Propeller (Next Question)

[Trevor_swfyc]

We have all been told about the phenomenon of prop walk. While I agree that I do not need to know how & why but it would be something to talk about and would surely impress at the next social evening.

As I see it each blade of the prop travels the same distance port > starboard as it does starboard > port so it should be in balance. So why does it walk ?

This effects power boats more than yachts due to I suspect the keel.

So its over to the prop prof's

Thanks Trevor

 

[Stingo]

Although it may not seem so, there is a water pressure difference between the top of the blade and the bottom i.e. the bottom blade is in deeper water and therefore has more bite when it turns.

Since your boat is stationary in the water and the rotation of the blade is 90 degrees to the centreline of the boat, the deep blade "walks" or initially moves in that direction until the boat starts moving forward or aft.



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[Col]

Also known as paddle wheel effect, for the reasons Stingo has mentioned.

<A target="_blank" HREF=http://www.arweb.co.uk/argallery/colspics> Cols Picture Album</A>

 

[Trevor_swfyc]

The water pressure is the same on each side of the prop blade so I cannot see why this should affect it when water is not compressable. I could understand this effect on a windmill as air is compressable and the mass near the ground is more than the mass higher up, therefore the force near the ground would be greater for the same wind speed. The density of water though is 1 at the surface and 1 at 2ft down.

Thanks for your explanation though.

Trevor

 

[DavidJ]

The pressure is released on the top side, seen by the movement of water on the surface and as you say it's incompressible so the underside pressure has no where to go so it 'bites'
David