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The U.S. Coast Guard (USCG) asked the committee to review ship stabil- ity regulations that exist outside of 46 Code of Federal Regulations (CFR) Subchapter S, to include 46 CFR Subchapters C and T, to determine if the standards reflect the current best practices for the vessels covered and if they warrant any clarifications, changes, or additions. In elaborating on the study charge, the USCG asked the committee to focus on the portion of the U.S. fleet operating inward of the Boundary Lines,1 which consists of smaller vessels in domestic service only, and not vessels that are internation- ally certified, ocean-going, and in coastwise service (as the latter operate outward of the Boundary Lines while traveling along the coast). Later it was decidedâwith USCG concurrenceâthat because the USCG is currently under a Notice of Proposed Rulemaking (NPRM) of the Subchapter C stability regulations that apply to certain fishing vessels, the committee would not review those regulations as part of this effort.2 This chapter begins with a short section that notes some places where specific Subchapter T stability requirements could be clarified. The re- maining focus of the chapter is on areas where the Subchapter T require- ments could potentially be supplemented by other standards, particularly to address concerns about flooding of smaller passenger vessels. According 1 As discussed in Chapter 2, Boundary Lines mark the divide between inland and offshore waters and generally follow the high tide waterline. Inward of the Boundary Line is gener- ally understood to mean partially protected and protected coastal and inland waters (see 46 CFR, part 7). 2 See https://www.federalregister.gov/documents/2016/06/21/2016-14399/commercial- fishing- vessels-implementation-of-2010-and-2012-legislation. 5 Improving the Stability Regulations Outside of Subchapter S 51
52 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE to USCG statistics, flooding accounted for almost 12% (or 66 cases) of the 562 reported marine casualties in the passenger vessel fleet in 2017.3 âFloodingâ was the third most common casualty for the passenger vessel fleet after âMaterial Failure/Malfunctionâ (23.5%) and âLoss/Reduction of Propulsion/Steeringâ (20.6%). It is notable that flooding was not among the three most common causes of casualties in other segments of the inspected fleet, including barge, cargo, outer continental shelf, research, school, and towing vessels. SUBCHAPTER T SIMPLIFIED STABILITY CLARIFICATIONS Vessels that meet certain characteristics (in 46 CFR 178.310, 178.320, and 178.325) may be certified using the simplified stability and subdivision criteria in Subchapter T. The simplified criteria are intended as an alterna- tive to the Subchapter S requirements, which require sophisticated stability software, accurate vessel plans, and an inclining experiment or detailed deadweight survey. Set up as proof tests or simple empirical calculations, the simplified criteria are intended to be more conservative than corre- sponding criteria in Subchapter S, as noted in the USCGâs Marine Safety Manual, Volume 4.4 Vessels covered by Subchapter T are generally smaller mono-hull craft of similar proportions, sometimes adapted from other services, and for which detailed plans may not be available. As vessels became larger, more sophisticated, and more capable (but still admeasuring less than 100 gross register tons), they would need to comply with the stability and subdivision standards in Subchapter S. The âbreak pointsâ between the two sets of stan- dards are, in the committeeâs view, adequately reasoned and appropriate. Table 5-1 identifies specific parts of Subchapter T that the commit- tee believes warrant clarification. The candidate changes are intended to make the regulations easier to understand, but would not change them substantively. 3 In 2017, âFloodingâ accounted for 11.8% of all reported marine casualties in the fleet. See Figure 10 on p. 6 at https://www.dco.uscg.mil/Portals/9/DCO%20Documents/5p/ CG-5PC/ CG-CVC/CVC1/AnnualRpt/2017DomesticAnnualReport.pdf. 4 See Chapter 6, Section 6.E.9.c on pp. 6â69 at https://media.defense.gov/2017/Mar/29/ 2001723819/-1/-1/0/CIM_16000_9.PDF.
