It seems to me that there are many ways to tie to a bull rail as there are people who have ever tried to secure their vessel to one of these splinter infested dock structures. In the Pacific Northwest they are a way of life. I've studied most every hitch I have seen and had just about settled on one when Bill Douglass showed me this method at Auke Bay, Alaska. It was shown to him by an anonymous captain, so I have therefore dubbed it the Douglass Hitch after the Captain who taught it to me.
Give it a try.