Laboratory Test Requirements for Marine Shock Isolation Seats

October 1, 2022

Naval Surface Warfare Center, West Bethesda, Maryland

This research provides preliminary guidance for laboratory testing of marine shock isolation seats. The purpose of the test is to demonstrate the effectiveness of a passive seat in reducing simulated wave impact loads in a laboratory before installation in a high-speed planing craft.

Small craft that operate at high speeds in rough seas subject the crew and passengers to wave impacts that may cause extreme discomfort. Craft designers therefore often include shock isolation seats to mitigate these negative effects. Current design practice is to install seats that employ springs and dampers (i.e., shock absorber) or leaf-spring assemblies as protection mechanisms. They are referred to as passive seats because the spring-damper assembly responds to individual wave impacts with no active elements that change in real time to adapt to the environment.

Spring-damper assemblies are also employed as protection mechanisms on land vehicles (e.g., tractors, trucks, automobiles, buses) and on-board larger ships for mine blast protection. Experience has demonstrated that dynamic environments are different, and that seats designed for land vehicle or mine blast applications may not protect against the unique characteristics of wave impacts on small highspeed craft. Seats have been installed in craft only to find out during subsequent seakeeping trials that they provide little to no protection or amplify base input motions. There is, therefore, a need to provide guidance on how to simulate these unique wave impacts in a laboratory test that will demonstrate the mitigation performance of passive shock isolation seats prior to installation in a high-speed craft.

High-speed craft motions include all six degrees of freedom: three translational (i.e., heave, surge, sway) and three rotational (i.e., pitch, roll, and yaw). During severe wave impacts in head seas the largest accelerations are in the vertical (i.e., heave) direction, but the other response degrees of freedom are just as important for people. Feedback from personal experiences indicates that any force out of plane with the vertical axis of standing or sitting that induces body torque or bending can be just as punishing as the vertical shock input. Simulation of fore-aft accelerations during a laboratory test is recommended herein as a test option achieved by an angle insert below the seat. Specific guidance for required off-axis testing (i.e., not just vertical testing) will be included in future revisions as data becomes available.

This guide is applicable to shock isolation seats used in high-speed planing craft. The test criteria presented herein are intended for planing craft ranging from 7-meters to 30-meters in length. The test procedures are intended only for passive seats with no active sensors or mechanisms for real-time adaptation to the dynamic environment, and no use of the occupants’ legs to mitigate an impact. In addition to protection mechanisms, shock isolation seats universally offer ergonomic features that provide differing degrees of comfort. This test standard addresses only the protection characteristics of seats; seat ergonomics is not addressed.

This work was performed by Michael R. Riley, Kelly D. Haupt, Dr. H. C. Neil Ganey, and Dr. Timothy W. Coats for the Naval Surface Warfare Center. For more information, download the Technical Support Package (free white paper). NSWC-0004

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This Brief includes a Technical Support Package (TSP).

Laboratory Test Requirements for Marine Shock Isolation Seats

(reference NSWC-0004) is currently available for download from the TSP library.

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