  We propose to apply an "effective boundary condition" method to the problem
of chiral propulsion. For the case of a rotating helix moving through a fluid
at a low Reynolds number, the method amounts to replacing the original helix
(in the limit of small pitch) by a cylinder, but with a special kind of partial
slip boundary conditions replacing the non-slip boundary conditions on the
original helix. These boundary conditions are constructed to reproduce
far-field velocities of the original problem, and are defined by a few
parameters (slipping lengths) that can be extracted from a problem in planar
rather than cylindrical geometry. We derive the chiral propulsion coefficients
for spirals, helicoids, helically modulated cylinders, and some of their
generalizations using the introduced method. In the case of spirals, we compare
our results with the ones derived by Lighthill and find a very good agreement.
The proposed method is general and can be applied to any helical shape in the
limit of a small pitch. We have established that for a broad class of helical
surfaces the dependence of the chiral propulsion on the helical angle $\theta$
is universal, $\chi\sim \cos\theta\sin 2\theta$ with the maximal propulsion
achieved at the universal angle $\theta_m = \tan^{-1}(1/\sqrt{2})\approx
35.26^\circ$.
