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Subject: Understanding Prop Walk
(Posted on Feb 13, 2010 at 11:20AM ) Tags:
We continue to survey folks about what they would like to know more about and a common thread takes us back to docking.  We're so happy to put more tools in your docking toolbox.  Today we dive into what that propeller is doing for you (or against you).  Knowing some propeller basics will help dramatically.

Prop walk goes by different names, but that's what we call it around here because the P-effect or P-Factor has us sounding less cool.  We say a boat "walks to port" in reverse.  Okay - what are we talking about?

Cut to its simplest element on the 'need to know' basis, does the boat pull to port or starboard when you are going backwards?   Prop walk is best tested in the middle of a wide open space from a standstill (with no wind or with the stern facing what wind you do have).  With the wheel 'a midships,' apply a big shot of reverse and see where she goes.  A right hand propellor will normally send the boat back to port and a left hand will send the boat generally to starboard.

Beyond which direction, prop walk varies by boat based on some set up factors that you can't do much about other than understand.  The angle of the propeller shaft and the size/type of the propeller are two factors that determine how much prop walk you should experience on a particular boat.

What you can control is the timing and location of your use of the throttle in reverse gear.  The higher you rev the engine in reverse, the more you get to experience prop walk.   From a standstill, it's all prop walk.  As you start moving backwards, you will start to gain some directional control as water starts flowing across the rudder.

Behind the scenes, it boils down to discharge current.  In forward, all the engineering is aimed at pushing the discharge current as straight backwards as possible.  In reverse, the discharge current ends up traveling off to one side or the other, based on the rotation direction of the prop and the magnitude depends on other setup variables.

 

How to you check and get a clue before you go?  Ask someone knowledgeable about the boat OR, with the boat securely tied to the dock, engage reverse power and look for the discharge current coming off the rudder.   If you see a lot of current coming out to starboard, you know the boat will 'walk to port' in reverse.  

With a pair of motors, the reason the motors rotate in opposite directions is to cancel the effects of propeller discharge.  Using more throttle on one engine or another can work in your favour as you understand these effects.

Our crew can help explain how to make prop walk your friend.  Docking clinics and courses can help you master these skills on the water.