If it helps you, when I went onto sea going tugs as a junior mate, I was always taught that it was the weight of the tow wire which pulled the towed vessel. There for we always kept the bight of the wire below the surface of the water, never allowing to lift out, as if it did there was always a strong chance of parting the wire. Harbour work yes you always maintain tension on the wire and never snatch it, but then usually the wire is as strong or greater in breaking strain than the bollard pull. I have seen some spectacular breaks in tows, if there is a synthetic part to the system take cover, most wires are preformed wires and usually just drop. In the days before towing winches, most ships would insist on using there mooring warps rather than the tugs tow system, this reduced the towing fees, an often would come down the end of a nylon rope?
Here is another interesting fact for tug enthusiasts, When a tug is engaged by a ship, when its tow is connected, any damage caused by the tug to anything. The towed ship in law is held responsible, as in law it is considered part of the towed vessel, an there have been some expensive claims paid out to other vessels and parts of docks etc over the years.