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Review of U.S. Coast Guard Vessel Stability Regulations (2018)

Chapter: 2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS

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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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Suggested Citation:"2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS." National Academies of Sciences, Engineering, and Medicine. 2018. Review of U.S. Coast Guard Vessel Stability Regulations. Washington, DC: The National Academies Press. doi: 10.17226/25258.
×
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11 report. See Section 5 for additional details on and potential options for addressing these data issues. Organization of the Report The report that follows is divided into four main sections and supporting appendixes. Section 2 discusses vessel stability and the current state of the stability regulations under Subchapter S. Section 3 identifies some near-term and longer-term options for stability-related improvements that USCG can pursue. Section 4 discusses the issue of lightweight changes for passenger vessels over time and presents options for industry to monitor weight changes better. Section 5 discusses other stability regulations and standards, advanced methods of assessing dynamic stability in severe conditions, the state of casualty data and analysis for assessing vessel stability, and stability-related information available to vessel operators. The appendixes contain additional details and reference information that supplement the material in the main body of the report. 2. VESSEL STABILITY AND USCG-ISSUED STABILITY REGULATIONS Vessel Stability Vessel stability is the ability of a vessel to return to an initial upright position after being disturbed by an outside force. A stable vessel is one that has sufficient righting forces at that moment to counter the forces working to capsize it, so it remains upright. The buoyant forces acting on the vessel’s hull as it heels (i.e., leans or tilts) create a vessel’s righting forces. Capsizing forces can originate from many sources, including natural forces such as wind, waves, and the accumulation of ice, or operational and loading conditions such as improperly stowed cargo, the free surface effects of slack tanks, the lifting of gear and cargo over the side, or the tripping forces from towing a barge. The interaction of these many forces will determine a vessel’s stability and its ability to remain upright (see Figures 1a and 1b).

12 Figure 1a A vessel with sufficient righting forces. Figure 1b A vessel with insufficient righting forces. A vessel’s stability is not static, and the interaction between a vessel’s righting forces and any capsizing forces is dynamic and will constantly vary throughout a vessel’s voyage. Although a vessel may start a voyage with sufficient positive stability (i.e., remaining upright), this is no guarantee that a vessel will maintain sufficient positive stability for the entire voyage. A stable vessel must have sufficient righting forces to counter all of the forces that are working to capsize it not only throughout an entire voyage, but also throughout a vessel’s entire lifetime. To assess a vessel’s stability properly requires the measurement of a vessel’s dynamic stability, currently accomplished using two methods: (1) the vessel’s metacentric height (GM), which is a pure static stability measurement, and (2) the vessel’s righting arm curve, which is a quasi-dynamic method. The metacentric height for vessels, first understood in the mid-1700s, was used for the initial stability criteria because it was a practical and simple stability calculation to perform. A vessel’s hull is a complex shape that required a lengthy series of calculations to analyze its true dynamic stability characteristics. However, the vessel’s metacenter (M) could be calculated from only the vessel’s displacement and waterplane inertia, allowing a designer or

13 shipyard of that period to perform the required initial stability calculations. Vessel stability results from the association of the downward force of the vessel’s weight centered at the center of mass (G) and the upward force of the buoyancy centered at the center of the immersed volume. As a vessel heels, the location of the center of buoyancy (B) moves and the path of its motion can be determined by the center of curvature of this curve. The metacenter (M) is this center of curvature (see Figure 2). Figure 2 Metacentric height, as a measure of initial static stability. NOTE: B = center of buoyancy; G = center of gravity; M = metacenter. The metacentric height is calculated as the distance between the ship’s center of gravity (G) and its metacenter (M), where a larger metacentric height suggests a greater initial stability. As long as the center of gravity is below the metacenter, a vessel will have positive initial stability. Although the metacentric height is only an indication of a vessel’s stability at its upright condition, such a stability measurement was generally not a problem in the early 19th century as most vessels had very similar hull forms and operational uses. The metacentric height of any new vessel was usually similar to the previous “successful” vessel, with only small incremental changes. As naval architecture advanced, allowing more varied vessel designs, the use of only metacentric height for gauging a vessel’s stability became less reliable. New vessels began to