STABILITY REGULATIONS OUTSIDE OF SUBCHAPTER S 53 TABLE 5-1 Potential Options for Updating Subchapter T Stability and Subdivision Requirements Cite Topic Comment Candidate Change PART 178âINTACT STABILITY AND SEAWORTHINESS PART 178, Subpart AâGeneral Provisions 178.115 Applicability to existing vessels Reviewed; no comment PART 178, Subpart BâStability Instructions for Operating Personnel 178.210 Stability Information Reviewed; no comment 178.215 Weight of passengers and crew Current wording in paragraph (b) is unclear: âthe owner of each vessel must provide the master with the total test weight.â âProvideâ could be taken to mean the owner must actually furnish test weights to the master Consider changing the word âprovideâ to the word âinformâ â¦ the owner of each vessel must inform provide the master of the total test weight. 178.220 Stability booklet Reviewed; no comment 178.230 Stability letter or Certificate of Inspection stability details Reviewed; no comment PART 178, Subpart CâIntact Stability Standards 178.310 Intact stability requirementsâ general Reviewed; no comment 178.320 Intact stability requirementsâ nonsailing vessels Current wording in paragraph (a) is unclear about requirement for tri-hull vessels (tri-toons) In paragraph (a), consider adding subparagraph (3) to allow simplified stability test of tri-hull vessels (tri-toons) 178.325 Intact stability requirementsâ monohull sailing vessels Current wording in paragraph (a)(7) is unclear about the use of the word âgreaterâ Suggest changing the word âgreaterâ to âlongerâ to avoid any ambiguity as to whether the regulation refers to width of the cockpit or its length continued
54 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE Cite Topic Comment Candidate Change 178.330 Simplified stability proof test (SST) 1. Current wording in paragraph (a)(4)(iv) after âThe vertical centerâ is unclear 2. Also, in paragraph (d)(6), the regulation mentions âcatamaran.â What about other multi- hull vessels? 1. Suggest adding âof gravityâ after âThe vertical center,â to read: âThe vertical center of gravity for the total test weight mustâ¦.â 2. Suggest adding âor tri- hull vessel,â to read: âOn a non-sailing flush deck catamaran or tri-hull vessel that is propelledâ¦.â 178.340 Stability standards for pontoon vessels on protected waters General Comment: The standards address longitudinal and transverse immersion separately, but could corner-crowding be considered? PART 178, Subpart DâDrainage of Weather Decks 178.410 Drainage of flush deck vessels Reviewed; no comment 178.420 Drainage of cockpit vessels Reviewed; no comment 178.430 Drainage of well deck vessels Reviewed; no comment 178.440 Drainage of open boats Reviewed; no comment 178.450 Calculation of drainage area for cockpit and well deck vessels Reviewed; no comment PART 178, Subpart EâSpecial Installations 178.510 Ballast Reviewed; no comment PART 179âSUBDIVISION, DAMAGE STABILITY AND WATERTIGHT INTEGRITY PART 179, Subpart AâGeneral Provisions 179.15 Incorporation by reference Reviewed; no comment 179.115 Applicability to existing vessels Reviewed; no comment TABLE 5-1 Continued
STABILITY REGULATIONS OUTSIDE OF SUBCHAPTER S 55 Cite Topic Comment Candidate Change PART 179, Subpart BâSubdivision and Damage Stability Requirements 179.210 Collision bulkheads Reviewed; no comment 179.212 Watertight bulkheads for subdivision and damage stability Guidance about potential openings for propulsion components is unclear or lacking Add provision for watertight bulkheads around potential openings for propulsion components 179.220 Location of watertight bulkheads for subdivision Reviewed; no comment 179.240 Foam flotation material 1. Believe in paragraph (b)(8), the intended word was âofâ rather than âorâ 2. Text in this paragraph is unclear about foam used for bottom flotation and buoyancy or fender collars around rigid-inflatable boat (RIB) craft 1. Suggest the second sentence read: The effective buoyancy of 881 kilograms per cubicâ¦ rather than âor 881 kilograms per cubicâ¦.â 2. Clarify paragraph to include foam used for bottom flotation and buoyancy or fender collars around RIB craft 179.310 Collision bulkheads Reviewed; no comment 179.320 Watertight bulkheads Reviewed; no comment 179.330 Watertight doors Reviewed; no comment 179.340 Trunks Reviewed; no comment 179.350 Openings in the side of a vessel below the bulkhead or weather deck. Reviewed; no comment 179.360 Watertight integrity Reviewed; no comment TABLE 5-1 Continued POTENTIAL SUPPLEMENTAL REQUIREMENTS As part of its review of the Subchapter T requirements, the committee examined requirements from other countries that apply to similar small passenger vessels. This review revealed several potential gaps or shortcom- ings in the existing Subchapter T requirements that apply to such vessels (open type or carrying not more than 49 day-service passengers). The gaps have to do with intact and damaged stability hazards unique to this vessel class, and include the absence of (1) a minimum required upright freeboard
56 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE when a vessel is intact, (2) standards for survivability when a vessel is intact but in swamped condition, and (3) standards for survivability when a hull is penetrated or a seawater connection is breached. Finally, consideration is given to the need for more USCG guidance on dynamic instabilities of high-speed craft not covered by current regulations. In considering potential supplemental requirements, the committee re- searched existing intact and damaged stability standards for small passenger vessels in other countries. The primary purpose of doing so was to identify additional stability standards required for these types of vessels that are not currently required by Subchapter T. The cited stability standards are illus- trative. Other international standards are available to the USCG as mod- els, such as International Organization for Standardization (ISO) 12217, which provides methods to evaluate intact stability for nonsailing boats and sailing boats of hull lengths greater than or equal to 6 m.5 Although not considered here, it might be advisable for the USCG to understand the format and technical aspects of the stability standards of other countries when developing any new standards. In doing so, the USCG might want to consider whether the standards are applicable to a vessel (such as mini- mum or maximum lengths, defined operating areas or sea states, or number of passengers on board) and whether they are prescriptive (i.e., specified minimum freeboard values or a minimum range of positive stability) or performance based (i.e., required to survive a given sea state). Minimum Required Upright Freeboard A vesselâs freeboard is the vertical distance between the waterline and the freeboard deck, which is ânormally the uppermost complete deck exposed to weather and sea, which has permanent means of closing all openings in the weather part thereof.â6 While there are many technicalities to this defi- nition for vessels with nonstandard freeboard deck configurations, these do not change the basic meaning of freeboard. Currently there are no absolute minimum upright freeboard requirements for small passenger vessels sub- ject to Subchapter T. However, there are three intact stability standards that are freeboard-based and applicable to these vessels: (1) the Simplified stabil- ity proof test (SST; see 46 CFR 178.330), (2) the weather criteria (46 CFR 170.170), and (3) the passenger heel criteria (46 CFR 171.050). The SST only specifies that the upright freeboard cannot be immersed by an amount based on the type of vessel (e.g., open, cockpit, decked) when heeled by 5 For example, Transport Canada currently requires power-driven passenger vessels of more than 6 m and less than 15 gross tons and carrying 12 or fewer passengers to comply with the requirements of ISO 12217 Part 1. 6 46 CFR Part 42.13-15(i).
