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Practical Impact-Exposure Testing - 8 Pages

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Practical Impact-Exposure Testing

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Taking the measures required under the European Union’s 2002 Vibration Directive, Boomeranger Boats, a builder of specialized high-speed RIBs in Finland, tests two models of shock-mitigating seats to determine which will best reduce whole-body impacts on boat operators. Text by Jussi Mannerberg Photographs and illustrations courtesy Boomeranger Boats Built in Finland, the 31.2' (9.5m), 6,614-lb (3,000-kg) Boomeranger Special Ops C-3500 open RIB, powered by twin 300-hp Mercury Verado outboards, is designed for high-speed commercial or government service. As such, the boat is subject to the European Union’s directive limiting worker exposure to impacts and vibrations. It requires adoption of the best available shock-mitigating technology to protect crews working in extreme marine environments. Editor’s Note: Professional BoatBuilder has devoted significant editorial space in recent issues to exploring the efforts of naval architects and engineers to model and under­ stand the slamming and impact accelerations that fast planing boats and the crews that operate them are exposed to. “Analyzing Accelerations” Parts 1 and 2 appeared in PBB Nos. 140 and 141, respectively. Those articles detailed what we know about the specifics of seakeeping in highspeed craft, what we should be able to model during the design phase, and what tests and data would help designers better predict vertical accelerations over a range of speeds and sea states. The following article is a practical account of how a designer and builder of high-speed professional-grade RIBs addresses vertical accelerations in its existing models. Its author, boatbuilder Jussi Mannerberg, tells how the company he manages, Boomeranger Boats (Loviisa, Finland), measures slamming loads on hulls and assesses impact exposure of professional boat crews to meet the requirements of the European Union’s Vibration Directive. This article is based on a similar paper Mannerberg presented at the 2012 High Speed Boat Operations Forum in Göteborg, Sweden. —Aaron Porter oomeranger Boats Oy has built professional high-speed rigid inflatable boats (RIBs) since 1991. In the last two years, we’ve seen an increasing number of potential buyers inquiring whether the boats comply with the European Union Directive on Whole Body Impact and Vibration, a standard implemented to assure the health and safety of workers in EU member nations. Designed to limit exposure to vibration in all workplaces, the Jaakko Pitkäjärvi/Courtesy Boomeranger Boats Practical Impact-Exposure Testing

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Jaakko Pitkäjärvi/Courtesy Boomeranger Boats Designers at Boomeranger tested two models of shock-mitigating seats to determine which better dampened the vertical and lateral accelerations crews are likely to be exposed to in high-speed, rough-water operations. The seats were installed in the forward position ahead of the helm station, and the crew riding in them were monitored as the boat was run through a variety of maneuvers in different wind and sea states. directive specifies the responsibilities of employers to assess risk and exposure to vibrations, plan and implement control...

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To measure the forces on the crew, the accelerometers were bound tightly to their torsos in kidney belts worn inside survival suits. The method has been validated against bone-anchored sensors and was found to be suitably accurate for impacts up to 16–20 Hz. Our test platform was a boat from the standard Boomeranger product line—a 31.2' (9.5m) Boomeranger Special Ops C-3500 open RIB powered by twin 300-hp Mercury Verado outboard engines; dry weight 6,614 lb (3,000 kg); deadrise 26°. We tested two different types of shock-mitigation jockey-style seats featuring vertical as well as limited...

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Accelerometer readings of the 24 highest impacts were isolated from the broader data to focus on how the shock-mitigating seats handled them. Data reveal instances in which Seat A seemed to amplify the impacts delivered to the hull. Figure 1. Values for 24 Highest Impacts 14 Hull Our measurement of hull impacts and the comparison of seats were focused on vertical accelerations (gz). Lateral and longitudinal accelerations were monitored by the accelerom­ters e on the subjects and on the deck with System 2. In addition, two video cameras recorded the entire run. One camera recording in slow...

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Overlaid data of a 2.3-second interval show that Seat A is already bottoming out and amplifying impacts at as little as 2 g. Blue – Hull/Deck Red – Seat A Green – Seat B was 17–26 knots from the southwest, and significant wave heights were from 3.3' to 8.2' (1m to 2.5m). The boat was driven at a variety of headings with speeds adjusted to what was considered safe for the subjects and comfortable by the crew. The maximum peak impact recorded on the deck/hull was 7.2 g (Figure 1). The Sed(8) value calculated from the hull-mounted accelerom­ters e was 2.84 MPa. This is significantly lower than...

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Blue – Hull/Deck Red – Seat A Green – Seat B Time (seconds) Overlay of the highest recorded impact suggests that the crew member in Seat A was subjected to more than 12 g. Note that the limit for the accelerometer was 11.5 g, thus the flat peak of the graph. Extrapolation of the graph indicates a peak in excess of 14 g. cumulative root-mean-square values over an 8-hour period—for the boat was 15.0 m/s1.75. This is lower than the daily exposure limit value of 21 m/s1.75, but higher than the daily exposure action value of 9.1 m/s1.75 stated in the EU directive. Impacts measured were similar...

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Figure 4. Mean Peak Values for Impacts Above 2g, 3g, and 4g 10 9 8 Hull Seat A Seat B Seat B, 4.0 g was the highest impact Comparing mean peak values for the entire data set to recorded. those for impacts greater Overlaying deck and seat data than 2 g, 3 g, and 4 g reveals that in the higher-peak deck Figure 4. reduced Values that impacts, Seat B significantly Mean Peak confirmsfor the differences Impacts Above A in impacts to the subject, while Seat 2g, 3g, and 4g seat performance were amplified in multiplied impacts up to a factor of rougher conditions. three. In the lower range of deck...

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