14 have significantly different hull forms from previous designs and with the metacentric height’s inability to measure the vessel’s stability at larger angles of heel, stability issues became more problematic. During the mid-19th century, naval architecture developed a newer quasi-dynamic method for measuring vessel stability that used the relating of righting energy to the area under the righting arm curve. A vessel’s righting arm curve is a graphical display of its righting arms, ranging from the initial upright position to high angles of heel, which covers all conditions a vessel is expected to encounter over its lifetime. As previously noted, the forces working to right the vessel are created solely by the buoyant forces on the vessel’s hull. The buoyant forces are pushing upward over the vessel’s entire hull surface situated below the waterline. By calculating the center of these buoyant forces for each heel angle, the center of buoyancy (B) is determined. The righting arm then is the horizontal distance between the center of buoyancy pushing up and the center of the vessel’s weight, or center of gravity (G), pushing down (see Figure 3). Figure 3 Righting arm (RA), also referred to as GZ (the horizontal distance between the center of gravity [G] and the vertical line through the center of buoyancy [B]).

15 Each heel angle then is a static snapshot of the vessel’s stability. When multiplied by the vessel’s displacement, the area under the curve for each heel angle determined the energy required to heel the vessel to that given heel angle. Although the metacentric height only measured vessel stability at the upright condition, the righting arm curve calculated stability over a broad range of heel angles, from the initial upright condition to larger angles of heel (see Figure 4). Figure 4 Righting arm curve. NOTE: Righting Arm (RA) (or GZ) is the horizontal distance between the center of gravity (G) and the vertical line through the center of buoyancy (B). The righting arm curve provided a method that could measure a vessel’s available righting energy, offering a useful measure of the vessel’s stability at the critical higher angles of heel that the metacentric height method could not do reliably. The Finnish researcher Jaakko Rahola was one of the first to develop stability assessment methods based on characteristics of the vessel’s righting arm curve and criteria to assess vessels through a wide range of heel angles and determine those that provided an adequate level of stability. By analyzing the righting arms of several vessels that had capsized in Finnish waters, Rahola was able to determine a minimum required righting arm at two points on the righting arm

16 curve, 20 and 40 degrees of heel. Although requiring more staff to perform the calculations, this approach allowed more viable stability criteria as minimum righting forces at high angles of heel were specified and applied to newer generations of vessel designs. In a reformatted state, Rahola’s stability criteria form the basis of today’s generation of quasi-dynamic stability criteria. The introduction of computers allowed for the efficient calculation of the righting arms, which now allowed the righting arm curves to be viably created. This allowed Rahola’s two-point minimum righting arm criteria to be converted into minimum required areas under the curve to critical angles of heel. The use of the area under the righting arm curve as opposed to Rahola’s two righting arm points offered several improvements in measuring a vessel’s stability. The most important of these improvements is that the real-life highly dynamic stability interactions could be better modeled with practical quasi-dynamic methods. By overlaying curves that represent the forces acting to capsize the vessel on the righting arm curve, which is acting to right the vessel, further advancements in available stability criteria were developed. An example is the severe wind and heel criterion. This criterion was developed to measure the stability of a vessel laying to in beam seas during an extreme storm. This criterion uses three overlays on the righting arm curve to approximate the dynamic motions a vessel would experience in a storm’s high winds and large beam seas (see Figure 5).4 4 See http://www.imo.org/en/KnowledgeCentre/IndexofIMOResolutions/Maritime-Safety-Committee- %28MSC%29/Documents/MSC.267%2885%29.pdf.

17 Figure 5 Righting arm curve approximating severe wind and rolling on a vessel. SOURCE: USCG, presentation to the committee by Jaideep Sirkar, March 2018. Similar heeling arm overlay methods are used to determine the effects of operational capsizing forces on a vessel’s stability, such as towing a barge or lifting a weight over the side, or the effects of water on deck from a boarding sea. Most stability-related concerns associated with today’s vessels are covered by the current generation of quasi-dynamic stability criteria. Overview of Shipping Regulation Ships, boats, and non-self-propelled vessels are regulated to improve safety and prevent damage to the environment depending on their service, the nation in which they are registered, where they travel, and their routes. Recreational boats have their own set of regulations and are not addressed in this report; neither are government and military vessels. Ensuring vessels have sufficient stability to remain upright and seaworthy in the expected conditions in which they will operate is one of the key ways to ensure vessel safety and prevent environmental damage. As much of shipping is by its nature international, moving goods and passengers from nation to nation on the oceans and seas of the world or extracting resources from the sea or from under the sea, it is necessary to have an international-based regulatory system applicable to all

