Loading Conditions for Exposed-Waters Towing Vessels Regulated under Subchapter M

A set of five vessel loading conditions was developed for exposed-waters towing vessels in support of compliance with applicable stability regulations invoked under the U.S. Code of Federal Regulations (CFR) Title 46, Subchapter M. These loading conditions are envisioned as a starting framework for the naval architect or third-party organization when pursuing a U.S. Coast Guard stability letter with the fewest operational restrictions. These conditions do not represent required operational scenarios. For each loading condition, variable loads based on both tank location and tank contents were specified with the goal of encouraging conservative stability evaluations, while maintaining a level of realism to the resulting vessel attitude at each condition. Use of these developed loading conditions as a replacement for the nearly forty-year-old McGowan and Meyer conditions is anticipated. Using 3D models and General HydroStatics stability software, three vessels representative of modern exposed-waters towing vessels, but designed before the enactment of Subchapter M, were tested against 46 CFR Subchapter S stability criteria at each loading condition. Results of the analysis are presented for each vessel and for each applicable Subchapter S criterion. As expected, vessels not designed for Subchapter M/Subchapter S stability regulations can have trouble passing using the proposed loading conditions. The authors experimented with simple changes to the tank geometry of these pre-Subchapter M vessels, creating compliance with nearly all stability criteria for all loading conditions. Based on relevant literature and the results of this work, it is recommended that, for conservatism, the free-to-trim method be used for stability analysis regardless of the loading conditions applied. It is recommended that if an exposed-waters towing vessel passes the applicable Subchapter S stability criteria using the loading conditions developed in this work, then the vessel should be considered for a stability letter with minimal operational restrictions.