Page 30: of Maritime Reporter Magazine (September 2013)

30 Maritime Reporter & Engineering News ? SEPTEMBER 2013 For some time now, military agencies and specialist organi- zations around the globe have driven the evolution of high speed watercraft. As a trickle-down ef- fect fast response craft now serve roles once typically serviced by displace-ment designs including pilot boats, dive boats, crew boats and offshore sup- port. During high speed operations the repeated shocks (RS) and whole body vibrations (WBV) of this harsh envi-ronment are transmitted to the passen-gers, operators and crew aboard, cre-ating avenues of owner liability if not addressed.While economic considerations and fast transport of personnel are core com-mercial objectives [1], long term expo-sure to RS & WBV pose health risks to personnel. Quantitative evidence from military studies indicate that high-speed boat crews have a hospitalization rate that compares to construction workers [CN], Þ remen [FN] and air crews [AN] within the military community [3]. Self-reported injuries among US Navy special operations crews reveal the ex-treme nature of the environment. Of those surveyed, 64.9% reported at least one injury [3]. Some operators reported multiple injuries. Typical of high speed craft operators, injuries ranged from sprains & strains to chronic pain and severe stress fractures [3]. Other stud-ies indicate the rate of injury on board U.S. Navy high speed craft is six times the overall U.S. Navy average [5]. A large percentage of those are acute and chronic lumbar spinal injuries present-ing both immediate and long term rami-Þ cations to the effected crewmen [6]. Most waterborne injuries can be placed in the category of ?motion in-duced? caused by the human body?s re- action to the physical work associated with self-mitigating the shocks during transit [4]. Motion induced fatigue is more clearly deÞ ned as physical im- pairment that results from exposure to WBV and RS for long periods of time, including deÞ ciencies in vigilance, perception, decision-making and reac-tion time. General discomfort, fatigue, post-transit degradation of performance and motion sickness when operating at low speeds are also among the wide spectrum of issues associated with mo-tion induced fatigue [2].As the exposure effects to extreme motions and repeated high accelera-tion slam impacts in high speed craft are becoming better known, increasing operational capabilities facilitated by enhanced watercraft designs no longer means that the vessel is simple to op-erate within the full operational enve-lope. Awareness of these issues have compelled speciÞ ers and designers to consider best design practices of shape, form, function, ergonomics and seating solutions to counter the physical stress-es that shock and vibration energies im- part to the crew and passengers.The roles of passengers also increas-ingly need to be considered by boat crews as they are being transported and perform tasks both at speed and once on-station [1]. Passengers will beneÞ t from having a pre-voyage brieÞ ng - knowing what to expect and how to act in reducing their potential for injury. As on-board systems become more complex the competencies demanded of operators move to a higher level and are more like those required by a heli-copter crew where effective situational awareness and command & control be-come crucial for performance and safe-ty. With new and retro Þ tted craft being capable of out-performing the crew it is essential that designers focus on human factors to ensure that they can operate the craft at the edge of its operating envelope and still ensure operational success and safety for the crew and pas-sengers [1]. Reducing RS and WBV experienced by boat crews by slowing down, bet-ter training, and novel hull designs is widely recognized, but a great deal of research in the area of shock mitiga-tion, has revolved around seating sys-tems [7]. Notwithstanding the boat?s structural limits, shock mitigating seats are also easily retroÞ tted onto existing watercraft to effectively upgrade the operator?s environmental operational envelope. The variability in operating conditions and human factors makes it difÞ cult to design an ideal shock mitigating sus-pension system [4]. High Speed Craft (HSC) seat suspension design places unique challenges on manufacturers due to the diversity of maritime operat-ing conditions. Fast craft can, on the same voyage, experience variations from calm seas at harbor, to potentially violent rough waves away from shore. Current automotive suspension systems are not appropriate for use in marine seats. For example, the sprung mass in automotive applications remains rela-tively constant when compared to ma-rine seating, which can vary 300% or more. Similarly, ranges of environmen- tal inputs in automotive applications are not comparable considering the ex-tremes of sea conditions. As a result, approaches to marine seat design differ greatly from other vehicles. Most shock mitigating seat designs currently use a passive suspension system to absorb the shock transmit-ted through the hull. Even the most advanced passive suspension systems can only be tuned for a relatively nar- row range of conditions to operate ef-fectively. Typical occupant weight and sea state must be assumed during the design and speciÞ cation phases of a project and may not be suitable for all operations. Some passive systems have the ability to be manually adjusted, but this may be difÞ cult while underway, at THE HUMAN FACTOR Shock & Vibration Repeated Shock and Whole Body Vibration Awareness Mark D. Lougheed (SNAME) is the Engineering Projects Leader ? Technology Develop- ment for CDG Coast Dynam-ics Group Ltd. Mark?s career spans 25 years as a high per-formance military and com-mercial craft designer. E: [email protected] #9 (26-33).indd 30MR #9 (26-33).indd 309/3/2013 10:13:52 AM9/3/2013 10:13:52 AM