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33 For DUKWs, the greatest risk to passenger safety is flooding from the failure of through-hull penetrations, hull damage, or overtopping waves. Ideally, DUKW vessels would have the capability to remain afloat and upright even when flooded. This capability has been advocated by the National Trans- portation Safety Board (NTSB) and in proposed legislation. However, Sub- chapter T regulations do not require this capability in small passenger vessels the size and passenger complement of DUKWs. In addition, achieving this level of buoyancy on DUKW vessels has proven to be difficult. This chapter describes the risks to DUKWs that can lead to flooding, the results of that flooding, studies and countermeasures that have been tried in the past, and actions the United States Coast Guard (USCG) can take to reduce the likelihood and consequences of flooding. Specific solu- tions explored target increasing a vesselâs buoyancy and removing the risks posed by driveshaft boot seals and the shortcomings of the Higgins Pump, both original to the WWII design. Mitigations for the risks posed by drain plugs and valves and engine cooling air vents are also discussed. Ultimately, additional actions and regulatory guidance are needed to reduce the likeli- hood of flooding and, when flooding occurs, to mitigate its effects. FLOODING AND CASUALTY EVENTS USCGâs Marine Information for Safety and Law Enforcement (MISLE) database lists a total of 11 âflooding-initialâ events from 1999â2021, and flooding was also the cause of all the DUKWs lost to sinking. For each of 3 Flooding and Survivability
34 OPTIONS FOR IMPROVING THE SAFETY OF DUKW TYPE AMPHIBIOUS VESSELS the four DUKW sinking accidents, the principal factors contributing to their loss were as follows: â¢ The Miss Majestic had a driveshaft boot seal that came loose after an incorrect repair. â¢ The Seattle Stretch Duck was missing a large drain plug after a repair. â¢ The DUKW 34 was run over by a barge and pushed under water. â¢ The Stretch Duck 7 (2018) had progressive flooding from large waves swamping the vessel through the bow engine cooling air vents and over the low freeboard on the sides. Flooding-initial events not leading to sinking include the Stretch Duck 7 (2015), which experienced partial flooding after a harder than normal entry into the water. The failure of shaft seals on two Truck Ducks operating in Tumon, Guam, allowed water to flood into the âsea chest box,â a small watertight compartment, which did not compromise the overall surviv- ability of the vessels. As reported to the committee, five Boston Ducks also experienced flooding-initial events during the span of 1999â2006, although little detail is available about the causes. The two sinking events in Liverpool, United Kingdom, were also pre- ceded by flooding. Wacker Quacker 4 sank in January 2013 after suf- fering a steering failure at the same time a hull drain plug was missing. The Higgins Pump masked the flooding from the missing drain plug until they stopped the engine during passenger evacuation and could not get it restarted. Wacker Quacker 1, which sank in March 2013, suffered hull breaches after its propeller hit a discarded car tire. Its passengers and crew abandoned into the water.1 The principal factors that led to these flooding and sinking events are characteristic of many DUKWs. DUKWs are considered âopen boats,â and as such are more susceptible to catastrophic swamping from boarding seas. In addition, DUKWs do not have a proper âship shapeâ bow; rather, they have a flat or âscowâ bow and a truck hood. The bow design worsens a DUKWâs ability to go through large waves without taking on water. Some DUKWs have large openings on the bow for engine cooling air intake/ exhaust vents. The WWII DUKWs and Stretch Ducks have low freeboards, ranging from 12 to 24 inches at their lowest point. Finally, because DUKWs also travel on land and have other unique design features, the vessels have multiple hull penetrations below the waterline. They may have multiple 1 MAIB, 2014, Very Serious Marine Casualty Report NO 32/2014, December. https://assets. publishing.service.gov.uk/media/54c1722240f0b6158d00002b/MAIBReport_32-2014.pdf.
FLOODING AND SURVIVABILITY 35 shaft seals and hull drain plugs that are potential large flooding points if damaged, repaired incorrectly, or inadvertently left open. However, considering the actual casualty events, no DUKW to date has been lost only because of insufficient intact stability. In all cases, the DUKWâs initial intact stability had been compromised by progressive flood- ing of some type before the DUKW was lost. Stability for a boat is the abil- ity to return to an upright position after experiencing a force such as wind, waves, or the forces resulting from the loading and unloading of cargo and passengers. Intact stability, as opposed to damage stability, is the initial stability of the vessel before any incidents that could weaken it.2 REGULATIONS AND NVIC 1-01 Regulations and policy documents applicable to DUKW stability and sea- worthiness cover intact stability and watertight integrity. Because DUKWs carry fewer than 50 passengers and are less than 65 feet in length, they do not need to meet damage stability standards (46 CFR 179.212). The Navigation and Vessel Inspection Circular 1-01 (NVIC 1-01) policy of us- ing bilge pumps during a flooding emergency is also intended to mitigate the risk of flooding. Intact Stability Standards Under Subchapter T (46 CFR 178.310), DUKWs may use either the Simpli- fied Stability Test (SST) or the intact stability criteria contained in 46 CFR Subchapter S, Subdivision and Stability. The choice of standard is depen- dent on several conditions regarding the boatâs size, arrangement, and ser- vice. DUKWs that only operate on âprotected waters,â as per NVIC 1-01, can opt to use SST. DUKWs with routes on âpartially protected waters,â must use the intact stability criteria given in Subchapter S. The owners of DUKWs operating on âprotected watersâ can also elect to use the stability criteria specified in Subchapter S. Because DUKWs are considered open boats, they are subject to addi- tional intact stability criteria under both SST and Subchapter S. Under 46 CFR 175.400, the definition of an open boat is âa vessel not protected from entry of water by means of a complete weathertight deck, or by a combina- tion of a partial weathertight deck and superstructure that is structurally suitable for the waters upon which the vessel operates.â 2 National Academies of Sciences, Engineering, and Medicine (NASEM), 2018, Review of U.S. Coast Guard Vessel Stability Regulations, Washington, DC: The National Academies Press, p. 11. https://doi.org/10.17226/25258.
