Award Date

5-1-2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Committee Member

Douglas Reynolds

Second Committee Member

Brendan O'Toole

Third Committee Member

Mohamed Trabia

Fourth Committee Member

Woosoon Yim

Fifth Committee Member

Nader Ghafoori

Number of Pages

302

Abstract

High-speed craft (HSC) are utilized in military and para-military maneuvers that are usually conducted in harsh environments. The Special Warfare Combatant-craft Crewmen (SWCC), a sub-unit of the United States Naval Special Warfare Command (NAVSOC), use an advanced HSC, known as the Combatant Craft Assault, to conduct coastal patrol and interdiction, and for infiltration and exfiltration of Navy SEALs. The United States Coast Guard, and the Drug Enforcement Agency (DEA), employ HSC for maritime patrol and to pursue ‘go-fast’ boats operated by contraband runners. In most of these intense applications, the mode of operation of the HSC results in planing. Planing occurs when the weight of the craft is largely supported by hydrodynamic lift, as opposed to hydrostatic lift. That is, the force that is normal to the fluid flow, not the buoyancy force (upthrust), is the force that sustains the partially immersed craft. The severity of these operations, in conjunction with the planing phenomena, create extreme dynamic environments that subject the HSC occupants to mechanical oscillations and shock pulses due to wave slam events. A wave slam is the violent impact between a waterborne craft and an incoming wave, and the shock created by the impact is the predominant cause of musculoskeletal injuries sustained by HSC operators and passengers.

Over the years, shock mitigation seats have been installed in HSC to diminish the rate of injury among on-board personnel. The consensus is that the design of a shock mitigation seat is based on finding a balance between comfort (from the seat cushion material) and shock mitigation capability. However, the varying characteristics of shock loads experienced at sea, makes it challenging for seat manufacturers to develop a seat that performs satisfactorily across the response spectrum of interest.

In addition, currently, a universally accepted measurement system to classify, or rate, shock mitigation seats does not exist. Consequently, it is difficult for potential shock mitigation seat clientele to make an informed decision when considering seats for a specific operating environment. Ultimately, the goal of this study was to develop a repeatable procedure to quantify the performance capabilities of a shock mitigation seat that would facilitate identifying its operational envelope. In addition, this endeavor also sought to develop a computer based numerical model of the system to validate the empirical results, and to characterize the dynamic response of the seat. The test protocol was based on requirements presented in a proposed ISO Standard, as well as guidance developed from research that was conducted by the United States Naval Surface Warfare Center Carderock Division (NSWCCD). However, en route to accomplishing the objectives, two questions – the answers to which were integral to the outcome of the study – had to be addressed.

The first query explored if the use of a single impact drop tower was a plausible method to simulate the rapidly applied vertical shock that is associated with a wave slam event experienced by an HSC. The NSWCCD has recommended using a mass-spring-damper model as the basis for the analytical representation of a shock mitigation seat. Hence, the second inquiry aimed to verify if the [HSC]-[seat-occupant]-[wave] global system could be appropriately modeled using the mass-spring-damper analytical representation of the seat.

Testing revealed that a drop tower, with certain hardware configurations, was a feasible option to simulate the vertical shock that is associated with a wave slam event. The tower was fully capable of testing to a severity threshold level of four (6.12 g), according to the recommended testing levels for military and commercial craft outlined in the ISO and NSWCCD documents. Using the tower, the seat was rated for use in vessels ranging from HSC Class 3 to HSC Class 4-3 for operating conditions that match severity threshold level four. The mass-spring-damper model proved to be a credible approach to the analytical representation of the shock mitigation seat.

A numerical analysis, conducted with SDOF and 2DOF models based on the mass-spring-damper concept, produced simulated acceleration data that agreed with the experimental data collected during the drop tests. The simulated acceleration and position data was effective in validating the empirical data, and a consolidated analysis using both the SDOF and 2DOF approaches provided an acceptable characterization of the dynamic response of the seat.

Keywords

mitigation; seat; shock

Disciplines

Mechanical Engineering

File Format

pdf

File Size

8500 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

Download


Share

COinS