Structural Strength of Work Boats and High Speed Crafts with Pre-fabricated, Floating Panels in Aluminium

Abstract

Aluminium is a material commonly used for smaller boats and high speed crafts due to its low weight. Traditionally the hull construction is performed in a similar manner as that of steel ships, with longitudinal stiffeners fitted through cut-outs in the transverse web frames and welded to the shell plating. This requires much fitting and welding, making the production of hulls a slow and expensive task due to the manual labour needed.

An alternative method for construction of aluminium hulls is to extrude panels consisting of both the shell plating and stiffener. These pre-fabricated panels are then welded directly to the transverse frames, thus reducing the manual labour related to hull production significantly.

This thesis continues the work previously performed by Jon Englund on floating panels. He found that the stresses will increase significantly in a floating frame structure compared to that of a traditional, fixed structure. It was discovered that the main challenge of a floating frame structure is out-of-plane bending stresses occurring in the stiffeners webs due to frame deflections.

By use of finite element analyses and analytical calculations, a compartment of the JumboCat 60 is analysed, and proposals for achieving acceptable stresses are given. The stiffener stresses mentioned above are found to be drastically reduced by increasing the bending- and axial stiffness of the transverse web frames. Local stress concentrations are found in the intersection between stiffener and web frame.

Nonlinear finite element analyses show that substantial strain-hardening can be achieved in the stiffeners webs through cyclic loading. The loss of strength due to welding may thus be partly recovered.