Ideally the ship should have some additional, low LOD so that it doesn't disappear when you decrease the object detail setting.
However, even with that setting maxed, the ship can disappear when its centre gets off the screen. The fix isn't complicated, but I don't remember it right now.
So you can do two things. Either wait for another dev to post the answer here, or use the search button on Starport, or Freelancer Archived.