# Side forces: rake effect and prop walk quantified

Discussion in 'Surface Drives' started by sandhammaren05, Feb 2, 2021.

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### sandhammaren05Senior Member

The lift on a prop blade has two components orthogonal to thrust, a radial component and an azimuthal one. Both average to zero when the prop is fully submerged. Positive prop blade rake generates a downward force (relative to the waterline) when the radial force is averaged over a revolution in surface piercing. The rake needed to maintain a given trim angle can be estimated from scaling, given a baseline. The azimuthal force under partial submersion averages over one revolution to the side force that causes 'prop walk'. For a rh rotating prop the side force points starboard. Both forces have maximum effect for half-submersion. The qualitative description and math for a purely helicoidal blade are given in ch. 5 of the Jan. 30, 2021, revision of https://www.amazon.com/Hydrodynamics-Speed-Water-Piercing-Propellers/dp/B085D873JK. Adding camber and cup to the mean camber surface of the blade will not change the conclusions.

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### DogCavalrySoy Soylent Green: I can't believe it's not people

Hi Sandhammaren05. I hope your book is getting the attention it deserves. As far as I know, it is the only book on surface drives. And if it is written with the same lucid clarity as this post , I'm sure it's a pleasure to read, even for someone not designing for surface piercing props.

Tell me: do you discuss SPP's in a lower speed regime? For example, around 15 knots, like A Hickman's first experiments?

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### sandhammaren05Senior Member

Sorry not to answer earlier, I was 'on vacation' from the web site. I think that low speeds are pretty well covered in, e.g., Oloffson's measurements, where the depth Froude nr. is =O(1). In contrast with photos of the thin helical wake slung of blades on F1 tunnel boats, there's enormous splash on exit and reentry of a blade.

In the book I should have included the explicit equation for relating camber along a blade arc to progressive pitch. We're using that now to try to understand better how to matheematize what I did to blade leading edges with hammer and anvil.

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### DogCavalrySoy Soylent Green: I can't believe it's not people

That would be well worth doing. I'm picturing the range between zero and Vmax on top fuel drag boats, which of course use surface props and six speed transmissions. The first few moments there might well be that splashing you describe, and those teams improve their performance by tiny increments these days. The designs are fairly mature.

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