  In this paper we revisit the problem of the tidal interaction occuring
between a protostellar accretion disc and a secondary point mass following a
parabolic trajectory. We model the disc response analytically and we compare
our results with three-dimensional SPH simulations. Inviscid as well as viscous
hydrodynamics is considered. We show that in a viscous system the response
derived from inviscid considerations is predominant even for the highest
estimates of an anomalous disc shear viscosity. The angular momentum lost from
the disc during the encounter is derived from linear theory, for distant
fly-bys, as well as the changes to the disc orientation expected in
non-coplanar encounters. It is shown that the target discs can become warped
and precess by a small amount during non-coplanar encounters. This small
precession is shown to give rise to a relative tilt of the disc which is always
more important for determining its final orientation than is the change to the
orbital inclination. We discuss the implications of our results for
protostellar accretion discs and planetary systems.
