Whole Body Vibration (WBV) is one of the hazards of operating small, high-speed craft (HSC) including passenger, light cargo, search-and-rescue, law-enforcement and rigid hull inflatable boats (RHIB) in choppy water.
WBV is the shaking or jolting of the human body through a supporting surface, usually a seat or floor in a vessel. Occupational health researchers have discovered that the forces can cause long-term health and work-performance problems. Naval architects are developing solutions.
“The best way to safeguard your people and passengers is to reduce the source of the vibration and shock, through good hull design,” said Alan Cartwright, head of marine engineering at Multimarine Composites Ltd. (MCL), builders of Powercat boats. The company uses Trelleborg Confor foam that controls vibrations and shocks where suspension seats cannot be used.
Though much more expensive, suspended decks or wheelhouses are starting to be considered.
Deck matting designed to reduce vibration exposure is another option.
“WBV exposure on planing craft is usually caused by continuous ‘hammering’ from short steep seas or wind-against-tide conditions,” said a marine guidance note from the British Maritime and Coastguard Agency. There is no definitive design to mitigate WBV in small vessels, the agency said.
Courtesy Tampa Yacht Manufacturing
Aboard Tampa Yacht Manufacturing’s boats, shock-mitigating seats with a special suspension system isolate the passengers from the vibration through the use of mounts attached to the vessel.
The Canadian Centre for Occupational Health and Safety (CCOHS) notes that WBV can cause fatigue, insomnia, stomach problems, headache and “shakiness” shortly after or during exposure. Long-term exposure is more problematic. According to the CCOHS, after daily exposure over years, WBV can affect the entire body and result in health disorders. Many studies have reported decreased performance in workers exposed to WBV. This includes the demanding ability to handle a HSC under severe sea conditions.
Studies of bus and truck drivers found that exposure to WBV — combined with body posture, postural fatigue and dietary habits — could have contributed to circulatory, bowel, respiratory, muscular and back disorders.
The operating environment for HSCs results in the most challenging conditions for mitigating WBV. Boat operators who experience shock impacts regularly may not receive a serious acute injury on any single trip. Over time, they may experience cumulative effects. The CCOHS says the time from first exposure to appearance of symptoms can range from a couple of years to up to 16 years.
Because physicians usually are not trained to diagnose vibration-related disorders, WBV is often not considered as the reason for a problem. There are no objective clinical tests to measure the impairment. This can present a problem in verifying claims for disability and worker’s compensation.
Dr. Trevor Dobbins, of Human Sciences & Engineering Ltd. in the U.K., has found that using a properly designed suspension seat versus a fixed one can reduce oxygen uptake by about 35 percent and heart rate by about 13 percent.
The British guidance note said the shape and hydrodynamic performance of the hull can reduce the impact of vertical movements. A deep-V hull will cut through the water better than a flatter-bottomed vessel. However, if the vessel heels significantly as it falls off the wave, the vessel will land on a flat part of the hull, resulting in severe slam. The use of multihulls and hydrofoils may provide a more comfortable ride.
In the HSC manufacturing sector, boatbuilders including Tampa Yacht Manufacturing are well aware of the problem. Bob Stevens, Tampa Yacht Manufacturing chief executive, said WBV is considered in the company’s current and future designs. Tampa Yacht has cooperated with Dobbins’ research, he said.
“We at Tampa Yacht install shock-mitigating seats and are mindfully aware of MMI (Man Machine Interface) and operational fatigue,” Stevens said.
MCL Powercats have hull forms to reduce vibration and impact at the fundamental source — as close to the waves as possible. Both the MCL Powercat 525 and 990 have catamaran hulls to reduce slamming at high speed in choppy waters. Likewise the British-built C-Fury Patrol RIB’s hull was designed with shock and WBV in mind. C-Fury’s hull design has a steep deadrise, small planing faces, eight spray rails, a hydrofoil and two very large tunnels with semicircular tops to permit airflow in all conditions to minimize the potential for hydraulic locking.
Because WBV-limiting technologies are just appearing and operators will live with current craft for many more years, awareness and training are key ingredients. FRC International, named by its expertise on Fast Response Craft, offers two FRC WBV awareness courses, occasionally in the U.S. and Canada.