Prop-walk

[rod fox]

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

Rod Fox

[CDBarry]

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.

[duluthboats]

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

[Corpus Skipper]

the shaft is connected to the boat, so that force is transferred to the boat.

Now that's my speed!

[duluthboats]

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

[Eric Sponberg]

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

Eric

[yipster]

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

Originally Posted by Eric Sponberg

Here is an easy way to answer your question. When the engine is idling and the propeller is turning slowly, a clockwise-turning propeller will "walk" the stern in the direction it is turning. You can imagine the propeller as a wheel that is rolling, and a clockwise-turning prop will "roll" to starboard, taking the stern of the vessel with it. That necessarily points the bow to port, and you want to counteract that tendency. Therefore, you want the thrust from the propeller to create a moment in the opposite direction. A moment is always a force x distance. In this case, force is propeller thrust, and distance is the distance of the shaft line offset from the hull centerline. A port moment (shaft line to port) will cause a turning moment to starboard. Therefore, a clockwise propeller should be offset to port, and a counter-clockwise propeller should be offset to starboard.

Eric

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

[Eric Sponberg]

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