These tools allow for more detailed insight into the structure in real world sailing load cases. This information can then be used to optimise the structure. Combined with modern methods of manufacturing this can lead to a lower weight, higher stiffness and a stronger structure. Underneath a recent example is described.

Alshark 37

Lightweight Structures B.V. performed a full structural analysis with its FEA capabilities and will take care of the vacuum infusion process.

Stresses on the hull

Stresses on the bulkheads

Water pressure loads

Deformation in the harbour

Better mechanical properties from vacuuum infusion
Lightweight Structures B.V. started developing an alternative structural design methodology for yachts after the implementation of vacuum infusion at Conyplex and Standfast Yachts in 1997. This process results in laminates with a significant higher fibre volume content than the previously used hand lay-up process.

identical laminates!	hand lay-up	vacuum infusion
fibre weight content (%)	60	75
laminate thickness (mm)	6.5	4.8

Vacuum infusion results in weight reduction by resin saving and in a better and more consistent laminate quality. The naval architect involved however concluded that the lower laminate thickness would affect the yacht’s safety and requested to increase the number of glass plies in the hull. A discussion was started, also with Lloyd´s Register in Rotterdam, about which laminate properties are required for getting a safe yacht.

Scantling rules
Lloyd’s scantling rules are based on analytical analysis formulas of flat rectangular plates, prescribed minimum material properties, assumed loads and safety factors. These four items provide a balanced set of semi-empirical rules for designing safe crafts. The rules allow for alternative methods to proof the structural strength.
For example, based on tests, a yard may use higher material stiffness or strength data.
Also, replacing the flat plate analysis by a Finite Element Analysis (FEA) is an option to proof structural strength. This method is generally accepted in other industries. However, FEA normally requires skilled people and is relatively time-consuming. Therefore, not many yacht designs are FEA based.

Lightweight Structures B.V. is convinced that FEA based engineering, when available at lower costs, will lead to better designed and safer yachts, weight and cost reduction and better insight into a yacht’s structural behaviour.

A yacht’s structure
For the primary loads (water and rigging loads) the structure is mainly loaded as a membrane. Stringers/bulkheads are only required locally to introduce point loads from chain plates, mast and keel. The hull shape and deck curvature cause, without additional stringers, significant displacements under these loads.
The super structure do not significantly contribute to the yacht’s stiffness. The super structure behaves as a cut out in the deck and needs to be reinforced.
Slamming loads determine the required core thicknessand core density of a sandwich laminate, and the maximum allowed panel size. For this reason additional bulkheads or stringers may be required.
Deck loads (green water) require additional bending stiffness of the deck or limited panel size.

Based on this analysis a simplified parametric model for a yacht’s internal structure and deck was defined. The internal structure consists of bulkheads with symmetrical cut outs and transversal and longitudinal stringers. The super structure consists of a simplified box-like structure. Laminate properties, calculated from ply properties, are assigned to the structural elements.

Mechanical loads
Lightweight Structures B.V. normally applies the following load cases: sailing under heel with sinus shaped waves (sagging condition, heel angle and wave height to be chosen), pretension of the rigging and several load cases associated with local loads that are not necessary in static equilibrium (green water and slamming loads).

To apply these loads and to define adequate boundary conditions it was decided to include rudder, keel, mast and stays in the structural model. This approach has two advantages. Firstly, since the rigging and the hull act as a statically indeterminate structure, it gives a better prediction of the structural behaviour of the yacht. Secondly it requires only a few input parameters with which all other internal loads are calculated.

The input parameters for a load case are: heeling angle, wave height, rig pretension scheme, back stay force, mass of the yacht and high non-structural masses (e.g. keel, tanks and engine). Based on the yacht’s equilibrium, the water line and trim angle from the heeling angle are calculated. Since the hull shape is available this results in the righting moment and the heeling force. This heeling force acts on the rig and provides the right loading of the chain plates. In this way the amount of input from the designer is minimised.