18 vessels that travel the high seas. The London-based International Maritime Organization (IMO),5 an arm of the United Nations, largely fills this function. See Appendix C for a description of the IMO. IMO issues a wide range of regulations covering most aspects of shipping safety and environmental protection for a full range of vessel types. The IMO-issued regulations include SOLAS, the International Convention on Load Lines, the International Convention for the Prevention of Pollution from Ships (MARPOL), and many more conventions, codes, and resolutions. These are enforced by the signatory nations, usually through their maritime agencies acting as the Flag Administration (nation in which the vessel is registered has jurisdiction over the vessel) and by the port nations (Port State) where vessels may call upon acting to ensure noncompliant vessels do not create hazards in their waters. Most vessels on an international voyage or ones that pass through international waters (such as U.S. vessels traveling between the U.S. mainland and Hawaii) are subject to the IMO-issued regulations. The IMO regulation most applicable to this study is the Intact Stability Code, issued in 2008 and updated by amendments several times since then (see Appendix C for a more detailed discussion of the IMO Intact Stability Code). The IMO-issued stability regulations are quite robust and are updated regularly, but they will not be addressed directly in this report, except for instances in which they may be adopted into the U.S. stability regulations or may offer a good reference for updating the U.S. regulations. Vessels that operate inside the waters of a single nation (or association of nations, such as the European Union) are generally subject only to the regulations of that nation or association. This applies to most vessels registered in the United States, as they operate only in domestic 5 For additional information on the IMO, see http://www.imo.org/en/About/Pages/Default.aspx.

19 waters. In the mid-20th century, the United States designated USCG as the agency with responsibility for regulating and inspecting ships and commercial vessels registered in and operating within areas under the jurisdiction of the United States. It also fulfills the role of the U.S. Flag Administration under the IMO regulations. The CFR contain USCG-issued regulations. Vessel safety, operation, and stability regulations are found primarily in Title 33, Navigation and Navigable Waters, and Title 46, Shipping. Title 33, Chapter I, contains the USCG-issued regulations, Subchapters A to S. These relate to a wide range of topics, including vessel personnel, aids to navigation, navigation rules of the road, maritime security, vessel operating regulations, pollution, and ports and waterways, among others. Some of these regulations relate to safe operation, which includes vessels remaining stable, but this Title does not contain regulations related to the determination of vessel stability. In Title 46, Chapter I covers USCG-issued regulations, which are contained in Subchapters A to W. These regulate the design, documentation, and operating personnel of U.S. vessels. Chapter III pertains to Great Lakes pilotage. Of interest to this study are the U.S. stability regulations contained in Title 46, Chapter I, Subchapters C, S, and T. Subchapter S is entirely related to subdivision and stability and is the location where USCG has assembled most of its stability regulations to facilitate their use and understanding. Subchapter T covers small passenger vessels and for the convenience of its users, stability-related regulations for these types of vessels are also included. Subchapter C covers uninspected vessels, which include most fishing vessels, and for fishing vessels the stability-related regulations are included in this Subchapter in Part 28, Subpart E. They apply to fishing vessels 79 ft or more in length without a load line that were built or substantially altered after September 4, 1991.

20 There are many stakeholders with an interest in stability regulations, since maintenance of stability is one of the key factors in ensuring vessel safety and avoidance of environmental damage. The stakeholders include the thousands of vessel owners and operators, the vessel personnel and passengers, the cargo owners and shippers, the communities along the shores who can be affected by vessel accidents, and the government agencies tasked with enforcing the regulations and carrying out search and rescue. All have an interest in efficient and effective stability-related regulations that ensure that vessels remain upright and stable at all times. The U.S. stability regulations should be updated and improved and include the interests and benefits of all the stakeholders. Subchapter S, Subdivision and Stability Subchapter S of Chapter I of Title 46 of the CFR applies to all of the inspected fleet, passenger and cargo vessels alike, with the exception of most small passenger vessels covered by Subchapter T. Some Subchapter T small passenger vessels—based on their size and type—may be required, however, to meet certain portions of the Subchapter S stability requirements in lieu of those given in Subchapter T. As of April 2018, the Subchapter S fleet includes more than 5,900 vessels: 539 passenger vessels and 5,375 cargo vessels.6 These numbers are from the MISLE database and are summarized in Appendix B (see Tables B-5 and B-6). The passenger vessels are categorized by passenger allowance and include 122 subchapter H vessels, 400 subchapter K vessels, and 17 subchapter R vessels. The cargo vessels, categorized by propulsion, include 617 self-propelled vessels and 4,758 non-self-propelled vessels and cover six inspection subchapters. A minor portion of this fleet can be described as ocean-going, or engaged in 6 From Table B-5: 6,106 total passenger vessels minus 5,567 Subchapter T vessels equals 539 passenger vessels subject to Subchapter S. Table B-6 displays the 5,375 cargo vessels.