36 OPTIONS FOR IMPROVING THE SAFETY OF DUKW TYPE AMPHIBIOUS VESSELS The SST contained in 46 CFR 178.330 is in essence a simple proof test performed on the DUKW at the loading (i.e., operating) condition specified in the regulations. The DUKW is first loaded with fixed weights that rep- resent the weight and location of the maximum number of passengers and crew that will be permitted on board. The lowest freeboard of the DUKW from the waterline to its gunwale is then measured. A portion of the fixed weights is then moved out board to one side to represent the effect of wind and passenger heeling on the DUKW. As an open boat, a DUKW will pass the SST if less than one-quarter of the upright freeboard is subsequently submerged when the weights are moved. In general, using Subchapter S involves performing an inclining or other suitable means to accurately determine the DUKWâs lightship char- acteristics of weight and vertical and longitudinal center of gravity. With this information a detailed set of stability calculations is performed to show compliance with the three standards. Subchapter S requires DUKWs to meet stability criteria for vessels of unusual proportion and form (46 CFR 170.173), for weather (46 CFR 170.170), and passenger heeling requirements (46 CFR 171.050). The weather criteria and passenger heeling criteria were developed for mechani- cally powered vessels of ordinary proportions and form with flush decks (170.170(d)). USCGâs Marine Safety Center (MSC) requires additional calculations to be submitted in accordance with 170.170(d) for open boats that lack flush weather decks, including DUKWs, as follows3: â¢ Weather criteria: The regulation, 46 CFR 170.170, limits the maxi- mum heel angle to 14 degrees or one-half of the upright freeboard submerged, whichever is less. The limit used by USCG for DUKWs is 14 degrees or one-quarter of the upright freeboard submerged, whichever is less. â¢ Passenger heeling criteria: The regulation, 46 CFR 171.050, limits the maximum heel angle to 14 degrees or the angle of heel at which the deck edge is first submerged, whichever is less. The limits used 3 These limits, which apply to all inspected open passenger boatsânot just DUKWsâwere first put forth in an internal November 2007 MSC Memorandum (Serial: H1-0703309), which was confirmed by an internal September 2008 USCG CG-5212 (now CG-ENG-2) Memoran- dum. No formal guidance, however, was issued by USCG until the 2018 revision of the âMSC Plan Review Guidelines (PRG), Review of Stability for Small Passenger Vessels (T) Procedure Number: H1-01.â https://www.dco.uscg.mil/Portals/9/MSC/PRG/PRG.H1-01.2018.04.20. Review_of_Stability_for_Small_Passenger_Vessels_(T).pdf. For a more complete history of the implementation of the stability criteria to DUKWs by USCG and a summary history of the development of small passenger vessel stability regulations in general, refer to Sections 1 and 2.5 of USCGâs âMSC Technical Report: Analysis of Open Boat Stability Standard Applied to DUKW Amphibious Vessels,â September 3, 2020.
FLOODING AND SURVIVABILITY 37 by USCG for DUKWs is 14 degrees or one-half of the upright free- board submerged, whichever is less. The additional MSC requirements for weather and passenger heel- ing criteria for open boats were included in an internal memo as early as Novem ber 2007, but not issued as formal USCG guidance until 2018. Watertight Integrity DUKWs are subject to two regulations that address watertight integrity. 46 CFR 179.350, covering openings in the side of a vessel below the bulk- head or weather deck, regulates the typical thru-hull engine exhausts and overboard discharges from the onboard bilge pumps. It also covers any sea chest seawater connections, but these would not be typical for DUKWs as they do not need to be fitted with a fixed seawater firefighting system. 46 CFR 179.360 regulates watertight integrity and covers any hatches or doors on a Subchapter T passenger vessel. For the typical DUKW this would be the air intake and exhaust vents used for the DUKWs propulsion engine air-cooled radiator. Flooding/Sinking Events The committee reviewed the DUKW sinking and flooding casualty events with respect to current regulations and policy. NVIC 1-01 was created after the Miss Majestic sank when a driveshaft boot seal failed after an improper repair to the seal, which had also been previously modified from the original design. NVIC 1-01 addresses shaft boot seals with two recommendations: (1) install restrictor plates in way of the shaft boot seals to slow the rate of uncontrollable flooding from a shaft boot seal failure, or (2) install a carrier bearing in place of the shaft boot seal. This would completely seal the shaft penetration from potential flooding. Neither of these recommendations are mandatory, although the NVIC 1-01 recommends a detailed procedure for inspecting the shaft boot seals, if present, and removing them if their condition appears questionable.4 NVIC 1-01 addresses missing drain plugs, the cause of the sinking of the Seattle Stretch Duck, only as an inspection item that âshould be ex- amined verifying proper fit and function. After the completion of all hull exams for credit these vessels should be operated in the water to ensure watertight integrity.â5 4 NVIC 1-01, pp. 20, 25. https://www.dco.uscg.mil/Portals/9/DCO%20Documents/5p/5ps/ NVIC/2001/n1-01.pdf. 5 NVIC 1-01, p. 20.
38 OPTIONS FOR IMPROVING THE SAFETY OF DUKW TYPE AMPHIBIOUS VESSELS For watertight integrity, which was compromised by the Stretch Duck 7âs large splash event in 2015, the regulations in 46 CFR 179.360 cover the engine room compartment vents arrangement. Because the Stretch Duck 7 had been certificated for operation on âprotected watersâ only, the engine compartment was only required to be able to be made âweathertight,â not watertight. Current stability regulations and policies do not directly address a DUKWâs ability to resist boarding seas, which sank the Stretch Duck 7 in the 2018 storm. Indirectly, the minimum freeboard requirements do tie into potential wind/wave conditions for a given operating route. Bilge Pumps Bilge systems on small passenger vessels would not normally be expected to handle emergency flooding events. However, USCG in NVIC 1-01 recom- mends that DUKW vessels have bilge pumps with the capacity to reliably âoffset uncontrolled flooding of the largest penetration in the hull until the vehicle can be safely beached.â NVIC 1-01 also raises concerns about the appropriateness of the original WWII high-capacity Higgins Pump for tour operations.6 SURVIVABILITY, STABILITY STANDARDS, AND FLOODING USCG considers it critical to DUKW survivability to mitigate the risk of flooding through means other than relying only on the intact stability requirements in the Subchapter T regulations and their accompanying implementation policy. Although USCG is aware that many DUKWs fail to meet the intact stability standards, as discussed below, the fact that no casualties have been attributed solely to insufficient intact stability has led MSC to conclude that âthere is likely a combination of stability standards and opera tional restrictions that would provide a level of safety equivalent to that intended by the regulation.â7 This conclusion is consistent with the sound application of the concept of survivability, which calls for considering survivability from all aspects, including equipment, outfitting, operational procedures, as well as design. In addition, many DUKW owner/operators have incorporated design changes to improve hull form and vital system survivability inclusive of, and often beyond, Subchapter T regulations. MSC conducted an in-house intact stability review of DUKWs in 2018. The study re-assessed an in-service DUKW at âintermediateâ (i.e., less 6 NVIC 1-01, p. 32. 7 Marine Safety Center (MSC), USCG, 2020, âMSC Technical Report: Analysis of Open Boat Stability Standard Applied to DUKW Amphibious Vessels,â September 3.
