Designing Shock-Mitigating Seats for Boat Operators

Products for nautical applications face an unusual set of design challenges. The corrosiveness of salt water can cause premature degradation, and the impact of fast-moving vessels bouncing up against forceful ocean waves can also damage equipment.

The U.S. Coast Guard is one agency, in particular, that needs reliable equipment for its boats. One of its vessels, the Kathleen Moore, has an overall length of 153 feet, 6 inches and a maximum speed of 28 knots. That’s the equivalent of 32.2 mph. For comparison, the average speed of a fishing boat is 9 knots, a sailboat is 7 knots, and a cruise ship is 20 knots.

The Kathleen Moore is a “cutter,” used to patrol territorial waters and enforcement activities. Like many other U.S. Coast Guard vessels, it is equipped with shock-mitigating seats. Designed by SHOCKWAVE of British Columbia, the seats significantly reduce shock absorbed by boat personnel.

“They are the go-to seats for most military and commercial boat users around the world and are mandated on a lot of government contracts,’’ said Rob Pletscher, Engineering Manager at SHOCKWAVE. “The seats work well in reducing stress for people who are on the vessels.”

The physical stress vessel operators experience can be significant. The faster the boat, the more likely the physical harm to the driver.

“If your seat isn’t designed to take abuse, you might be the one taking the abuse,’’ Pletscher said. “Traveling even at moderate speeds without seat suspension can cause cumulative back and neck/pain injuries. Wave shock impact can be unforgiving, and over a long enough period, it can leave you debilitated and unable to take the boat out whatsoever.”

The test engineers face in designing seats is creating a suspension system that maintains stability but also reduces friction and allows for some latitude in movement.

“The biggest challenge is keeping things as tight as possible but also moving as freely as possible,’’ Pletscher said. “The way to make a good suspension system is to reduce the friction in it. But when you reduce the friction, you also need to keep it tight enough and reduce slop. The biggest challenge is weighing those two. We must keep things tight enough so that the seat is not sloppy but also keep it free enough so that you can get better performance out of your seat. It’s a balancing act.”

The other critical challenge was identifying products that can withstand corrosion in a saltwater environment and remain durable under the stress of high-impact movement.

Designing shock-mitigating seats requires knowing the tolerances of the materials used. The components need to work as a cohesive unit.

“You need to be very careful with the components,’’ Pletscher said. “When you have components that interact with each other, you have to look at machining tolerances, minimums, and maximums for all the components that touch each other. You have to make sure there’s no interference because you can’t have two things occupy the same space at the same time, or else they’ll be jammed together.”

Tolerances that are too tight would create rigidity and lead to friction. Components that are too loose will result in instability of the seat. Finding the precise values required painstaking research and testing.

“We spent a lot of time on the design side to make sure that we have the absolute minimum clearances between components while taking the manufacturing tolerances into consideration,’’ Pletscher said. “Then we look at components that are easier to manufacture at tight tolerances to help reduce the overall stack up, which is basically the addition of gap components.”

SHOCKWAVE solved issues with corrosion and tolerance preferences with support from igus, the manufacturer of high-performance plastics. Components from igus support suspension with limited friction and moisture absorption while also allowing for vertical and horizontal seat adjustments. Nearly 38,000 bushings from igus are used annually by SHOCKWAVE.

“On our high-volume product line, the main issue we tried to solve was to get a bushing that didn’t swell when it’s exposed to water,’’ Pletscher said. “We wanted a bushing that could handle high loads but was also dimensionally stable so that the tolerances could be tight enough while also keeping the cost down.”

Linear guides from igus are used primarily in SHOCKWAVE’s commercial and military products. The guides are corrosion-resistant, durable, and like all igus products, maintenance-free and require no lubrication.

“A lot of the linear cars that exist on the market have steel balls inside the mechanism,’’ Pletscher said. “If you want to keep the profile low, you need some space because the steel ball can’t touch two surfaces at the same time. They’re not going to work properly. The igus slides don’t have any steel components, so corrosion is not an issue. And there are no rolling mechanisms. That allows for a lot tighter tolerances so that you don’t have that slop in the seat where it’s rocking from one side to the other.”

The genesis of SHOCKWAVE’s shock-mitigating seats developed when company founder and CEO David Smith cruised from Victoria, B.C. to Glacier Bay, Alaska in 2002. With seats that included igus components on board, Smith breezed down Canada’s Pacific Coast in only 10 hours, setting a speed record in the process, with far less physical stress compared to previous long seafaring journeys.

Thomas Renner writes about engineering, architecture, building, construction, and other trade industry topics for publications throughout the U.S. For more information, visit here .

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