The double sheet bend or double becket hitch is much better than the square knot. The square knot tends to weaken the line quite a bit more than a double sheet bend. The single sheet bend can come undone under certain conditions, which is why I recommend the double sheet bend instead, as it is much more secure.
Also, keep an eye on the line for chafe, no line is ever going to be thick enough if chafe isn't controlled. A good source for heavy duty chafe protection is your local fire department. Used fire hose is very good as a chafe protection material.
Last edited by sailingdog; 05-01-2006 at 11:00 PM.