STABILITY REGULATIONS OUTSIDE OF SUBCHAPTER S 57 either a passenger or wind-heeling moment. The SST does not specify what the minimum upright freeboard should be, apart from the requirement that it not be immersed. The alternative intact stability criteria and the weather and passenger heel criteria also specify that the upright freeboard cannot be immersed beyond a certain distance when heeled by either a passenger or wind-heeling moment. These criteria also have a specified maximum heel angle that cannot be exceeded. None of the standards and criteria, however, are specific in identifying an absolute minimum freeboard in any condition for the small passenger vessels. Although there are some requirements for minimum freeboard to the deck in well deck and cockpit vessels in Subchapter T, 178.420 (drainage of cockpit vessels) and 178.430 (drainage of well deck vessels), these require- ments are not absolute in specifying a minimum required freeboard for all circumstances. For example, the cockpit deck of such vessels operating on protected waters must be above the deepest load waterline. For cock- pit vessels operating on partially protected or exposed waters, there is a minimum cockpit deck freeboard requirement of 10 inches unless the vessel meets certain Subchapter S requirements, in which case this distance can be lower. In all cases, there is no minimum freeboard requirement for the vesselâs gunwale or deck at edge, which is important for keeping boarding seas out of the vessel. Because of the format of the Subchapter T stability regulations, the freeboard of passenger vesselsâparticularly smaller passenger vessels not longer than 65 ft and operating in protected or partially protected watersâ tends to be proportional to the length of the vessel; that is, the smaller the vessel, the lower the freeboard, and the larger the vessel, the higher the freeboard. Although the vessels have adequate intact stability for the expected maximum passenger and wind-heeling moments and ânormalâ sea conditions, smaller vessels may not have adequate protection against potential unexpected boarding seas or large wakes from larger vessels operat ing nearby. The absence of a minimum upright freeboard that takes into account a wider variety of sea conditions can be problematic. Although the SST and weather criteria are directly proportional to vessel size with respect to passenger and wind heeling, the seas encountered by a passenger vessel are based on the wind and wave conditions in a particular location and not directly proportional to vessel size. Minimum freeboard requirements for small passenger vessels can be found in the standards and regulations of other countries; examples are pro- vided in Box 5-1. The purpose of these requirements is to provide a minimum level of protection against unexpected boarding seas that could be experi- enced on the waters where the vessel normally operates, including protection against wakes from larger vessels operating nearby in congested areas.
58 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE BOX 5-1 Examples from the Regulations of Other Countries of Minimum Required Freeboards Australian National Standards for Commercial Vessels Part C Section 6 Subsection 6AâIntact Stability Requirements Regulation 188.8.131.52.2âSpecifies a minimum freeboard to the lowest weathertight deck for passenger vessels based on the vesselâs length. The minimum required freeboards range from 150 mm (6 in) for vessels of 6 m (20 ft) or less to 250 mm (10 in) for vessels over 10 m (32 ft). Part C Section 6 Subsection 6BâBuoyancy and Stability After Flooding Specifies vessel use categories and operational areas. Danish Maritime Authority Order No. 1012 Regulation 5âFreeboard Conditions and Marking Part (5) specifies a minimum freeboard for passenger vessels exclusively oper- ated in port areas or on lakes: for decked vessels, a freeboard of not less than 300 mm (12 in), and for open vessels, a freeboard of not less than 250 mm (10 in). Specifies operating areas for ports and lakes. Danish Maritime Authority Technical Regulation No. 2 Existing Passenger Ships Engaged on Domestic Voyages Part 2 Intact StabilityâSection 4 Part (2) specifies each individual vessel shall have a minimum freeboard assigned by the Danish Maritime Authority âin consideration of the shipâs trade area.â Part (3) specifies a minimum freeboard in all cases of at least 0.15 m (6 in). Danish Maritime Authority Technical Regulation No. 10 Small Vessels Carrying a Maximum of Passenger Regulation 5âFreeboard Conditions Part (1) specifies a minimum freeboard of at least 5% of the vesselâs breadth. An absolute minimum freeboard of least 0.20 m (8 in) with a minimum of 0.30 m (12 in) for vessels trading in the Greenland area is also specified. Safety Code for Passenger Ships Operating Solely in United Kingdom (UK) Categorized Waters MSN 1823 (M) Annex 7âThe Heeling Test and Freeboard Measurements Regulation 9âFreeboard Measurements Part (9.1) specifies a minimum mean loaded freeboard to the deck edge at the side for passenger vessels based on the vesselâs length. The minimum required freeboards range from 380 mm (15 in) for vessels of 6 m (20 ft) or less to 760 mm (30 in) for vessels over 18.3 m (60 ft).
