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Prop walk - what causes it?

  1. Apr 15, 2012 #1
    I've been puzzling over this problem all day so i'm hoping someone can come up with some better ideas than I have:

    When you put a boat that has a single propeller in reverse it will also turn. The direction in which it turn is based on the direction that the propeller rotates; if it rotates to clockwise (when viewed from the back) the boat will turn left, if it rotates anti-clockwise the boat will turn right. Basically it seems that the lower part of the propeller rotation has more 'bite' that the upper.

    There seem to be a few theories out there based on the effect:

    1) The greater pressure on the lower part of the pressure generates more drag [as far as i can see the hydrostatic pressure shouldn't have any effect on the drag].
    2) The greater density of water generates more drag on the lower part of the propeller [would be true if water wasn't incompressible].
    3) Asymmetric thrust or the p-factor, similar to that experienced by aircraft. This creates a torque due to the difference in blade velocities caused by the propeller being mounted at an angle to the water flow. [this seems plausible when the boat is moving, but prop walk is most noticeable when the boat is stationary]

    My thoughts are there is some interaction between the hull and propeller wake, but I can't seem to find any solid facts to base this on and I've found no scientific papers or books even mentioning prop walk. Does anybody have the answer?
  2. jcsd
  3. Apr 15, 2012 #2
    the propeller also has torque, which is acted on the boat... that is why when the propeller turns counterclockwise, the boat rotates clockwise until the ballast forces cancel out the torque and the boat is in an equilibrium of leaning ot the right, and it appears to be turning to the right

    the fact that the effect is more evident when the boat is stationary would support this, because the prop would be working harder when it is trying to move water which is stationary relative to itself, so it is requiring more torque and therefore also causing more torque on the boat, and the boat does not have the stabilizing effect of the hull moving forwards in water
  4. Apr 16, 2012 #3
    I think you're referring to 'torque roll'. Prop walk produces a yawing effect on the boat, which seems to be produced by a side-ways force generated by the propeller (or maybe a torque around the vertical axis of the boat). I guess a torque around the vertical axis of the boat could be generated by a propeller shaft that is at an angle to the horizontal plane, but I doubt this would be a significant effect.
  5. Apr 17, 2012 #4
    The way I understand it, this is only a problem on props that are angled down from the horizontal axial plane of the boat, and it is this angle which causes an asymmetrical angle of attack. Depending on direction of rotation, one side will 'bite' more than the other on each revolution because the pitch of the blade is changing on the port side vs. the starboard in relation to the horizontal, pulling the boat around. Three-bladed props seem to have more of a problem than two. Keeping the prop perpendicular to the water's surface, or using a feathering prop, seems to reduce the problem. Note--I am no expert; just what I've read.
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