Engineering Design Driven by Models and Measures: The Case of a Rigid Inflatable Boat

Rigid-hulled inflatable boats are extremely practical and popular nowadays, offering an effective conciliation among usability and costs. Their stable and seaworthy behavior is guaranteed by performing hydroplaning hulls coupled with unsinkable inflated tubes. At the same time, their design is often based on tradition and preconceptions. In this article, both numerical methods and experimental mechanics techniques are proposed as an essential way for supporting the designers in decisive tasks. Three different situations are detailed where a numerical or an experimental approach shows its benefit inside the engineering design process: firstly, permitting investigation of the behavior of materials driving the fiberglass selection; then measuring the levels of stress and strain in the hull during sailing; and finally, using information as a base for developing numerical models of the hull slamming in waves. Even if the discussion is focused on a rigid inflatable boat, a large part of these considerations is relevant beyond this particular case.

Engineering Design Driven by Models and Measures: The Case of a Rigid Inflatable Boat

Rigid-hulled inflatable boats are extremely practical and popular nowadays. They offer a effective conciliation among usability and costs. Their stable and seaworthy behaviour is guaranteed by performing hydroplaning hulls coupled with unsinkable inflated tubes. At the same time, their design is often based on tradition and preconceptions. Rarely, the design assumptions are validated by the reality or, even, by deeper investigations. In this article, both numerical methods and experimental mechanics techniques are proposed as an essential way for supporting the designers in their decisive tasks. Three different situations are detailed where a numerical or an experimental approach shows its benefit inside the engineering design process: firstly permitting to investigate the behaviour of materials driving the fiberglass selection; then measuring the levels of stress and strain in the hull during sailing; finally, using all available information as a base for developing numerical models of the hull slamming in waves. Even if the discussion is focused on a rigid inflatable boat, large part of its considerations is relevant beyond this special case.

Drop Tests and Modeling of Water Entry of Air-Cavity Hull Sections

Marine vessels sailing through waves may experience very high loads during slamming events. The present study addresses a novel air-cavity hull configuration that contains air trapped between rigid side hulls and a platform. Laboratory drop tests have been conducted with two-dimensional sections imitating air-cavity hulls. The platform vertical position was the main variable parameter. Time-dependent accelerations and pressure measured at the platform center are reported. Peak values of these variables and their occurrence times are identified. The captured air pockets are found to reduce maximum slamming loads. A simplified mathematical model is applied for simulating the initial phase of the air-cavity hull water entry. The obtained results can be used for seakeeping assessment of air-cavity ships as well as validation of more sophisticated mathematical models for hull slamming.

Hull Slamming

This report presents an in-depth review of the current state of knowledge on hull slamming, which is one of several types of slamming problems to be considered in the design and operation of ships. Hull slamming refers to the impact of the hull or a section of the hull as it reenters the water. It can be considered to be part of a larger class of water entry problems that include the water landing of spacecraft and solid rocket boosters, the water landing and ditching of aircraft, ballistic impacts on fuel tanks, and other applications. The problem involves the interaction of a structure with a fluid that has a free surface. Significant simplifications can be achieved by considering a two-dimensional cross section of simple shape (wedge, cone, sphere, and cylinder) and by assuming that the structure is a rigid body. The water is generally modeled as an incompressible, irrotational, inviscid fluid. Two approximate solutions developed by von Karman (1929, “The Impact on Seaplane Floats During Landing,” NACA Technical Note NACA-TN-32) and Wagner (1932, “Uber stoss und Gleitvorgange an der Oberache von Flussigkeiten,” Z. Angew. Math. Mech., 12, pp. 192–215) can be used to predict the motion of the body, the hydrodynamic force, and the pressure distribution on the wetted surface of the body. Near the intersection with the initial water surface, water piles up, a jet is formed, and the solution has a singularity in this region. It was shown that nearly half of the kinetic energy transferred from the solid to the fluid is contained in this jet, the rest being stored in the bulk of the fluid. A number of complicating factors are considered, including oblique or asymmetric impacts, elastic deformations, and more complex geometries. Other marine applications are considered as well as applications in aerospace engineering. Emphasis is placed on basic principles and analytical solutions as an introduction to this topic, but numerical approaches are needed to address practical problems, so extensive references to numerical approaches are also given.