21 international commerce, and accordingly must comply with international stability regulations for legal operation (such as SOLAS, International Convention on Load Lines, and High-Speed Craft Safety), in addition to those regulations found in the CFR. Those international regulations are developed and maintained by IMO, to which the United States contributes significant scientific and leadership efforts. However, the majority of the U.S. fleet is subject only to the stability regulations of the CFR, since the vessels operate in U.S. waters defined as protected (rivers); partially protected (lakes, bays, and sounds); and exposed (coastal). Subchapter S was implemented as a Final Rule by USCG in 1983. The purpose of creating this new subchapter was to consolidate the stability-related regulations from the various inspection subchapters into one location. At the time, it was believed this would simplify the regulations and facilitate their use by industry and regulators.7 Since its initial adoption, Subchapter S has been updated several times, with the last revisions incorporated by a Final Rule in December 2010. This 2010 Final Rule also included changes to the passenger vessel inspection Subchapters H, K, and T. The changes were initiated by the March 4, 2004, capsizing of the small passenger pontoon vessel M/V Lady D in the Inner Harbor of Baltimore, Maryland. The casualty highlighted that the weight for passengers in the regulations was too low considering the increased weight of the average U.S. passenger.8 Subchapter S is divided into Parts 170 to Parts 174, with each Part further divided into Subparts that address a specific stability topic, vessel type, or both. Part 170 is applicable to all vessels and covers intact stability along with general requirements. Part 171 applies to 7 See Federal Register, Subdivision and Stability Regulations; Final Rules. Vol. 48, No. 215, Nov. 4, 1983. pp. 50996–51053. (http://cdn.loc.gov/service/ll/fedreg/fr048/fr048215/fr048215.pdf). 8 See Federal Register, Passenger Weight and Inspected Vessel Stability Requirements; Final Rule. Vol. 75, No. 239, Dec. 14, 2010. pp. 78063–78092. (https://www.gpo.gov/fdsys/pkg/FR-2010-12- 14/html/2010-30391.htm).

22 Subchapters K and H passenger vessels and covers subdivision and weathertight and watertight (WT) integrity. Part 172 applies to intact and damage stability of vessels carrying bulk cargoes. Part 173 and Part 174 contain special stability rules, with Part 173 pertaining to vessels by use (such as lifting) and Part 174 pertaining to vessels by type (such as liftboats). Passenger vessel stability requirements (both intact and damage) for Subchapter T are still located in Subchapter T. A listing of all the parts and subparts of Subchapter S is provided in Table 2. Table 2 46 CFR Chapter I, Subchapter S—SUBDIVISION AND STABILITY Part 170 Subpart A–D General requirements, etc. Part 170 Subpart E Intact stability criteria Part 170 Subparts F–I Specific requirements—Weights, centers, WT doors, etc. Part 171 Subpart A Passenger vessels—General Part 171 Subpart B Passenger vessels—Intact stability Part 171 Subpart C Passenger vessels—Subdivision and damage stability Part 171 Subparts D–H Passenger vessels—Specific requirements for domestic Part 172 Subpart B Bulk cargoes—Grain Part 172 Subpart C Bulk cargoes—Barges carrying oil (Subchapter D) Part 172 Subpart D Bulk cargoes—Vessels carrying cargo—33 CFR 157 Part 172 Subpart E Bulk cargoes—Barges carrying hazardous liquids (Subchapter O) Part 172 Subparts F, G Bulk cargoes—Ships carrying hazardous liquids, gases Part 172 Subpart H Bulk cargoes—Great Lakes bulk carriers Part 173 Subpart B Vessel use—Lifting Part 173 Subpart C Vessel use—School ships, including sailing Part 173 Subpart D Vessel use—Oceanographic research Part 173 Subpart E Vessel use—Towing Part 174 Subpart B Vessel type—Deck cargo barges Part 174 Subpart C Vessel type—Mobile offshore drilling units (MODUs) Part 174 Subpart E Vessel type—Tugboats, towboats Part 174 Subpart G Vessel type—Offshore supply vessels (OSVs) Part 174 Subpart H Vessel type—Liftboats Part 174 Subpart I Vessel type—Hopper dredges, working freeboard Part 174 Subpart J Vessel type—Dry cargo ships Examples of Content and Organizational Issues with Subchapter S Stability Regulations Writing regulations is a difficult task, for it requires being specific in requirements, staying within legal and regulatory limits, having a thorough understanding of the topic (so that correct

