Human impact exposure on fast boats
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HUMAN IMPACT A good rule of thumb is that anything that hurts is potentially dangerous.! Slamming at speeds in rough conditions can be dangerous. Derogations from the EU directive can be granted, but only on a national level, and only on the condition that all reasonable measures are taken to reduce the impact exposure. This includes, for example, using the proven best shock mitigation technologies available. The EU directive does not apply to privately used leisure boats. ISO 2631 This is the original standard for whole body vibration. Not relevant. Human impact exposure on fast boats Carl Magnus Ullman presents us with a walk-through guide to human impact exposure, with questions answered and myths dispelled. M ost boaters know that slamming at speeds in rough conditions can be dangerous. The topic of injury risks on slamming fast craft is still gaining increased recognition, even among leisure boat owners. This is not least due to media attention on a number of accidents where boat owners, as well as innocent paying passengers, have been injured in severe slam events. Research is ongoing, mainly sponsored by military and coastguard agencies in various countries. A number of standards exist, as well as different methods of boiling down exposure data to single-figure values. Hugo Montgomery-Swan asked me to clarify the subject and point out what is relevant to know and to understand. First of all, acute injury does not come as a result of vibration, but as a result of impact. This is well known in the scientific community. So why has ‘whole body vibration’ come into this field? It is simply because there have been no standards regulating or limiting the exposure to impact. So the EU committee just had to use the closest ones they could find: ISO standards developed for lorries and forestry machines. Vibration is not good. Extensive exposure can cause problems over time, adding to aging of discs and cartilage, but this is not of great relevance for the risk of acute injury – especially not for leisure boating people. Let’s forget vibration, whole body vibration, WBV and similar acronyms and focus instead on impacts. The most severe injuries, seen as a result of bad slamming events, include fractures to vertebrae and extremities and rupture of intervertebral discs, even in the neck. Distortions are common but normally less serious. How bad a slamming is acceptable? A good rule of thumb is that anything that hurts is potentially dangerous. Pain will usually be the first indicator of an injury. It is wise not to accept travelling in HS craft in such a way that you experience severe discomfort or pain, regardless of whether this appears at 4g or 13g impacts. It is also dangerous to claim that certain wave conditions and directions are more dangerous than others. Injuries can occur regardless of wave direction, and the risk is proportional to the level of energy in the slam. Standards EU Directive 2002/44/EC Does the EU directive apply? Yes – it is the law in all EU countries and applies to boats being operated professionally – as well as in tourist/ joyride applications. It defines the maximum level of exposure you are allowed to subject employees or paying passengers to. These limit values are so strict that they are normally exceeded within minutes on an 8–10 m boat doing 35 knots in 0.7m waves. So when someone claims that, with this or that boat, or this or that seat, you will comply with the EU directive, it is just not true! It will only be true if you don’t leave the dock on a windy day. ISO 2631:5 This is an updated version, with algorithms tweaked to give higher values, if there are a significant number of high-level impacts in the raw data. The problem is that it doesn’t show much difference. Single severe impacts disappear completely. This standard should not, according to the authors, be used for anything producing impacts over 4g. Not relevant. Annex 10 of 2000 HSC CODE, International Code of Safety for High-Speed Craft. This annex is put in place to make sure crew and passenger seats, on boats and ferries, do not break or become detached from the deck, under collision or when running a ground. It has no bearing on the function or performance of suspension seats. The limits for structural failure are lower than the forces acting on HS boats under severe impact. It deals mainly with horizontal forces. Seats claimed to comply with this standard are known to have failed, with injured operators as a result. MCA Marine Guidance Note MGN 436 This is a new document (Sept 2011) based on understanding of new research and technologies. The recommendations are relevant! Terms and units used in the standards RMS or Root Mean Square is defined as ‘the square root of the arithmetic mean of the squares of a set of values, used as a measure of the typical magnitude of a set of numbers, regardless of their sign’. In this context, it is used as an average value of how much energy is transmitted through vibration over a period of time. It does not say anything about single impacts and is normally not affected by single impacts. POWERBOAT & RIB MAGAZINE Tech - Shock Mitigation.indd 4 103 20/12/2012 17:03

FEATURE SHOCK MITIGATION The beauty of today’s new technology is that measuring impacts now is so easy that basically anyone can do it. Above: Royal Australian Navy RIB fully loaded with gear and crew (on heavier boats having softer peak impacts). Main: Deep-V hull displacing water effectively. Applying RMS value limits, it is not dangerous to fall out of a window on the sixth floor, because, over a period of one minute, you have not received more energy exposure than you should be able to sustain … VDV, or Vibration Dose Value, is a single-figure value for cumulative RMS values over an...

HUMAN IMPACT RIGHT: FIG.1 GPS and Impact track of Impact measurement presented directly from the DaccR. Green peaks are below 4g, yellow peaks are 4-8g, red peaks are over 8g. FIG.2 Impact measurement in rough seas done with two DaccR (Dyena Accelleration Recorder), one on the console and one on the the driver. Red curve is g-force on person on suspension seat. Blue curve is g-force on boat. Green curve is speed over ground. FIG.3 Illustrative way of showing the size and number of impacts. The height of the column is the nubmer of impacts at each g-level. The position of the column is the...