Why Does a Boat Turn When in Reverse?

[kezman2000]

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?

 

[carmatic]

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

 

[kezman2000]

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.

 

[NeuronsAtWork]

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.

 

[alphy]

Hello,

I can provide some insight into the phenomenon of prop walk and its causes. Prop walk is the term used to describe the turning motion of a boat when the propeller is in reverse. This turning motion is caused by a combination of factors, including the design of the propeller, the direction of rotation, and the interaction between the propeller and the surrounding water.

One of the main causes of prop walk is the asymmetrical design of most propellers. As you mentioned, the lower part of the propeller has a larger surface area and therefore creates more drag compared to the upper part. This difference in drag creates a torque or turning force on the boat, causing it to turn in the direction of the propeller rotation.

Another factor that contributes to prop walk is the angle of the propeller in relation to the water flow. When the propeller is mounted at an angle, it creates an asymmetric thrust or p-factor, similar to the one experienced by aircraft. This thrust can also contribute to the turning motion of the boat.

The shape and design of the hull can also play a role in prop walk. The hull creates a wake or disturbance in the water, which can interact with the propeller and affect its performance. This interaction can cause the boat to turn in a certain direction, depending on the design of the hull and the propeller.

In summary, prop walk is a complex phenomenon that is caused by a combination of factors, including the design of the propeller, the direction of rotation, and the interaction between the propeller and the surrounding water. More research is needed to fully understand and explain the mechanics of prop walk, but these are some of the main factors that contribute to it.

I hope this helps in your understanding of prop walk. If you have any further questions, please don't hesitate to ask.