23 measures are applied to achieve the desired outcome, while avoiding unexpected consequences), and writing in a form that is easily understood by both users and regulators. The following list contains examples of content and organizational issues with the current Subchapter S. The list is a summary, with more details provided in Appendix D. 170.173, Criterion for Vessels of Unusual Proportion and Form It is not clear what the base point is for applying the range of righting arms for vessels with inherent list in some operating conditions. 170.295, Special Consideration for Free Surface of Passive Roll Stabilization Tanks, and 172.030(b), Grain Exemptions for Certain Vessels These two parts are difficult to understand, contain confusing requirements and definitions, have formula typographical errors, and employ simplified formulas designed for pencil and paper calculations. Downflooding Definition There are about nine locations in Subchapter S with definitions of downflooding. These definitions appear to vary and can refer to any opening, any opening that cannot be closed or cannot be closed rapidly (without defining rapidly), and watertight and weather-tight closures. Permeability Factors There are multiple definitions of permeability factors that should be used for various types of calculations and sometimes these vary for the same space depending in which part it is referenced. Tank Runoff in Damage Stability Calculations This is not addressed consistently in accordance with modern damage stability calculation methods that allow tank runoff to be included in the calculation.

24 Reliance on Simple Static Stability Criteria In some parts of Subchapter S, simple static GM-based stability criteria, such as in 170.170 Weather Criteria, 171.050 Passenger Heeling Criteria, 172 Subpart E Tank Barges Carrying Hazardous Liquids, and 173.095(b) Towline Pull Criteria, are used for checking vessel stability. Certifying a vessel meets the minimum required initial GM does not ensure the vessel has sufficient long-range stability as it heels, which is critical to overall vessel stability safety. For example, a vessel with low freeboard may have a high initial GM, but it could quickly lose stability when heeled and could easily capsize. Outdated or U.S-Focused Citations of Other Regulations and Standards When a U.S. regulation cites an international regulation, ASTM International (formally known as the American Society for Testing and Materials) standard, American Society of Mechanical Engineers (ASME) standard, or some other standard, the specific version of the regulation or standard is cited in the CFR. To allow otherwise would mean U.S. regulations could change without following the regulatory rulemaking process if the cited external reference is revised. When a specific regulation is cited, U.S. regulations can be outdated, which is difficult for modern vessels that may need to comply with both a new and older version of the regulation. In addition, the CFR tends to cite only U.S. standards, which can be difficult for shipping, since some of it is international and equipment is purchased from sources around the world, where U.S standards are not followed. An example of this is watertight doors, for which an ASTM standard is referenced in the CFR, but many available doors are only certified to international or class standards. USCG Stability Policy and Guidelines Contained in Multiple Documents and Locations Having policy guidance distributed across multiple documents and locations creates problems:

25 1. There are multiple document types, each containing policy decisions. These documents may contain additional guidance other than stability, so finding the stability-related material is difficult. 2. The documents are not available at a single location. 3. Many documents are not searchable. 4. There is not a single database summary of the documents. 46 CFR Subchapter T—Small Passenger Vessels Stability Regulations Subchapter T regulations cover “small” passenger vessels, which are those that measure less than 100 gross tons (U.S. regulatory) and carry 150 or fewer passengers on day trips or 49 or fewer passengers on overnight trips. Originally Subchapter T covered all passenger vessels measuring less than 100 gross tons regardless of the number of passengers, and Subchapter H covered all passenger vessels over 100 gross tons. The change occurred as Subchapter T vessels were becoming significantly larger and carrying far more passengers than originally intended. The Subchapter K passenger vessel regulations were implemented to cover those passenger vessels that measure less than 100 gross tons (U.S. regulatory) and carry more than 150 passengers on day trips or more than 49 passengers on overnight trips. Both Subchapter H and Subchapter K passenger vessels intact and damage stability requirements are now contained solely within Subchapter S. However, at the request of the end users, Subchapter T retained the stability requirements for those vessels within the Subchapter T regulations, which apply to smaller vessels that often operate in less demanding conditions. Subchapter T stability regulations are more focused on simple calculations and simple tests than the Subchapter S regulations, which are based on traditional stability calculation methods.