FLOODING AND SURVIVABILITY 39 than the maximum number of passengers permitted) loading conditions using the more stringent âopen boatâ freeboard limits set forth in USCGâs internal 2007/2008 Memorandums. (See section above on âIntact Stabil- ity Standards.â) The DUKWs included in the study failed to comply with the stability criteria at some of the intermediate loading conditions, which prompted a review of all of the DUKW stability calculations submitted to MSC between 2007 and 2018. This review found that the DUKWsâ stability was being assessed using the less stringent flush deck freeboard limits for the weather criteria, 170.170, and the passenger heeling criteria, 171.050, instead of the more stringent open boat freeboard limits given in USCGâs internal 2007/2008 Memorandums. A further review found that many of those DUKWs also failed to meet the weather criteria at intermediate load- ing conditions when using the more stringent open boat freeboard limits. Despite the DUKWs performance against the stability standards, MSC concluded that âDUKW COIs [Certificates of Inspection] typically list opera tional restrictions, including limiting route exposure, length of voyage, passenger seating, and acceptable weather and sea states. These restrictions complement the vesselsâ stability characteristics, further mitigating the risk of flooding and sinking.â8 The optimum solution to the flooding risk is to prevent flooding events from occurring in the first place. The final sections of this chapter and Chapter 4 on Operating Areas will examine solutions to prevent flooding. The next section explores solutions for preventing sinking after flooding occurs. REMAINING AFLOAT AND UPRIGHT DURING FLOODING Providing the means for a DUKW to remain afloat and upright after flood- ing occurs will significantly mitigate the risks associated with flooding events of any type. Remaining afloat and upright can protect passenger safety in almost any flooding scenario a DUKW may typically encounter. This would include boarding seas over the bow or sides or through the engine cooling air vents and flooding through hull penetrations. The committee reviewed the fitting of flotation foam internally, the addi tion of external inflatable bladders or the installation of external floats, and the use of a combination of internal and external buoyancy enhance- ments. Except for the internal fitting of flotation foam alone, the committee found that all the solutions could potentially work. 8 MSC, USCG, 2020, âMSC Technical Report: Analysis of Open Boat Stability Standard Applied to DUKW Amphibious Vessels,â September 3.
40 OPTIONS FOR IMPROVING THE SAFETY OF DUKW TYPE AMPHIBIOUS VESSELS Flotation Foam The committee notes that fitting flotation foam in the hull is not a viable option to keeping a DUKW afloat and upright in case of a swamping or hull breaching incident. Although installing flotation foam has been used in numerous small recreational and small inspected craft to make them âunsinkableâ if swamped or capsized, for DUKWs this approach has been studied by both USCG and by the United Kingdomâs Marine Accident Investi gation Branch (MAIB) and found to be impractical. The space avail- able below the passenger deck is insufficient for the foam volume required. Moreover, installing foam could create potential fire hazards and increase the difficulty of maintaining a DUKWâs hull, internal components, and mechanical systems. After the sinking of the Stretch Duck 7, MSC prepared an engineering study on the feasibility of installing foam in this vessel. It calculated that a total of 426.4 cubic feet of foam would have been required to keep the Stretch Duck 7 afloat. The maximum volume available below the passenger deck was calculated to be 271.6 cubic feet using a permeability factor of 85 percent (machinery spaces). The remaining required foam, 195.5 cubic feet, was calculated to fill approximately 50 percent of the passenger seating space. These volumes are theoretical maximums; the actual space available for flotation foam would be significantly less because clearance is required around equipment, shafts, etc., for inspection, maintenance, and repair. Since MSC determined that installing flotation foam was not feasible, they made no calculations to see if the DUKW would also remain upright in the swamped condition.9 Flotation foam has also been tried for DUKWs operating in the United Kingdom, where it has failed to prevent sinking. Instead, the foam became a fire hazard. UK regulations require that DUKWs have 110 percent reserve buoyancy and remain upright and stable in case of a hull breach or swamp- ing.10 Some UK DUKWs chose to fit internal foam to comply with this reserve buoyancy requirement. Subsequent sinkings revealed the approach inadequate. The sinkings of the Wacker Quacker 4 in March 2013 and the Wacker Quacker 1 in June 2013 happened after suffering hull breaches. A fire on another DUKW, the Cleopatra, was attributed to the flotation foam coming into contact with rotating machinery and catching fire.11 The United Kingdomâs MAIB subsequently conducted an investigation that included a series of real-world stability tests and flooding trials on the 9 MSC, 2020, MSC Memorandum (Serial: H1-2001479), May 13. 10 UK DUKWs are certified by the Maritime and Coastguard Agency (MCA) and are required to meet the stability criteria given in the Merchant Shipping Notice 1699 M (MSN1699M). 11 MAIB, Very Serious Marine Casualty Report NO 32/2014, December 2014. https://assets. publishing.service.gov.uk/media/54c1722240f0b6158d00002b/MAIBReport_32-2014.pdf.
FLOODING AND SURVIVABILITY 41 Wacker Quacker 1. When fitting the vessel with the required volume of foam to meet the 110 percent reserve buoyancy requirement, it was found that not all of the foam fit under the passenger deck and distributed the rest in the passenger cabin. After simulating the actual hull breach by remov- ing pipe caps, the vessel took about 5 minutes to fully flood. It remained afloat and upright with a starboard list. The study did not fully evaluate if a swamped DUKW would remain upright when subject to the effects of passengers escaping over the sides.12 Inflatable Bladders Retrofitting DUKWs with inflatable bladders could be a feasible solution to preserve buoyancy when flooding. However, there are disadvantages to this method, chiefly additional costs and labor that also need to be taken into consideration. In addition, the bladders may reduce the vesselâs maneuver- ability in the water. Inflatable bladders are already in use for amphibious passenger vessels, albeit to provide additional intact stability, not positive flotation during flooding. Companies have developed vessels (USCG inspected) with a flex- ible bladder on each side of the hull near the waterline. The bladders are deflated for road operations to minimize the vesselâs width and are inflated before entering the water. These vessels are purpose built for sightseeing tours and provide combined water and land tours, similar to DUKWs. An example of a bladder deflated for operation on land is found at this web- site.13 An example of when the bladder is inflated for operation in the water can be found at the following website.14 To explore the feasibility of installing inflatable bladders on DUKWs, the committee calculated the required bladder volume and size to achieve reserve buoyancy, based on USCGâs foam reserve buoyancy study for the Stretch Duck 7. Assuming the maximum feasible length of a bladder on a DUKW is 26 feet, providing equivalent reserve buoyancy as noted in the USCG study would require two 30.6-inch in diameter by 26-foot long bladders (see Table 3-1). Although retrofitting a DUKW with bladders is feasible with todayâs technology, the bladders would be fairly sizable when inflated. The advan- tages and disadvantages of installing inflatable bladders to provide reserve buoyancy are as follows. 12 MAIB Accident Report # 32/2014. 13 Example of a deflated bladder: https://trolleyboats.net/wp-content/uploads/2020/01/ 100_0384.jpg. 14 Example of an inflated bladder: https://trolleyboats.net/wp-content/uploads/2020/01/ duckboat020-1024x766.jpg.