STABILITY REGULATIONS OUTSIDE OF SUBCHAPTER S 59 Two important features of these regulations are that they (1) require a minimum freeboard measured to either the vesselâs deck edge at side or the lowest weathertight deck based on vessel type and (2) are based on the geographical area where the vessel will be operated. The reason for the first feature is that vessel design is relevant to the risk of casualty should seas board the vessel. Open vessels have the highest risk should this hap- pen, whereas flush deck vessels with adequate deck drainage will have a lower risk and consequently can have a lower minimum freeboard. Cockpit, well deck, and open vessels need a minimum âheight of sideâ7 to protect against boarding seas, while cockpit and well deck boats specify a minimum freeboard to the deck to protect against back flooding through deck drains and freeing ports. The second feature helps account for differences in risks associated with an operating areaâs prevalent and extreme wave and wake conditions. It merits noting that Danish regulations refer to âtrading area;â the Australian code specifies âoperational area;â and the UK code discusses âcategorised watersâ as a way to mitigate known risks when addressing minimum freeboard requirements for some passenger vessels operating in different geographical areas. Swamped Vessel Survivability Small passenger vessels of the open type that carry not more than 49 pas- sengers or that have relatively large cockpits or wells can be susceptible to swamping if exposed to large seas. For these vessels, there are currently no regulations in 46 CFR Subchapter T that provide for the survivability of the vessel if swamped. A concern is that such vessels may not remain afloat upright and would either capsize or sink and force the passengers and crew into the water. There are documented cases of small passenger vessels not having enough freeboard and being swamped or overwhelmed by larger than expected waves in what were considered to be sheltered waters (i.e., those designated as protected or partially protected by the local Officer in Charge, Marine Inspection). A recent example is the swamping of the DUKW8 excursion vessel Stretch Duck 7 during a severe thunderstorm on Table Rock Lake, Missouri, on July 19, 2018, resulting in 17 fatalities. It merits noting that the USCG has swamping criteria for Rigid Hull Inflatables (RHI) and Rigid Hull Foam Collar (RHFC) vessels. These cri- teria are listed in Marine Technical Note (MTN) 01-08, âMarine Safety 7 âHeight of sideâ with respect to an open boat means the distance between the waterline and the lowest point of the gunwale. The clear height should be measured to the top of the gunwale or capping or to the top of the wash strake if one is fitted above the capping. 8 The letters DUKW originate from General Motors manufacturing code: D = designed in 1942; U = utility; K = all-wheel drive; W = dual-tandem rear axles. See https://en.wikipedia. org/wiki/DUKW.