26 Because of these differences, the report describes Subchapter T regulations in more detail to show some of the differences. Subchapter T, Part 178 Intact Stability and Seaworthiness contains the intact stability standards, while Part 179 Subdivision, Damage Stability Watertight Integrity Requirements contains the damage stability standards if applicable to the particular vessel. Subchapter T Intact Stability Requirements—Part 178 In Subchapter T there are basically two intact stability classes as defined in 178.320 and 178.325: those vessels using a simplified stability proof test and those using portions of the Subchapter S stability requirements. The use of the simplified stability proof test is limited to vessels in the following general classes:  Power Mono Hulls and Catamarans: Vessels that are less than 65 ft in length, carry fewer than 151 passengers, and have no more than one deck above the bulkhead deck except for a pilothouse.  Sailing Mono Hulls: Vessels that are less than 65 ft in length, carry fewer than 150 passengers, and have no more than one deck above the bulkhead deck, excluding a pilothouse. In addition, the simplified stability proof test is limited to those sailing vessels not operating on exposed waters, sailing in daylight hours only, carrying 49 or fewer passengers, having an angle of downflooding of more than 60 degrees, having a cockpit less than one-fifth the length on deck, and not having an unusual type, rig, or hull form. The vessel classes previously listed are not mandated to use the simplified stability proof test. At the owner’s option, he or she may use the stability standards listed in Subchapter S, as

27 they may provide a better passenger loading capability. All other vessels must use the listed portions of the intact and, if applicable, damage stability standards of Subchapter S. Intact Stability by Simplified Stability Proof Test For those vessels that are permitted to use a simplified stability proof test, and if the owner chooses to do so, there are three versions contained within Subchapter T: power vessels, in 178.330(a)(b); sailing vessels, in 178.330(a)(b)(c); and pontoon boats on protected waters, in 178.340. Power and sailing vessels are similar in that the vessel is placed in a specified loading condition with fuel and water tanks at approximately three-fourths full and the weight of the maximum number of permitted passengers, crew, and gear represented with movable weights such as barrels of water. For the pontoon boat permitted to use a simplified stability proof test, conditions are similar to the other two vessels, except fuel, water, and sewage tanks should be either empty or full. The weights are initially placed to simulate the vessel’s normal trimmed conditions and vertical center of gravity, except for the pontoon boats, which are placed on the centerline to minimize trim and heel. For all vessels, except the pontoon boats, the freeboards are then measured at specified locations. The vessel is then subjected to a heeling moment, with the movable weights calculated from either the maximum passenger heeling moment or the maximum wind heeling moment. The resulting freeboards are then measured. If the resulting freeboards meet the minimum conditions specified in Subchapter T, the vessel is assumed to have sufficient intact stability for safe operations. The pontoon boats use a similar movement of the weights and checking of freeboards, but with a much simpler process that requires no heeling moment calculations or minimum remaining freeboard calculations.

28 Intact Stability by Subchapter S Regulations For those vessels not permitted or whose owner chooses not to use the simplified stability proof test, Subchapter T, in 178.310 and 178.325, prescribes the parts of the Subchapter S regulations that must be met. The parts of Subchapter S that must be met depends on the type of vessel, power or sail. In general, the Subchapter S sections listed are:  Power Mono Hulls and Catamarans: These vessels must meet Sections 170.170 Weather Criteria, 170.173 Criterion for Vessels of Unusual Proportion and Form, and 171.050 Intact Stability Requirements for a Mechanically Propelled or a Non–Self- Propelled Vessel.  Sail Mono Hulls and Catamarans: In addition to the sections required for powered vessels, these vessels must also meet either 171.055 Intact Stability Requirements for a Mono Hull Sailing Vessel or 171.057 Intact Stability Requirements for a Catamaran Sailing Vessel. Subchapter T Damage Stability and Watertight Integrity Requirements Part 179 Subchapter T requires damage stability standards for certain classes of vessels depending on the vessel’s length, number of passengers carried, operating route, and hull material of construction. As with the intact stability requirements, those standards may be found in Subchapter T or in sections of Subchapter S. The damage stability standards are divided into two items, a collision bulkhead and watertight subdivision bulkheads. Collision bulkheads are required for the following classes of vessels, either power or sail:  179.210 (a)—All vessels greater than 65 ft in length regardless of number of passengers carried, route permitted, or hull construction material.