42 T A B L E 3 -1 I nfl at ab le B la dd er S iz e an d Fl oa t Si ze : E st im at io ns f or a S tr et ch D uc k L in e D es cr ip ti on V al ue U ni ts Fo rm ul a N ot es 1 St re tc h D uc k 7 L ig ht sh ip 17 ,9 20 lb s Fr om S ta bi lit y L et te r 2 36 P as se ng er s 6, 66 0 lb s 18 5 lb s pe r Pa ss en ge r 3 2 C re w M em be rs 37 0 lb s 18 5 lb s pe r C re w M em be r 4 Fu el a nd M is ce lla ne ou s 46 4 lb s 40 G al lo ns F ue l an d 12 0 #â s M is c 5 To ta l Fu ll L oa d D is pl ac em en t 25 ,4 14 lb s = (1 ) + (2 ) + (3 ) + (4 ) B ef or e Fl oo di ng 6 B uo ya nc y fr om S ub m er ge d H ul l 89 6 lb s = 0. 05 Ã ( 1) A ss um ed 9 5% P er m ea bi lit y 7 B uo ya nc y fr om A pp en da ge s 2, 81 4 lb s Fr om U SC G M em o H 1- 20 01 47 9 8 B uo ya nc y fr om S ub m er ge d Pa ss en ge rs 5, 86 8 lb s = (1 85 â 2 2) Ã ( 2) 22 l bs p er P as se ng er - P FD T yp e 1 9 B uo ya nc y fr om S ub m er ge d C re w M em be rs 32 6 lb s = (1 85 â 2 2) Ã ( 3) 22 l bs p er C re w M em be r - PF D T yp e 1 10 To ta l Su bm er ge d D is pl ac em en t 15 ,5 10 lb s = (5 ) â (6 ) + (7 ) + (8 ) + (9 ) A ft er F lo od in g 11 E st im at ed W ei gh t of B la dd er s an d E qu ip m en t 1, 00 0 lb s 12 M in im um T ot al R eq ui re d B uo ya nt F or ce 16 ,5 10 lb s = (1 0) + ( 11 ) 13 D en si ty o f W at er 62 .3 lb s/ ft 3 Fr es h W at er 14 M in im um T ot al B la dd er V ol um e R eq ui re d 26 5. 0 ft 3 = (1 2) / ( 13 ) C om bi ne d V ol um e B ot h Si de s of D U K W 15 M in im um B la dd er V ol um e pe r Si de 13 2. 5 ft 3 = (1 4) / 2 16 M ax im um P ra ct ic al B la dd er L en gt h 26 .0 ft D U K W H ul l = 33 F ee t 17 M in im um R eq ui re d B la dd er C ro ss -S ec ti on 5. 10 ft 2 = (1 5) / ( 16 ) 18 M in im um R eq ui re d B la dd er D ia m et er 2. 55 ft = 2 Ã (( 17 ) / 3. 14 ) ^ 0. 5 C ir cu la r C ro ss -S ec ti on 19 M in im um R eq ui re d B la dd er D ia m et er 30 .6 in ch = (1 8) Ã 1 2 C ir cu la r C ro ss -S ec ti on 20 E qu iv al en t B uo ya nt F lo at H ei gh t 36 .0 in ch A ss um ed V al ue 21 E qu iv al en t B uo ya nt F lo at W id th 20 .4 in ch = (1 7) Ã 1 44 / ( 20 ) N O T E : C om m it te e ca lc ul at io ns b as ed o n sp ec ifi ca ti on s fo un d in M SC M em or an du m S er ia l: H 1- 20 01 47 9 (M ay 1 3, 2 02 0) , d at a as o f A ug us t 3 , 2 02 1.
FLOODING AND SURVIVABILITY 43 Advantages â¢ Reserve buoyancy at 100 percent in the event of any flooding incident. â¢ Minimal likelihood that passengers would need to escape the vessel due to flooding, avoiding hazards including canopies and donning life jackets in cramped spaces while under way. â¢ Minimal downtime and disruption to operations if retrofit is in- stalled via a prefabricated package. â¢ No additional permits for operations on roads may be required because bladders can be deflated. â¢ Air compressors, already installed on some DUKWs to inflate tires, can be used to inflate/deflate the bladders. â¢ Crew can readily control inflation/deflation. â¢ No need to reduce passenger capacity to compensate for added weight because the bladders can be made slightly oversized. Disadvantages â¢ Acquisition costs of the bladders and supporting equipment. â¢ Additional costs for the long-term maintenance, storage, and repair of the bladders and supporting equipment. â¢ Requires the crewâs active involvement to inflate/deflate the bladders at the appropriate times. â¢ Increased resistance of the DUKW making it even slower, less maneuverable, and burn more fuel. â¢ Added weight may be an issue for over the road operations if maxi- mum vehicle weight limits are exceeded. Floats Buoyant floats installed on the sides of a DUKWâs hull is similar in con- cept to inflatable bladders. These floats could be permanently inflated bladders or molded foam-filled units, and they could be permanently mounted or removable, to be installed only for waterborne operations. Removable floats would add labor to a tour and require storage when not in use, but permanently attached floats would increase the width of the DUKW when operating on land unless some type of foldup configuration was used. Both permanently mounted floats and removable floats could be viable options. Removable, permanently inflated bladders have been used for reserve buoyancy on DUKWs that have operated overseas. Companies have fit- ted two horizontal clips on the DUKWs, to which floats were installed
44 OPTIONS FOR IMPROVING THE SAFETY OF DUKW TYPE AMPHIBIOUS VESSELS for waterborne operations. The floats numbered four units on each side and each was fitted with handles on the ends for ease of installation and removal.15 Using molded foam-filled floats would allow the floats to have a more compact, rectangular cross section, as opposed to circular cross section of permanently inflated bladders. For example, a circular bladder with a diam- eter of 30.6 inchesâthe minimum bladder size according to Table 3-1âis equivalent to a rectangular float with a height of 36 inches and a width of only 20.4 inches. A molded float could also have a streamlined âbowâ to re- duce waterborne drag. The advantages and disadvantages of non-inflatable buoyancy floats are, in general, similar to those for the inflatable bladder concept with some notable differences as follows. Advantages â¢ Reserve buoyancy at 100 percent in the event of any flooding incident. â¢ Minimal likelihood that passengers would need to escape the vessel due to flooding, avoiding hazards including canopies and donning life jackets in a cramped space while under way. â¢ Minimal downtime and disruption to operations if retrofit is in- stalled via a prefabricated package. â¢ If removable, no additional permits for operations on roads. â¢ No supporting equipment required, such as air pumps. â¢ No need to reduce passenger capacity to compensate for added weight because the floats can be made slightly oversized. â¢ Use of closed cell foam would allow the floats to retain their effective ness in the event of a collision or minor damage. Disadvantages â¢ Acquisition costs of floats. â¢ Recurring maintenance and repair costs. â¢ If removable, cost and time for the onboard crew or shoreside personnel to install and remove the floats. â¢ If removable, place to store the floats convenient to the launching ramp. If different ramps are used for entering and exiting the water, the cost, time, and labor to transport floats to the launching ramp. â¢ Increased resistance making the DUKW even slower, less maneuver- able, and burn more fuel. 15 Example of removable, permanently inflated bladders or molded foam-filled units: https:// www.irishtimes.com/polopoly_fs/1.3572804.1532203638!/image/image.jpg_gen/derivatives/ ratio_1x1_w1200/image.jpg.