60 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE Center Review of Rigid Hull Inflatable and Rigid Hull Foam Collar Ves- sels.â This MTN provides USCG-accepted alternative design standards for RHI and RHFC vessels that are considered equivalent to certain regula- tory requirements in Subchapter T and Subchapter S. The basic swamped criteria format is contained in Box 5-2. The intent of these standards is to have the swamped vessel remain afloat and upright and thus become the âlife raftâ for the passengers and crew as opposed to depending solely on the use of life jackets or other lifesaving appliances because the vessel sank. Here again, standards and regulations of other developed countries contain minimum survivability requirements for small passenger vessels when swamped. Examples are provided in Box 5-3. Swamped vessel survivability standards can apply to (1) open type ves- sels that have no self-drainage capability and that are subject to permanent flooding and (2) cockpit and well deck-type vessels that have large spaces that can be temporarily flooded. Although cockpit and well deck-type small passenger vessels might seem to present less risk from swamping than open passenger vessels due to their self-draining watertight decks, they are just as likely to be swamped as an open boat, at least for a short time until BOX 5-2 Summary of Coast Guard Swamping Criteria for Rigid Hull Inflatables (RHI) and Rigid Hull Foam Collar (RHFC) Vessels Swamping Criteria: The following must be satisfied when an intact RHI becomes swamped, or partially swamped, in any condition of loading. a. The point of least freeboard must not submerge in the static condition; b. There should be no appreciable change in vessel heel; c. The range of positive stability beyond equilibrium must be at least 5 degrees for protected waters, 10 degrees for partially protected waters, and 15 degrees for exposed waters; d. There must be at least 2.82 foot degrees of righting energy from the equilibrium heel angle to the angle of vanishing stability; and e. The maximum righting arm must be at least 0.33 ft. SOURCE: Marine Technical Note (MTN) 01-08, âMarine Safety Center Review of Rigid Hull Inflatable and Rigid Hull Foam Collar Vessels.â
STABILITY REGULATIONS OUTSIDE OF SUBCHAPTER S 61 BOX 5-3 Examples of Minimum Survivability Requirements for Small Passenger Vessels from Other Countries Australian National Standards for Commercial Vessels Part C Section 6 Subsection 6BâBuoyancy and Stability After Flooding Chapter 4âMeasures to Control Consequences of Swamping Regulation 4.2.1âDefines the classes of passenger vessel considered to be at ârisk of swamping.â In general, they are vessels of less than 6 m (20 ft) and those vessels between 6 m (20 ft) and 24 m (79 ft) with a âflooding risk categoryâ of II. Regulation 4.4.1âSpecifies the âdeemed-to-satisfyâ solutions to control the con- sequences of swamping. The acceptable deemed-to-satisfy solutions are based on whether it is a âseagoingâ vessel or one operating on âsheltered water,â as well as to what âflooding risk categoryâ it falls in. In general, there are two solutions: âbasic flotationâ and âlevel flotation.â Transport Canada Marine Safety TP 11717 Standards for the Construction and Inspection of Small Passenger Vessels Standard 6.4.2 provides minimum stability standards when in the defined swamped condition of a metacentric height of at least 0.05 m (2 in) and a mini- mum freeboard of 150 mm (6 in) to the top of any wells or cockpits. Transport Canada Marine Safety TP 10/2007 Passenger Vessel Operations and Damaged Stability Standards (Non-Convention Vessels) Part IIâDamaged Stability Part (6) provides minimum stability standards when in the defined swamped con- dition of a metacentric height of at least 0.05 m (2 in) and a minimum freeboard of 150 mm (6 in) to the top of any wells or cockpits. the water drains. Because of their drainage capability, the former vessels are assumed to have the ability to self-recover from a swamped condition. While such a recovery may be expected for minor flooding conditions, a major swamping event could have just as dramatic of an initial impact on a cockpit or well deck-type passenger boat as an open boat. As discussed next, the standards for each vessel type can differ somewhat to reflect the permanent flooded condition that open boats will experience as opposed to the temporary flooded condition that will be experienced by cockpit and well deck-type boats.
62 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE Swamped Survivability from Permanent Flooding of Open Passenger Vessels For the open type small passenger vessels, swamped survivability standards would need to achieve the following goals. 1. The vessel will remain upright with sufficient residual stability to meet a. A specified metacentric height (GM), b. The heeling moment from passengers hanging on one side of the vesselâs gunwale, or c. The heeling moment from large canopies and windbreaks if present. 2. The vessel will have a minimum residual freeboard at any location along its length. The intent of goal 1.a is to provide some measure of stability to counter the sea and wind conditions and to keep the vessel upright and usable as a life raft until rescue. Because the vessel is now swamped and the passengers are no longer contributing a significant load (they are mostly supported by their own buoyancy), only a fraction of the vesselâs original stability is likely required. The intent of goal 1.b is to account for two possible scenarios that are likely to occur during swamping or after swamping has occurred. One scenario is that some of the passengers are washed overboard during the swamping and are either attempting to get back into the swamped vessel or are clinging to a side of the vessel awaiting rescue. The other scenario is that some of the passengers are abandoning the vessel after the swamping and swimming to nearby vessels or to shore. Goal 1.c applies to small open passenger vessels whose canopies and windbreaks remain above the swamped waterline, presenting significant wind area. The objective in this case is to keep passengers and crew from becoming trapped under the canopy or windbreaks. The intent of Goal 2 is to minimize the shipping of additional water into the passenger space and to provide some measure of protection for the passengers against boarding seas while they await rescue. Swamped Survivability from Temporary Flooding of Cockpit and Well Deck Passenger Vessels Swamped survivability standards for cockpit and well deck passenger vessel types would therefore want to achieve the following goals similar to those of the open boat swamped survivability standards, including