29  179.210 (b)—All vessels less than 65 ft in length that (1) carry more than 49 passengers, (2) operate on exposed waters, (3) operate on partially protected waters and are more than 40 ft in length, or (4) are constructed of wood and operate in cold waters. The requirements for the location and construction of the collision bulkhead are contained in 179.310. All other Subchapter T passenger vessels are not required to have a collision bulkhead. Watertight subdivision bulkhead requirements, if applicable, are divided similar to the intact stability requirements into those standards given directly in Subchapter T or found in sections of Subchapter S. The general division of the requirements for each class of vessels are:  179.212 (a)—All vessels less than 65 ft in length that either (1) carry more than 49 passengers or (2) are constructed of wood and operate in cold waters. These vessels must meet Section 179.220 Location of Watertight Bulkheads for Subdivision.  179.212 (b)—All vessels greater than 65 ft in length regardless of number of passengers carried, route permitted, or hull construction material. These vessels must meet Sections 171.070 through 171.073 in Subchapter S. The remaining Subchapter T passenger vessels are not required to have watertight subdivision bulkheads. Damage Stability Section 179.220—Location of Watertight Bulkheads for Subdivision This damage stability standard uses a table of floodable length factors to calculate the maximum permitted watertight bulkhead spacing for a given location along the hull. The floodable length factors vary based on the location of the compartment in question's center between the forward and after perpendicular. This factor is plugged into a simple formula along with the vessel’s length on deck and the effective freeboard and depth of the vessel in way of the compartment in

30 question to get a maximum permitted watertight bulkhead spacing. This method does not require any of the extensive damage stability calculations needed for larger Subchapter K and H passenger vessels that require the use of computer-based software. When coupled with the intact stability's simplified stability proof test, this method allows for a small passenger vessel to have effective intact and damage stability standards that do not require complex calculations or the services of a naval architect. The method does have a drawback in that it is very conservative in maximum permitted bulkhead spacing. Section 179.212(a) does give vessel owners the option of using the Type II subdivision requirements in Subchapter S instead of Section 179.220. If the owner selects this option, he or she cannot use the simplified stability proof test even if this is permitted by Section 178.320 or 178.325. The owner must also meet the Subchapter S intact stability Sections 170.170 Weather Criteria, 170.173 Criterion for Vessels of Unusual Proportion and Form, 171.050 Intact Stability Requirements for a Mechanically Propelled or a Non–Self-Propelled Vessel, and 171.055 Intact Stability Requirements for a Monohull Sailing Vessel. Damage Stability by Subchapter S Type II Sections 171.070 to 171.030 The Subchapter T passenger vessels that must meet the Subchapter S Type II subdivision standards in Sections 171.070 to 171.073 are the larger classes of vessels over 65 ft in length. These vessels must meet the same subdivision standards that are required for the Subchapter K and H passenger vessels. These vessels by their length would not be permitted under 178.310 to use the simplified stability proof test for their intact stability standards. The vessels would have to use the intact stability sections in Subchapter S as previously discussed in this report.

Next: 3. OPTIONS FOR UPDATING U.S. STABILITY REGULATIONS »
Review of U.S. Coast Guard Vessel Stability Regulations Get This Book
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On September 12, 2018, TRB released a letter report that reviews regulations and policy documents that establish vessel stability requirements for U.S. Flag vessels. The review, conducted at the request of the U.S. Coast Guard Office of Design and Engineering Standards, considers options to make and keep stability requirements current, align them better with international standards, and improve their consistency, clarity, and usability. In addition to identifying a series of promising options for these purposes, the report makes recommendations to the Coast Guard on coordinating with industry advisory groups and collecting, managing, and analyzing data to inform regulatory decisions.

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