Late to the party, but thought I'd suggest an alternate approach in case someone else comes across this.
The answer above works, but its disadvantage is that the default is not added to Ransack's search object, so - if you are using a search form - the default selection is not shown in the form.
The following approach adds the default to the search object and therefore will appear in your search form.
def index
@search = Job.search(params[:q])
@search.status_cont = 'Open' unless params[:q] #or whatever, must use Ransack's predicates here
@results = @search.result
@job = @results.paginate(:per_page => 10, :page => params[:page])
end