Lord Kelvin's isotropic helicoid

Can a propeller be isotropic? Nearly 150 years ago, Lord Kelvin proposed the isotropic helicoid, but there are no published measurements on his particle. We 3D-printed his particle and unexpectedly found no measurable translation-rotation coupling. We explain these results by demonstrating theoretically and computationally that Kelvin's proposed coupling exists, but it is small since it is only due to a weak breaking of a symmetry of non-interacting vanes in Stokes flow.