  We use 3-dimensional SPH/N-BODY simulations to study ram pressure stripping
of gas from spiral galaxies orbiting in clusters. We find that the analytic
expectation of Gunn & Gott (1972) relating the gravitational restoring force
provided by the disk to the ram pressure force, provides a good approximation
to the radius that gas will be stripped from a galaxy. However, at small radii
it is also important to consider the potential provided by the bulge component.
A spiral galaxy passing through the core of a rich cluster such as Coma, will
have its gaseous disk truncated to $\sim 4$ kpc, thus losing $\sim 80%$ of its
diffuse gas mass. The timescale for this to occur is a fraction of a crossing
time $\sim 10^7$ years. Galaxies orbiting within poorer clusters, or inclined
to the direction of motion through the intra-cluster medium will lose
significantly less gas. We conclude that ram-pressure alone is insufficient to
account for the rapid and widespread truncation of star-formation observed in
cluster galaxies, or the morphological transformation of Sab's to S0's that is
necessary to explain the Butcher-Oemler effect.