FLOODING AND SURVIVABILITY 45 â¢ Added weight may be an issue for over the road operations if maxi- mum vehicle weight limits are exceeded. Combining Methods Because the required external bladders or floats would be fairly large rela- tive to a DUKWâs hull size, the committee also explored combining the installation of internal flotation foam with external bladders or floats. For Stretch Duck 7, the installation of internal flotation foam to 43 percent of the hull volume, reduces the required external flotation volume by approxi- mately 32 percent (see Table 3-2). FLOODING THROUGH HULL PENETRATIONS DUKWs have multiple hull penetrations below the waterline, including: â¢ Forward and aft driveshaft penetrations for the road drive axles. â¢ Maintenance/drain plugs and valves for the hull bilges and drive shaft tubes. â¢ A propeller shaft stuffing box for the waterborne propulsion system. â¢ A rudder stock gland for waterborne steering control. The committee focused only on the driveshaft penetrations and the bilge drain plugs/valves because they are not found on typical Sub chapter T boats. The propeller shaft stuffing box and rudder stock gland are typical marine equipment, and they have not been a source of flooding in DUKW accidents. As such, no changes are required of this typical marine equipment. Higgins Pumps and Bilge Systems The Miss Majestic and the Wacker Quacker 4 were both dependent on a Higgins pump at the time of their sinking. The combination of a Higgins pump and flooding through a hull penetration creates a higher risk of sinking. DUKWs during WWII were originally fitted with three bilge pumps to deal with flooding from hull penetrations and modest battle damage. The hand bilge pump had a capacity of approximately 25 GPM (gallons per minute). The transfer case-driven self-priming centrifugal pump had a capacity of approximately 50 GPM through a bilge suction manifold and was operated from the driverâs compartment.16 16 U.S. Army, 1944, âThe DUKW: Its Operation and Uses.â See https://cgsc.contentdm.oclc. org/digital/collection/p4013coll9/id/397.
46 T A B L E 3 -2 I nfl at ab le B la dd er a nd F lo at S iz es w it h Pa rt ia l In te rn al F lo ta ti on F oa m : E st im at io ns f or a S tr et ch D uc k L in e D es cr ip ti on V al ue U ni ts Fo rm ul a N ot es 1 St re tc h D uc k 7 L ig ht sh ip 17 ,9 20 lb s Fr om S ta bi lit y L et te r 2 36 P as se ng er s 6, 66 0 lb s 18 5 lb s pe r Pa ss en ge r 3 2 C re w M em be rs 37 0 lb s 18 5 lb s pe r C re w M em be r 4 Fu el a nd M is ce lla ne ou s 46 4 lb s 40 G al lo ns F ue l an d 12 0 #â s M is c 5 To ta l Fu ll L oa d D is pl ac em en t 25 ,4 14 lb s = (1 ) + (2 ) + (3 ) + (4 ) B ef or e Fl oo di ng 6 B uo ya nc y fr om S ub m er ge d H ul l 89 6 lb s = 0. 05 Ã ( 1) A ss um ed 9 5% P er m ea bi lit y 7 B uo ya nc y fr om A pp en da ge s 2, 81 4 lb s Fr om U SC G M em o H 1- 20 01 47 9 8 B uo ya nc y fr om S ub m er ge d Pa ss en ge rs 5, 86 8 lb s = (1 85 â 2 2) Ã ( 2) 22 l bs p er P as se ng er - P FD T yp e 1 9 B uo ya nc y fr om S ub m er ge d C re w M em be rs 32 6 lb s = (1 85 â 2 2) Ã ( 3) 22 l bs p er C re w M em be r - PF D T yp e 1 10 To ta l Su bm er ge d D is pl ac em en t 15 ,5 10 lb s = (5 ) â (6 ) + (7 ) + (8 ) + (9 ) A ft er F lo od in g 11 E st im at ed W ei gh t of B la dd er s an d E qu ip m en t 1, 00 0 lb s 12 E st im at ed W ei gh t of I nt er na l Fl ot at io n Fo am 20 0 lb s Fr om U SC G M em o H 1- 20 01 47 9 13 M in im um T ot al R eq ui re d B uo ya nt F or ce 16 ,7 10 lb s = (1 0) + ( 11 ) + (1 2) 14 V ol um e of I nt er na l Fl ot at io n Fo am 10 0 ft 3 43 % o f H ul l V ol um e fr om U SC G M em o 15 B uo ya nc y Pr ov id ed b y Fl ot at io n Fo am 5, 50 0 lb s = 55 Ã ( 14 ) 55 l bs /f t3 p er 4 6 C FR 1 79 .2 40 (b )( 8) 16 B uo ya nc y R eq ui re d fr om B la dd er s 11 ,2 10 lb s = (1 3) â ( 15 ) 17 D en si ty o f W at er 62 .3 lb s/ ft 3 Fr es h W at er 18 M in im um T ot al B la dd er V ol um e R eq ui re d 17 9. 9 ft 3 = (1 6) / ( 17 ) C om bi ne d V ol um e B ot h Si de s of D U K W 19 M in im um B la dd er V ol um e pe r Si de 90 .0 ft 3 = (1 8) / 2 20 M ax im um P ra ct ic al B la dd er L en gt h 26 .0 ft D U K W H ul l = 33 F ee t 21 M in im um R eq ui re d B la dd er C ro ss -S ec ti on 3. 46 ft 2 = (1 9) / ( 20 ) 22 M in im um R eq ui re d B la dd er D ia m et er 2. 10 ft = 2 Ã ( (2 1) / 3 .1 4 ) ^ 0. 5 C ir cu la r C ro ss S ec ti on 23 M in im um R eq ui re d B la dd er D ia m et er 25 .2 in ch = (1 8) Ã 1 2 C ir cu la r C ro ss S ec ti on 24 E qu iv al en t B uo ya nt F lo at H ei gh t 32 .0 in ch A ss um ed V al ue 25 E qu iv al en t B uo ya nt F lo at W id th 15 .6 in ch = (1 7) Ã 1 44 / ( 20 ) N O T E : C om m it te e ca lc ul at io ns b as ed o n sp ec ifi ca ti on s fo un d in M SC M em or an du m S er ia l: H 1- 20 01 47 9 (M ay 1 3, 2 02 0) , d at a as o f A ug us t 3 , 2 02 1.
