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This question already has an answer here:

There's all kinds of research on how to make something cut through the water with minimal resistance, but what if resistance was exactly what you wanted? What kinds of shapes create above-average resistance with water? Also, does the hydrophobicity of a material affect its fluid-resistance in water? Because hydrophobic materials are pretty jagged...but they also repel water. Note that this is different than drag as drag deals with aerodynamics.

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marked as duplicate by probably_someone, John Rennie, Kyle Kanos, Jon Custer, peterh says reinstate Monica Jul 8 '17 at 18:00

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  • $\begingroup$ Do you mean the shape with the highest drag coefficient that we know of? Currently there is no workable theory as to the drag coefficient of an arbitrary shape - everything we know is either empirical or only applies to very simple shapes. $\endgroup$ – probably_someone Jul 8 '17 at 5:43
  • $\begingroup$ I would think it's intuitive enough tat you could assume the highest "known" shape. Obviously I would not ask for data where there is none to be had. $\endgroup$ – user158288 Jul 8 '17 at 7:41
  • $\begingroup$ I probably wouldn't be too surprised. It is not a duplicate topic because that post is in reference to aerodynamics. I have no reason to think that I can make all of the exact same assumptions in fluid dynamics that are made in aerodynamics. $\endgroup$ – user158288 Jul 8 '17 at 7:47
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    $\begingroup$ You need to put constraints on the shape. Imagine a square frame supporting chicken wire inside. Half the spacing of the wires and drag will increase. Half it again… Or fix another identical frame behind the first one. And another one… and keep increasing the drag! $\endgroup$ – user154997 Jul 8 '17 at 8:43
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    $\begingroup$ If you have a given volume of material it is easy to show that there is no upper bound to the resistance: make it into a thin flat plate which is as thin as you like and hence has a radius as large as you like. You need to specify constraints on the shape as other comments have said. $\endgroup$ – tfb Jul 8 '17 at 9:44
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I agree with the response above that there is no workable theory for the drag coefficient of an arbitrary shape. However, I feel like I might can shed light on the "least efficient" shape to move through water.

Based on drag being the result of momentum conservation, my guess is that a flat plate moving "bulldozer" style through water is about as much resistance as I can think of offhand. If I were trying to have some form of effective braking system for a watercraft, the flat-plate approach would be my starting point.

The larger and more rigid the plate, the better of course.

With regard to the hydrophobicity part of your question: I've never calculated this (and don't know how to, frankly), but I strongly suspect that the hydrophobicity or hydrophilicity of the material plays an ignorable role in the drag. Actual physical features of the object surface might play a role, though. For example, is the surface smooth, or pelleted like a golf-ball. I'm sure the golf ball type surface would have more drag.

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