Towline and Tow Wire Selection
Selecting a towline from bollard pull: minimum breaking load rules, steel wire against HMPE, pennants, terminations, chafe and retirement.
Towline size follows from bollard pull through a documented rule. MSC/Circ.884 paragraph 12.11 sets the main towline minimum breaking load at 3.0 times bollard pull below 40 tonnes, at 3.8 minus bollard pull divided by 50, times bollard pull, between 40 and 90 tonnes, and at 2.0 times bollard pull above 90 tonnes. DNV expresses the same rule in kilonewtons with a utility factor running from 2.5 to 2.0 as the design force rises from 200 kN to 1,000 kN.
The circular also governs the rest of the arrangement. Shackles, rings and connecting items carry an ultimate load bearing capacity at least 50 percent above the minimum breaking load of the towing arrangement. Fibre rope pennants carry a breaking load of not less than twice the towline MBL below 50 tonnes of bollard pull and 1.5 times above 100 tonnes, interpolated between, in grommet construction with hard eyes. All wire rope terminations are hard eyes except at the drum end, all wire ropes in use have the same lay, and a spare towline meeting all main towline requirements is carried.
The full article will compare steel wire rope against high modulus polyethylene, cover creep, heat and chafe, and set out inspection, discard criteria and the record keeping that supports them.