The information contained in this explanation of what is propeller walk comes directly from the NauticEd Maneuvering and docking a sailboat under power sailing course which forms part of the requirement for the NauticEd Sailing Certification rank of Skipper.
You’ve learned to sail, sailed for a few years, and now you’ve upgraded to a bigger boat with an inboard engine. How frustrating! You want your new sailboat to go backwards but you keep going sideways. Welcome to propwalk. GRRRRR. This is not something that you probably got taught at your local sailing school because you learned on a smaller boat with maybe an outboard. Now you’ve got a much bigger and heavier boat and there are more expensive mistakes that can be made. Propwalk can be frustrating or you can understand it and use it to your advantage.
Let’s Understand Propwalk.
Imagine you’re walking up a spiral staircase. Each step is the same height and requires the same amount of energy to go up the next step.
Now imagine if the spiral stair case was tilted over 20 degrees. You’d find that as you went around the stairs they would be steeper on one side and flatter on the other. Or the stepper side cuts through more vertical space than the flatter side.
As we look at a propeller and the water flowing through it, the arc that the tips of the propeller follow relative to the moving water pushed by the propeller is a spiral shape, much like a spiral staircase.
Or another way to see it, is to observe the sweep of each blade as it passes through the water.
Now if we tilt the shaft of the propeller down, the spiral also tilts down.
But we have to put put a few prefaces on this. There must be no boat hull above to affect the initial flow of water, the water must be deep and the propeller can not be moving horizontally through the water – that’s a lot of prefaces and not reality. But here is what that would look like anyway.
Now let’s put the propeller in close proximity to the hull of the boat where the water tends to move horizontally. This is represented here by the imaginary sweep lines. Here then you can see that the up swinging reversing blade (green -starboard) cuts more of the flowing water similar to the tilting staircase example above.
In much the same way as the tilting staircase, the down sweeping blade cuts through less flowing water than the upsweeping blade. This creates more force on the up swinging side of the propeller than the down swinging side and thus a torque is produced on the prop shaft.
This results in the following forces and thus a resultant clockwise torque on the boat.
In a similar fashion, imagine yourself treading water in a swimming pool and your right arm swings in big circles while your left swings in smaller circles. Your body would move backwards but the action would also turn your face to the right and consequently your back to the left. The boat turns in exactly the same way.
In forward gear, the exact same phenomenon occurs, just in the opposite direction. However, we notice it much less because the water from the propeller is being pushed over the rudder which creates far greater forces and thus counteracts any tilt induced torque.
So in summary, a boat which has a counter rotating shaft, when in reverse, yaws clockwise (stern to the left) because the shaft is tilting downwards. Factors to reduce the effect include having a smaller diameter propeller or reducing the pitch (twist) of the blade or lengthening the shaft so that the water flow is further from the boat which would tend to allow the water flow to be more in line with the shaft. The effect is also reduced by a slower turning propeller IE less engine RPM.
An alternative way of mounting the propeller is called a saildrive and these are widely accepted in Europe. Saildrive systems have a horizontally mounted propeller shaft and therefore they do not create prop walk.
But there is no need to run out and go to the expense of converting your boat to a saildrive unit if you are experiencing frustration with prop walk. Because now that you understand the theory of propwalk, (more than 90% of sailors) all you have to do is practice the exercises in the NauticEd Maneuvering under power course a few times and you’ll have it licked.
So there you have it – now you’re 1 in a thousand sailors who understand how propwalk originates.
The boating rules and NauticEd’s Maneuvering and docking a sailboat under Power sailing course shows students how to take advantage of propwalk. Just imagine you’re trapped in an EXTREMELY tight marina. Exercise number 8 wil show you how to get out. Turn the boat one way and you’re in trouble, turn it the other way and use a combination of wheel and throttle and you’re out of there with out a scratch.
Take Maneuvering and docking a sailboat under Power course from one of the best sailing schools online – NauticEd.