FLOODING AND SURVIVABILITY 47 The Higgins pump, critical for survivability, is a self-priming pump that is chain-driven by the propeller shaft. The Higgins pump is always âonâ whenever the DUKW is traveling afloat, but its capacity depends on the propeller speed. At full propeller speed, the pump has a nominal capacity between 200 to 250 GPM. Slowing the engine significantly decreases the pumping rate, and stopping the engine stops all pumping. The Higgins pump takes suction from the center compartment only. This wartime design presents several hazards for DUKWs as passenger vessels. To get the full pumping capacity, the DUKW must be operated at maximum rated propeller speed (i.e., full throttle). In addition, the reli- ability of the chain drive that powers the Higgins pump depends on careful maintenance and frequent adjustments. Finally, the Higgins pump, like any bilge pump, is prone to clogging from the detritus inevitably lurking in every bilge. Clogged bilge pumps were such a concern during World War II that warnings about the importance of keeping bilges clean are found throughout the U.S. Armyâs DUKW operations manual.17 In addition, USCGâs requirements for bilge systems under Subchapter T do not intend for bilge systems on small passenger vessels to serve as the primary deterrent to a flooding emergency. The bilge system is to remove water from normal operations. The primary deterrents to prevent flooding are to be the hullâs structural integrity and watertight through-hull fittings.18 Driveshaft Boot Seals The committee notes that the original WWII design for driveshaft boot seals, especially in conjunction with a Higgins pump, presents a higher risk of flooding and sinking. DUKWs still using the original design should be modified. The best approach is to replace the boot seals with carrier bear- ings. In recognition of the high cost of replacement with carrier bearings, the committee offers guidance on actions short of replacement at the end of this section. All DUKWs have conventional truck axles for land-based operation. These axles are typically powered by conventional cardan-joint driveshafts from a transfer gearbox located in the DUKWâs hull. This arrangement necessitates some type of sealing mechanism where the driveshafts penetrate the DUKWs watertight envelope. The original WWII DUKWs used a some- what complex pivoting driveshaft tube that ran from the axle housing to a pivot joint at the hull. This driveshaft tube was made watertight with large rubber boot seals at each end, as depicted in Figure 3-1. 17 U.S. Army, 1944, âThe DUKW: Its Operation and Uses.â See https://cgsc.contentdm.oclc. org/digital/collection/p4013coll9/id/397. 18 NVIC 1-01; Subchapter T Part 192.5200.T A B L E 3 -2 I nfl at ab le B la dd er a nd F lo at S iz es w it h Pa rt ia l In te rn al F lo ta ti on F oa m : E st im at io ns f or a S tr et ch D uc k L in e D es cr ip ti on V al ue U ni ts Fo rm ul a N ot es 1 St re tc h D uc k 7 L ig ht sh ip 17 ,9 20 lb s Fr om S ta bi lit y L et te r 2 36 P as se ng er s 6, 66 0 lb s 18 5 lb s pe r Pa ss en ge r 3 2 C re w M em be rs 37 0 lb s 18 5 lb s pe r C re w M em be r 4 Fu el a nd M is ce lla ne ou s 46 4 lb s 40 G al lo ns F ue l an d 12 0 #â s M is c 5 To ta l Fu ll L oa d D is pl ac em en t 25 ,4 14 lb s = (1 ) + (2 ) + (3 ) + (4 ) B ef or e Fl oo di ng 6 B uo ya nc y fr om S ub m er ge d H ul l 89 6 lb s = 0. 05 Ã ( 1) A ss um ed 9 5% P er m ea bi lit y 7 B uo ya nc y fr om A pp en da ge s 2, 81 4 lb s Fr om U SC G M em o H 1- 20 01 47 9 8 B uo ya nc y fr om S ub m er ge d Pa ss en ge rs 5, 86 8 lb s = (1 85 â 2 2) Ã ( 2) 22 l bs p er P as se ng er - P FD T yp e 1 9 B uo ya nc y fr om S ub m er ge d C re w M em be rs 32 6 lb s = (1 85 â 2 2) Ã ( 3) 22 l bs p er C re w M em be r - PF D T yp e 1 10 To ta l Su bm er ge d D is pl ac em en t 15 ,5 10 lb s = (5 ) â (6 ) + (7 ) + (8 ) + (9 ) A ft er F lo od in g 11 E st im at ed W ei gh t of B la dd er s an d E qu ip m en t 1, 00 0 lb s 12 E st im at ed W ei gh t of I nt er na l Fl ot at io n Fo am 20 0 lb s Fr om U SC G M em o H 1- 20 01 47 9 13 M in im um T ot al R eq ui re d B uo ya nt F or ce 16 ,7 10 lb s = (1 0) + ( 11 ) + (1 2) 14 V ol um e of I nt er na l Fl ot at io n Fo am 10 0 ft 3 43 % o f H ul l V ol um e fr om U SC G M em o 15 B uo ya nc y Pr ov id ed b y Fl ot at io n Fo am 5, 50 0 lb s = 55 Ã ( 14 ) 55 l bs /f t3 p er 4 6 C FR 1 79 .2 40 (b )( 8) 16 B uo ya nc y R eq ui re d fr om B la dd er s 11 ,2 10 lb s = (1 3) â ( 15 ) 17 D en si ty o f W at er 62 .3 lb s/ ft 3 Fr es h W at er 18 M in im um T ot al B la dd er V ol um e R eq ui re d 17 9. 9 ft 3 = (1 6) / ( 17 ) C om bi ne d V ol um e B ot h Si de s of D U K W 19 M in im um B la dd er V ol um e pe r Si de 90 .0 ft 3 = (1 8) / 2 20 M ax im um P ra ct ic al B la dd er L en gt h 26 .0 ft D U K W H ul l = 33 F ee t 21 M in im um R eq ui re d B la dd er C ro ss -S ec ti on 3. 46 ft 2 = (1 9) / ( 20 ) 22 M in im um R eq ui re d B la dd er D ia m et er 2. 10 ft = 2 Ã ( (2 1) / 3 .1 4 ) ^ 0. 5 C ir cu la r C ro ss S ec ti on 23 M in im um R eq ui re d B la dd er D ia m et er 25 .2 in ch = (1 8) Ã 1 2 C ir cu la r C ro ss S ec ti on 24 E qu iv al en t B uo ya nt F lo at H ei gh t 32 .0 in ch A ss um ed V al ue 25 E qu iv al en t B uo ya nt F lo at W id th 15 .6 in ch = (1 7) Ã 1 44 / ( 20 ) N O T E : C om m it te e ca lc ul at io ns b as ed o n sp ec ifi ca ti on s fo un d in M SC M em or an du m S er ia l: H 1- 20 01 47 9 (M ay 1 3, 2 02 0) , d at a as o f A ug us t 3 , 2 02 1.
