Tug Stability Under Towline Load
2008 IS Code Part B section 2.8: self tripping and tow tripping heeling levers, escort area ratios, and how a recommendatory Part B binds a newbuild.
Tug stability under towline load is governed by the 2008 IS Code, resolution MSC.267(85), as amended. Resolutions MSC.413(97) and MSC.443(99) added scope subparagraphs 1.2.7 to 1.2.9 and definitions 2.27 to 2.31 to Part A, in force 1 January 2020. Part A gained no numerical tug criteria at all.
All the numerical criteria sit in Part B section 2.8, added by resolution MSC.415(97) and effective from 1 January 2020, and Part B is recommendatory. They apply to ships whose keel was laid on or after that date and to ships converted for towing after it. Part B 2.8.2.1 gives the self tripping heeling lever from towline pull, with a transverse thrust coefficient of 0.5 for conventional propulsion and a floor of 0.7 or 0.5 for azimuth arrangements depending on whether the tug tows over the stern or the bow. Part B 2.8.4.4 gives the escort criteria: area A at least 1.25 times area B, area C at least 1.40 times area D, and equilibrium heel not exceeding 15 degrees.
The full article will work through both heeling levers, the escort equilibrium, the stern freeboard and freeing port requirements, and the class and flag routes by which a recommendatory Part B becomes a condition of a newbuild’s notation.