STABILITY REGULATIONS OUTSIDE OF SUBCHAPTER S 63 1. The vessel will remain upright with sufficient residual stability to meet a. A specified minimum GM, or b. The heeling moment from large canopies and windbreaks if present. 2. The vessel will have a minimum residual freeboard at any location along its length. The survivability standardâs values could also be adjusted to reflect the effectiveness of the vesselâs self-draining capabilities. Vessels with more ef- fective drainage will spend less time with their stability compromised and consequently could have lower standards for Goals 1.a and 1.b. It may not be advisable for Goal 2âs minimum freeboard to be relaxed in this instance to minimize the recurrence of boarding sea swamping the vessel again while it is in an already partially swamped and thus vulnerable condition. Additionally, a requirement for the swamped vessel to survive with pas- sengers clinging to one side is not needed for cockpit or well deck vessels due to the temporary nature of their swamping situation. Any passengers washed overboard during the initial swamping will be recovered after the vessel has drained at least partially and recovered a significant portion of its preswamped initial stability levels. In addition, because this class of cockpit and well deck vessels is self-rescuing, no transfer of passengers to another vessel would be necessary while the vessel is in a temporarily swamped condition. Combining the Minimum Freeboard and Swamped Vessel Survivability Standards Minimum required freeboard and swamped condition standards work together to prevent a stability failure when operating in large seas. The dif- ference between the two is that one is proactive and the other is reactive. The minimum required freeboard provides safety by minimizing the chance of a boarding sea occurring in the first place, while the swamped surviv- ability standard would provide a measure of safety after a boarding sea has occurred. Because of this interrelationship, a connection between the standardsâ prescriptions would seem to be beneficial. For cases where an open boat is operating on a large lake, it might be appropriate for the open boat to be able to survive when the boat is completely full of water as the water cannot drain out of this type of open vessel and successive boarding waves will potentially flood the vessel fully. However, if the vesselâs freeboard was higher than the required mini- mum, then it may also be appropriate for some relaxation in the assumed swamped condition. Because the vessel has the higher freeboard, the chance
64 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE of a boarding wave is less and the amount of water boarding would also be reduced. Consequently, it may be appropriate to reduce the amount of water assumed trapped in the vessel when assessing the vesselâs swamped survivability, such as by assuming the vessel is only half full of water. Survivability from a Hull Penetration and Seawater Connection Breach Small passenger vessels certified under Subchapter T that are less than 65 ft in length, carry not more than 49 passengers, and operate on protected or partially protected waters do not have to meet subdivision requirements (with a few exceptions) (46 CFR 179.212). As previously noted, in 2017 flooding accounted for almost 12% of reported marine casualties in the U.S.-inspected passenger vessel fleet.9 Thus, if the vesselâs underwater hull watertight integrity is breached in any manner, there are no requirements in place to prevent the vessel from sinking. The only exceptions are for wood vessels constructed after March 10, 2001, that are operated on cold waters. The potential causes of damage to a small passenger vesselâs watertight integrity are limited. The four main causes of damage for all kinds of vessels are: 1. Failure of a seawater hull connection or failure of a pipe passing through a space that is connected to the sea. 2. Failure of the hullâs watertight envelope, such as a plate seam split- ting or a wood plank coming loose. 3. Grounding of the vessel on the sea bottom, which breaches the vesselâs hull. 4. Collision with another vessel that breaches the vesselâs hull. However, the likelihood of the last three causes occurring among small passenger vessels is reduced due to their unique design and operating profiles. The hull structural integrity is robust (the vessels are regularly hauled using a travel lift) and inspected on a regular basis, so hull failure is unlikely. Ground- ing is also unlikely, as this type of small passenger vessel is typically operated in a given area in which hazards are well known to the captains. Collisions are also unlikely, as small passenger vessels generally operate in good weather conditions and the vessels are highly maneuverable. Even if a collision were to occur, small passenger vessels with their robust hull structures can readily bounce off with no damage to their water tight integrity. However, the first of these potential causes of a vesselâs loss of water- tight integrity, failure of a seawater connection or seawater pipe, has more 9 See Figure 10 on p. 6 at https://www.dco.uscg.mil/Portals/9/DCO%20Documents/5p/CG-5PC/ CG-CVC/CVC1/AnnualRpt/2017DomesticAnnualReport.pdf.