48 OPTIONS FOR IMPROVING THE SAFETY OF DUKW TYPE AMPHIBIOUS VESSELS FIGURE 3-1 Example of a boot seal in a WWII DUKW. SOURCE: USCG, NVIC 1-01, p. 13. These seals are unlike conventional small craft propeller shaft seals, where there is no lateral or rotational movement of the seal itself. WWII DUKW shaft boot seals must handle both the rotational movement of the driveshaft tubes through the full travel of the road springs, as well as the longitudinal elongation of the driveshaft. These seals are critical to maintaining the watertight integrity of a WWII DUKW: Their failure essentially results in an approximately 4-inch diameter hole in the hull roughly 26 inches below the DUKWs waterline. According to USCG calculation, this 4-inch diameter hole would let in water at approximately 220 GPM.19 19 USCG, NVIC 1-01, p. 33.
FLOODING AND SURVIVABILITY 49 FIGURE 3-2 Miss Majestic rear driveshaft boot after sinking accident. SOURCE: NTSB/MAR-02/01, p. 15. See https://www.ntsb.gov/investigations/ AccidentReports/Reports/MAR0201.pdf. For DUWKs still equipped with a Higgins pump, removing flood water from a boot seal failure requires not only that the Higgins pump be opera tional, but the survival of the DUKW depends on the Higgins pump performing at its full rated capacity. When a boot seal and the Higgins pump both fail, sinking can happen in less than 10 minutes. The Miss Majestic suffered both when a new, improperly installed boot seal failed (see Figure 3-2) and the Higgins pump was inoperable because a drive chain sprocket key was missing, there was excessive play in the drive chain, and a portion of one of the impeller blades was broken off. The flooding rate was at least 170 GPM, which would put the stern deck awash within 7 minutes. The DUKW sank 15 to 60 seconds after the passengers and operator realized there was a serious problem and began attempting to escape.20 USCGâs NVIC 1-01 guidelines, created in response to the Miss Majestic sinking, addressed the issue of the driveshaft boot seals in three ways21: 20 NTSBâs accident report NTSB/MAR-02/01; DUKW Damage Stability Analysis, December 8, 1999. 21 USCG, NVIC 10-1, pp. 19â21 and 25.
50 OPTIONS FOR IMPROVING THE SAFETY OF DUKW TYPE AMPHIBIOUS VESSELS 1. Heightened inspections of the original-design boot seals during the required annual and drydock inspections; 2. Installation of a restrictor plate in way of the boot seal penetration; and 3. Installation of a carrier bearing in place of the boot seal. Although USCGâs increased inspection requirements are an improve- ment because they provide specific guidance on how to inspect the boot seal properly, inspections alone are inadequate. Inspections do not remedy the inherent vulnerability of the boot seals and the Higgins pump. Moreover, at this time inspections are only done once per year. The shaft boot seal that failed on the Miss Majestic replaced a torn boot seal that was only discov- ered on an earlier trip when the operator noticed that water was coming out of the Higgins pump overboard discharge. Given their fragile nature, boot seals still in use may warrant more frequent inspections. The installation of restrictor plates reduces the maximum flooding rate in the event of a boot seal failure, decreasing the risk of sinking. The restric- tor plate example in NVIC 1-01 reduces the GPM flowing through a failed boot seal from 220 to 30 GPM. Applying the restrictor plate to NTSBâs Miss Majestic analysis increases the time to swamping from 7 minutes to about 50 minutes. However, a DUKW with restrictor plates will still depend on its bilge system to prevent flooding, contrary to USCGâs requirements. The installation of carrier bearings in place of the boot seals completely removes the flooding potential from the driveshaft penetrations and is, by far, the safest remedy. This solution also brings the DUKWâs bilge system and the DUKW itself in compliance with USCGâs intention for Subchapter T passenger boats. However, given the high cost of installing carrier bearings, the commit- tee recognizes that other actions short of the ideal may be necessary, at least in the near term. These actions, which mitigate at least some of the risk of the WWII driveshaft boot seal arrangement, include: 1. Fit DUKWs that have the original WWII driveshaft boot seal arrange ment with at least restrictor plates, as per NVIC 1-01, as soon as possible. 2. Fit DUKWs that utilize restrictor plates with at least two indepen- dent bilge pumps (power source, float switch, discharge piping, and operatorâs visible/audible alarm), each capable of pumping twice the minimum estimated maximum flooding rate possible in the event of a boot seal failure. 3. Institute routine checks and cleaning of the bilges to minimize the clogging of the emergency bilge pumps.
