Rhino for computing Hydrostatics

I have Rhino installed on my MacBook Pro. I am trying to compute some properties of a sailboat hull using the hydrostatics mass analysis tool. The model is well-formed. I am running into a problem in that all I get is wetted surface with water plane = 0. None of the other measures (center of buoyancy, etc.) that hydro is supposed to compute are computed. I am somewhat new to rhino, so may be doing something stupid. Has anyone had experience working with the hydro analysis tool in Rhino?