Prop-walk

I know the effect of prop-walk. Can anyone give me a detailed theory of how and why ?

Rod Fox

 

The idea is that the water close to the hull is going slower than farther away. When the water goes into the upper and lower propeller blades at different speeds, the forces generated by the vector sum of the speed of the water in and the speed of the blade around are then different and act in different directions on the top an bottom blades. Since the top blade is going the opposite direction than the bottom, the net force is somewhat off the centerline.

 

I kind of like that “forces generated by the vector sum of the speed”. To me an easy way to look at it is the only thing keeping the prop from rolling off in the direction of rotation is the shaft, the shaft is connected to the boat, so that force is transferred to the boat.

Gary

 

Now that's my speed!

 

In this case I was being a smart a**, in matters related to props I should leave to Mr. Barry, I have thought about taking his class on prop selection. As with most things boaty, there is no simple answer.

Gary

 

See my answer to "Offset single prop--which direction" on this forum.

Eric

 

quik link to "Offset single prop--which direction"

now on a dual system i take it the tops of the props should be counter rotating outboard from the keel?

 

Yes, in a twin screw installation, the propellers should be, as viewed from the stern, clockwise on the starboard side and counterclockwise on the port side so that the tops of the props are turning outboard. The difference in the direction is usually taken up by the transmission, but sometimes it is in the engine itself.

Eric