward] blade, is passing through an area of resultant velocity which is greater than that affecting the left or upswinging [downward] blade. Since the propeller blade is an airfoil, increased velocity means increased lift. The downswinging [upward] blade has more lift and tends to pull (yaw) the aircraft’s nose to the left [boats stern to the left].”
This explains prop walk and now let’s look at the resulting prop wash and what effect is has. Back to the earlier observation that on a right handed prop the wash is seen on the starboard side of the vessel. This wash pushes on the starboard side and helps move the stern to the left. Another phenomena that helps push the stern to the left [while in the slip] is the wash that pushes against the slip finger.
For those that want to work this out using vector analysis please visit: http://www.av8n.com/how/htm/yaw.html#8-5 section 8.5 and http://www.meretrix.com/~harry/flying/notes/pfactor.html
There is also prop walk when the vessel is moving forward but most of us unconsciously correct for it by applying a bit of left rudder. On a right handed prop the forward walk is stern to the right. If you want to test this statement put the boat in forward and get it up to 5 knots and steer the boat on an absolute straight course. Now without turning the wheel shift into neutral and the boat will slightly make a turn. This is because while motoring forward you have trimmed out the effects of prop walk.
Bill Yawn holds a USCG Master’s License and a Commercial Pilots License and is a retired industrial engineering manager.