FLOODING AND SURVIVABILITY 51 4. Institute routine inspections of the emergency bilge pumps to ensure that they are fully operational. Drain Plugs and Valves The sinking of the Seattle Stretch Duck and the Wacker Quacker 4 were both tied to missing hull drain plugs. On the original WWII DUKWs, plugs and valves were installed for maintaining the transmission, transfer case, and other mechanical systems; draining rainwater when the DUKW was operated on land; and emergency draining a swamped DUKW once on land. (DUKWs have multiple bilge pockets that trap water.) Numerous accidents during World War II resulted from DUKW crews forgetting to install the plugs before commencing waterborne operations. In response, the U.S. Army specified that DUKWs should be retrofitted with plug racks and stenciled signs in the driverâs compartment to remind the crew to reinstall them. The U.S. Army also supplied a modified drain plug wrench to simplify and speed up plug removal and installation. Instructions and warnings on the proper use of drain plugs and valves were also scat- tered throughout the DUKW operations manual (see Figure 3-3).22 The problems with drain plugs continued into the 21st century. In 2001, a DUKW sank in Seattle after a large drain plug had not been re- installed. Fortunately, the master noticed the problem in time to safely disembark the passengers. USCG Subchapter T regulations require that all areas of the bilge be effectively drained under all practical operating conditions (Subpart E). In addition, most DUKWs are fitted with canopies that minimize rainwater ingress, and the vessels are often stored in a sheltered area when not in use. The benefits of having the hull drain plugs and valves are now far outweighed by the safety hazards they present if not reinstalled after main- tenance, repairs, and inspections. FLOODING THROUGH ENGINE COOLING AIR VENTS Because the majority of their operations during WWII were intended to be on land, the original DUKWs were fitted with an air-cooled radiator that provided engine cooling for both land and waterborne operations. On water, the required fresh air intake and exhaust vents are sources of flood- ing from large waves and heavy surf. The committee notes that the air vents required for engine cooling are a significant hazard. Installing keel coolers 22 U.S. Army, 1944, âThe DUKW: Its Operation and Use,â p. 11. https://cgsc.contentdm. oclc.org/digital/collection/p4013coll9/id/397.
52 FI G U R E 3 -3 U .S . A rm y m an ua l e xc er pt s on d ra in p lu gs a nd v al ve s. SO U R C E : U .S . A rm y, 1 94 4, â T he D U K W : It s O pe ra ti on a nd U se .â
FLOODING AND SURVIVABILITY 53 FIGURE 3-4 WWII DUKW with an open hood hatch. SOURCE: A. Moyers, Chattanooga Ducks. is one viable remedy, but keel coolers do not eliminate the need to ensure that air vents are securely closed at appropriate times. The original air-cooled radiator is located in the DUKWâs forward engine compartment and utilizes an engine-driven fan to move air through the radiator. Fresh air intake was normally provided through the operatorâs compartment, with exhaust vents located on the port and starboard sides of the operatorâs compartment. WWII DUKWs were fitted with closable covers for the air exhaust vents, and âbow surf platesâ could be set up as additional protection against flooding. The crew on WWII DUKWS could open a forward hatch in the hood if required for land-based operations. The U.S. Army operations manual requires that this hood be âclamped tightlyâ for all waterborne operations. DUKWs modified for commercial passenger use have either a large intake vent on the âhoodâ or a member of the crew props the forward hatch open to provide adequate cooling, particularly in high ambient tem- peratures (see Figure 3-4). This modification has contributed to DUKWs experiencing flooding, including the sinking of the Stretch Duck 7 in 2018. Caught in a storm with winds and waves exceeding the DUKWâs COI maxi- mum operating limits, the Stretch Duck 7 began to founder when water came over the vesselâs bow and entered through the spring-closed forward
54 OPTIONS FOR IMPROVING THE SAFETY OF DUKW TYPE AMPHIBIOUS VESSELS air vent. The spring closure mechanism was not strong enough to keep the force of the overtopping waves from intermittently opening the air vent.23 In 2015, the same DUKW had experienced flooding of the forward com- partment after entering the water harder than normal. Some DUKWs have fitted a keel cooler in line with the existing cooling water loop through the radiator. This minimizes the flooding hazard and allows for the easier fitting of fixed fire suppression systems in the engine compartment, which are required by Subchapter T. A keel cooler augments engine cooling during waterborne operations and still allows for radiator cooling during land operations. With this arrangement, the only action required by the crew is to close or open the forward hatch when entering or leaving the water. Although keel coolers can significantly improve the safety of DUKWs from flooding, the arrangement still requires the crew to open and securely close the forward air vent at the appropriate time. Given its very vulnerable location on the hood of the DUKW, it is critical that this be done correctly every time. SUMMARY The committee agrees with USCG that reducing the risk of flooding is criti- cal for DUKWs and that insufficient intact stability has not been the cause of the loss of a DUKW from sinking or capsizing to date. All sinkings or capsizings have been caused by progressive flooding. Given the limitations of the existing DUKW hull design, there is no simple method of effectively enhancing DUKW damage stability by changes to hull internals or adding flotation. Instead of focusing solely on intact stability, the survivability of DUKWs should be considered from all aspects and use multiple, comple- mentary methods. There are viable methods to retrofitting DUKWs to increase their re- serve buoyancy. However, fitting flotation foam in the hullâon its ownâ will not keep a DUKW afloat and upright during a flooding emergency. There is simply not enough space in the hull. In addition, packing the foam tightly in the hull increases the risk of fire. Installing inflatable bladders or attaching floats to the sides of the DUKW on their own or in combination with flotation foam in the hull are potential solutions. The original WWII driveshaft boot seals present a higher risk of flood- ing. Although fitting boot seals with restrictor plates mitigates this risk somewhat, the safest approach is to replace boot seals with carrier bearings. 23 NTSB, 2020, âSinking of Amphibious Passenger Vessel Stretch Duck 7, Table Rock Lake, near Branson, MO, July 19, 2018,â Marine Accident Report NTSB/MAR/20-01, April 28, pp. 50â51. https://www.ntsb.gov/investigations/AccidentReports/Reports/MAR2001.pdf.
FLOODING AND SURVIVABILITY 55 It is critically important to permanently seal or eliminate wherever practical all drain plugs and valves. For any remaining hull drain and main- tenance plugs, all maintenance and repair procedures need to be reviewed to ensure that plugs have been correctly reinstalled before the DUKW is allowed back into service. Spring-closed engine cooling vents are also a significant flooding haz- ard. Keel coolers are one solution that allow a DUKW to be safely operated with the forward vents and hatches securely closed while waterborne. If a DUKWâs engine cooling system cannot be safely operated with vents and hatches securely closed, the potential for waves high enough to broach the DUKWs bow warrants consideration during the determination of the DUKWâs safe operating area. Because of the higher risks of flooding, the current guideline in NVIC 1-01 that DUKWs have bilge systems capable of offsetting uncontrolled flooding, such as from driveshaft seal failures or missing maintenance plugs, is reason- able and advisable.