STABILITY REGULATIONS OUTSIDE OF SUBCHAPTER S 65 relevance to small passenger vessels due to the large number of seawater connection points that can be found on such vessels. These connections include â¢ Seawater supply for fire pumps â¢ Seawater cooling water supply for main engines, generators, and air conditioning units â¢ Seawater supply for deck washdown â¢ Propeller shaft seals or stuffing boxes â¢ Rudder stock seals or stuffing boxes â¢ Stern drive and sail drive sealing boots â¢ Nonintegral engine and air conditioning keel coolers â¢ Depth sounders and fish locating sonar transducers â¢ Stabilizer fins â¢ Retractable or movable keels on sailboats Due in part to the corrosive nature of seawater, potential failures of these connections can arise from conditions such as seawater pipe hull con- nection fractures, internal wastage of the pipes and fittings, and degradation of flexible hoses and hose connections. For example, a typical main engine with a seawater-cooled heat exchanger and water-cooled exhaust would have the following potential failure points: â¢ Seachest or seawater pipe hull connection â¢ Seacock or seachest valve (corrosion from dissimilar metals) â¢ Piping lines, fittings, and valves to the engine â¢ Flexible hose connection to the engine-driven pump â¢ Multiple flexible hose connections on the engine between the pump, cooler, and exhaust water discharge riser â¢ Flexible hose connections on the exhaust line after the discharge riser Propeller shaft and rudder stock penetrations for small passenger vessels are also âseawater connections.â These components typically consist of a stuffing box or dripless mechanical seal connected with a rubber flexible hose to the vesselâs stern or rudder tube. As with the seawater supply connec- tions, there are multiple failure points in each one of these propeller shaft or rudder stock hull penetrations. Starting from the seawater side, the potential failure points could be failure of the stern or rudder tubes from internal cor- rosion, the flexible hose from normal degradation of the hose over time, and the flexible hose securing clamps from corrosion or becoming loose. Additionally, small passenger vessels may employ âstern-,â âsail-,â or more recently âpod-â drives, which depend on a flexible seal, or âboot,â
66 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE between the inboard engine and the external propulsor. As with a propeller shaft or rudder stock penetration failure, the failure of a stern drive boot can create a large opening in the vesselâs watertight envelope for which there is currently no required means to isolate the failure point to prevent the loss of the vessel. A failure of any one of these seawater connections can create an un- manageable flooding situation in a small passenger vessel. Although the USCG has requirements for seacocks or seachest shutoff valves on all sea connections, on small vessels these devices are often hidden or not in read- ily accessible locations; susceptible to corrosion from dissimilar metals; and (like most sea valves) not exercised on a regular basis. As a result, the seacock or shutoff valve may not be able to be closed when needed in an emergency. Likewise, USCG requirements for bilge pumps on small passenger vessels can have limited effect on keeping a vessel afloat from a watertight hull integrity failure because they are designed mainly for draining incidental water accumulation rather than breaches in the vesselâs watertight integrity.10 Box 5-4 gives some examples of minimum survivability requirements in other developed countriesâ standards and regulations for small passenger vessels when subject to the kinds of hull envelope watertight failures previ- ously discussed. A primary aim of such standards is to control the conse- quences of local flooding and allow the vessel to remain afloat and upright with adequate stability while awaiting rescue. While the committee is not in a position to give specific guidance on how the USCG might design such standards, the focus would presumably be on requiring local watertight compartments or partial subdivision of a vesselâs hull. For example, such local subdivision requirements might target: â¢ Propeller shaft and rudder stock penetrations: They could either be located in small watertight compartments or be contained inside longitudinal and/or transverse watertight bulkheads. The bulk- heads would only need to be high enough to contain the potential flooding of the local space containing the hull penetration under normal operating drafts and trims with a small margin. Alterna- tively, rudder tubes could be extended so the gland or seal is above the waterline. 10 46 CFR 182.520 requires that vessels 26 to 65 ft long and that carry not more than 49 passengers either have (1) a fixed power bilge pump (10 gallons per minute [GPM]) and portable hand pump (10 GPM) or (2) a fixed hand bilge pump (10 GPM) and portable hand pump (5 GPM). Vessels under 26 ft are only required to have a portable hand pump (5 GPM). A 1-in diameter hole submerged 2 ft below the waterline will allow an inflow of approximately 27 GPM.
