M.Sc Thesis
M.Sc Student Moshkovich Yahav A Design Method for Planing Hulls,Considering Hydro- Elasticity, Dynamics and Nonlinear Structural Response Department of Mechanical Engineering Professor Nitai Drimer

Abstract

Planing is the most common concept of fast boats.  When a fast boat sails in head seas, it emerges off the water (all of it or just the front part) and lends.  While landing, the boat typically slams the water and the structure is applied to the slamming pressure, which is typically much more significant than the hydrostatic pressure.

The slamming pressure in fluid-structure interaction has been extensively studied from the beginning of the previous century, both analytically and experimentally.  In the last three decades, with the development of computers technology, this phenomenon has been studied extensively with numerical simulations. Although a variety of slamming problems were studied and numerical solutions has been published, the consideration of hydro-elasticity is yet rarely been applied by boat designers.

Practically, boats design is based on the rules that the marine classification societies develop. The rules of the different classification societies are similar and handle the slamming pressure by assessing a quasi-static pressure, which is applied to the structural elements (plates, longitudinal stiffeners, transverse frames).  These members of the hull structure are being represented as beams according to the linear beam theory.  The slamming pressure depends on the boat operational conditions, which are represented, for design practice, by the significant wave height (average of the highest one third of the waves in a record) and the sailing velocity.

Actually, the slamming is a violent fluid structure interaction, where dynamics, hydro-elasticity, and nonlinear structural effects might be important.  It is important to mention that the slamming load is the dominant load for the design of a planing boat hull.

This thesis presents a new and advanced design method for planing hulls, which considers hydro-elasticity and nonlinear dynamic structural analysis.  The suggested method combines rules calculations, analytical solution and nonlinear numerical analysis of fluid-structure interaction, which are implemented to a practical design procedure.  The basic design parameters for the procedure are: the drop velocity, the thickness of the boat bottom plate, the distance between the stiffeners and the deadrise angle.

The numerical analysis is based on Finite Element Method, applying the commercial code ABAQUS/CAE, with a Coupled Eulerian-Lagrangian (CEL) formulation for the fluid domain and Lagrangian formulation for the structure domain.

The fluid structure interaction simulations are not easy to implement and require specialty and verification.  A parametric analysis is applied to provide a database of 300 cases, which may be used by designers for a preliminary assessment of the load effect (deformation, strains, stresses) at representative transverse sections along the hull, within a practical range of the design parameters.

Few detailed examples of the suggested design method clarify and illustrate its application.  The design examples demonstrate saving of about 20% of the bottom plates thickness, relative to design by the rules.  Alternatively the spacing between stiffeners may be increased to save welding length in the boat production.