Redesign of a Composite Marine Shaft via the Novel Double-Double Layup

Composite drive shafts offer substantial weight and durability advantages over steel in marine propulsion systems, yet their design traditionally relies on computationally expensive metaheuristic optimisation of a combinatorially large, discrete ply-angle space. This study investigates the Double-Double (DD) family of laminates, a continuous, two-angle design framework, as an alternative methodology applied to the redesign of a composite propulsion shaft. A finite element model is first validated against two literature reference laminates. A stiffness-equivalent DD laminate is then derived analytically via lamination parameter matching, and its structural and dynamic performance is assessed against torsional stiffness, buckling, and modal criteria. The full continuous DD design space is subsequently mapped through a parametric finite element sweep to identify the optimal configuration, and extended and locally reinforced DD laminate approaches are examined where the standard architecture proves insufficient. Finally, the invariant Master Ply concept and trace-based scaling are employed to resolve remaining structural deficits through material substitution rather than repeated design space remapping, establishing a fundamentally different and computationally efficient design pathway for composite marine structures.