STABILITY REGULATIONS OUTSIDE OF SUBCHAPTER S 67 â¢ Seawater cooled engines: In this situation the engine could be con- tained inside longitudinal and/or transverse watertight bulkheads high enough to contain the potential flooding of the local space containing the engine(s) under normal operating drafts and trims with a small margin. â¢ Stern drive propulsion units: The inboard side of the stern drive system could be located in a small watertight compartment at the transom of the vessel. The compartment would be sized so that under flooding conditions the vessel has adequate minimum free- boards and stability levels to remain afloat and upright. â¢ Water jet drives: Water jet intake ducts, impeller housings, and other inboard-mounted components, if not integral with and of the same or heavier construction as the surrounding hull, could be enclosed by watertight bulkheads so that a breach of the housing will not flood the hull of the vessel. It is important to emphasize that these candidate solutions are offered only as examples for the USCG to consider. The committee was not able to examine each in depth. BOX 5-4 Examples of Minimum Survivability Requirements from Other Countriesâ Regulations for Small Passenger Vessels That Apply to Hull Envelope Watertight Failures Australian National Standards for Commercial Vessels Part C Section 6 Subsection 6BâBuoyancy and Stability After Flooding Chapter 5âMeasures to Control Consequences of Local Flooding Part 5.1 defines the sources of local flooding to include âsea water systems, skin fittings, holes in the shell due to corrosion, rot or leaking caulking, shaft seals, shaft failures, leakage through closing appliances, and structural failures.â Part 5.4 requires that âa space containing seawater systems, stern glands, rudder stock penetrations, thrusters, canting or retractable keels shall be separated from other spaces in the vessel by watertight boundaries.â Safety Code for Passenger Ships Operating Solely in UK Categorized Waters MSN 1823 (M) Regulation 6.1âWatertight Subdivision Part 6.1.2 requires âthe stern gland of every such ship shall be situated in a water- tight shaft tunnel or other watertight space.â
68 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE A Note About Dynamic Instabilities of High-Speed Craft High-speed, dynamically supported craft, such as air-cushion vehicles and hydrofoil boats, have been shown to exhibit various types of instabilities (see Cohen and Blount 1988, Table 1) that are not currently addressed in USCG regulations. Considered a function of speed, these instabilities are influenced by hydrodynamic pressures dominating the hydrostatic pressure (Codega 1994) and are not completely understood using a simplified zero speed hydrostatic analysis. The instabilities have been shown to exist in calm water and may be amplified in the presence of waves (Troesch and Falzarano 1992). The dynamic stability of high-speed craft can be studied in the design phase using theoretical and numerical analysis methods or systematic physi- cal model testing, but must be verified with full-scale trials guided by an understanding of the important variables that affect the phenomenon. If detected, an instability can be corrected using various methods (see Codega and Lewis 1987), but can only be avoided by operational limits as a last resort. Also understood is that these phenomena are affected by changing the loading and powering, or by operating the vessel in waves. Vessels that are marginally stable in calm water at low speed may become unstable at higher speed and in waves. Hard chine monohull planing hulls are the most common type of high-speed craft, but other vessel hull types exist (e.g., multihull vessels, round bottom high-speed vessels, Small Waterplane Area Twin Hulls, hydrofoil, and air cushion vehicles), each exhibiting their own type of instability. The International Maritime Organization High Speed Craft Code (IMO HSC) (IMO 2000) describes many of these stability issues and illustrates that other methods of demonstrating compliance with the requirements may be accepted, if the method chosen is shown to deliver an equivalent level of safety. Methods may include mathematical simulation of dynamic behavior, scale model testing, or full-scale trials. In addition, the IMO HSC lists 10 types of known stability hazards experienced by high-speed craft that should be verified by model or full-scale tests and/or calculations (see IMO 2000, p. 27). Vessels require such calculations and/or tests to demon- strate that they will return to an original state after a disturbance by one or more of the known stability hazards listed in the HSC. SUMMARY At the request of the USCG, the committee reviewed the current intact and damaged stability standards in Subchapter T (Part 178 Intact, Part 179 Sub- division) to determine if the standards reflect the current best practices for the vessels covered and if they require changes or additions. In general, the
STABILITY REGULATIONS OUTSIDE OF SUBCHAPTER S 69 committee found that the existing Subchapter T standards reflect the cur- rent best practices for the domestic passenger vessels covered, and that the transition between when a vessel can use the simplified intact stability tests and the simplified subdivision calculations (as opposed to the stability stan- dards in 46 CFR Subchapter S) is adequately reasoned. The committee did find instances in which the current regulations could be updated to improve clarity (as outlined in Table 5-1) and where they might benefit from some supplemental requirements. Informed by the discussions in this chapter, the USCG may want to consider whether it is desirable to add stability standards for some of the smaller passenger vessels to address stability hazards unique to these vessels. The additions could address minimum required freeboards to protect against boarding seas, survivability in a swamped condition, and local subdivision in the event of a seawater connection failure. In particular, candidate vessel types for such standards are those that have open, well, or large cockpit designs and those carrying not more than 49 passengers that have no subdivision requirements. Although the committee had limited time to discuss all other vessel types that may be candidates for supplements to existing stability regula- tion, it merits noting that high-speed, dynamically supported craft can exhibit instabilities that are not currently addressed in existing stability regulations. Accordingly, this too is an area for which the USCG could aim to provide guidance. REFERENCES Abbreviations IMO International Maritime Organization IMO MSC International Maritime Organization Maritime Safety Committee Codega, L. The Dynamic Stability of High Speed Craft. Professional Boat Builder, Octoberâ November 1994. Codega, L., and J. Lewis. Case Study of Dynamic Instability in a Planing Hull. Marine Tech- nology Society Journal, Vol. 24(2), 1987, pp. 143â163. Cohen, S., and D. Blount. Research Plan for the Investigation of Dynamic Instability of Small High-Speed Craft, Transactions of Society of Naval Architects and Marine Engineers, 1988. IMO. RESOLUTION MSC.97(73), Adoption of the International Code of Safety for High- Speed Craft, 2000 (2000 HSC Code) (adopted on December 5, 2000). Troesch, A., and J. Falzarano. Modern Nonlinear Dynamical Analysis of Vertical PlaneMotion of Planing Hulls, Journal of Ship Research, 1992.