  Collisions of gas particles with a drifting grain give rise to a mechanical
torque on the grain. Recent work by Lazarian & Hoang showed that mechanical
torques might play a significant role in aligning helical grains along the
interstellar magnetic field direction, even in the case of subsonic drift. We
compute the mechanical torques on 13 different irregular grains and examine
their resulting rotational dynamics, assuming steady rotation about the
principal axis of greatest moment of inertia. We find that the alignment
efficiency in the subsonic drift regime depends sensitively on the grain shape,
with more efficient alignment for shapes with a substantial mechanical torque
even in the case of no drift. The alignment is typically more efficient for
supersonic drift. A more rigorous analysis of the dynamics is required to
definitively appraise the role of mechanical torques in grain alignment.

Astrophysics of Galaxies