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Assuming induced supercavitation (like Supercavitating torpedo and Juliet Marine Systems Ghost) reduces water surface area does that reduce friction enough to submerge a surface drive propeller without a reduction in thrust if the reduction in surface area equals what would be on the surface?

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The idea behind surface props is to get the appendage drag down to nothing. All the shafting and shaft support structure is out of the water. This means the propeller is a ventilating type. The hypertorpedo and Ghost are also ventilated designs. The difference lies in the vapor pressure and vapor composition, and the differences in hydrodynamics of the these mixed-phase flows. Ventilated flows don't have the same problems with cavitiation bubbles eroding the surface of the propeller.

As far as propeller design goes, the devil is in the details. You have to account for static pressure (depth pressure). You have to account for the effective speed of sound in the different bubbly regions (the speed of sound in mixed phase flows can get stupid slow, so you end up with supersonic blade speeds and normal ogival blades don't work.) And you have to recognize that normal flow models just don't work. In order for bubbly flow models to work, you need to operate on a scale where for some representative length $\epsilon$, the flow properties change very little over this distance, but the distance is still very big compared to the bubble or cavity sizes. This condition simply isn't met in ventilated propellers.

So the basic idea is similar, you want cleavers, not ogives, but that is about all you can say. The rest is cut and try. Note that Ghost has the props out in front of the SWATH hulls. Ghost isn't anywhere near fast enough to want cavitation adapted props. And if memory serves, Hypertorpedos use props for cruise, but are basically just rockets at speed. Their prop regime is likely also noncavitating, but I'm not sure about that.

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  • $\begingroup$ Would artificially ventilating a submerged surface drive achieve the same friction reduction as being out of the water? $\endgroup$ – Benjamin Fenigsohn Aug 18 '19 at 20:04

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