Full structural simulation of an IMOCA-class racing yacht keel — assessing stress, displacement, and regulatory compliance across self-weight and frontal grounding impact scenarios.
Overview
This study used ANSYS to simulate the structural response of a racing yacht keel — a fin-and-bulb assembly — under two distinct load cases: the sustained gravitational self-weight of the keel and bulb acting on the fin, and a dynamic frontal grounding impact. Both scenarios were assessed against IMOCA Class Rules for 2028, which mandate an overall safety factor of 5.0 and a heightened factor of 6.5 in structural attachment zones such as the root-fin and fin-bulb interfaces.
With a steel yield strength of 800 MPa, this translates to a maximum permissible von Mises stress of 123.07 MPa in the attachment regions. The simulation results were validated against independent analytical hand calculations, and a mesh convergence study was conducted across node counts from approximately 8,000 to 65,000 to confirm result reliability.
Case 1
The first load case applied the combined self-weight of the fin and bulb assembly as a gravitational body force. With the bulb mass of 2,486 kg at the fin tip, the cantilevered fin experiences significant bending stress along its length and a measurable lateral deflection at the tip.
Case 2
The second load case simulated a frontal grounding event — a critical scenario in offshore racing where the keel strikes a submerged obstruction. In this case, the safety philosophy shifts: rather than preventing any deformation, the criterion is to permit controlled plastic deformation that dissipates impact energy, analogous to automotive crumple zones.
With a safety coefficient of 1 relative to the 16% breaking strain, the fin attachment zone is designed to yield plastically before fracture — protecting both the vessel's hull and crew from the sudden transfer of impact force. A brittle fracture at this joint would be catastrophic.
Regulatory compliance — IMOCA 2028
| Region | Safety Factor Required | Permissible Stress | Simulated Stress | Status |
|---|---|---|---|---|
| General structure | 5.0× | 160.0 MPa | 185.6 MPa (Case 1) | ⚠ Review |
| Attachment zones (root/bulb) | 6.5× | 123.07 MPa | 125.99 MPa (fin, Case 2) | ⚠ Marginal |
| Frontal impact — breaking strain | 1.0× (16% strain) | Plastic deformation permitted | Strain = 0.001156 | ✓ Pass |
Mesh Verification
To confirm the reliability of the FEA results, a mesh convergence study was performed by systematically reducing element size and recording the change in key stress outputs. As the node count increases — meaning smaller, more numerous elements — the solution converges towards the true continuum behaviour. Effective convergence was observed at approximately 40,000 nodes (mesh size 0.035 m), beyond which the stress values stabilise to a negligible difference.
Two independent datasets were compared across the same nodal range: the maximum z-direction normal stress under self-weight, and the fin equivalent von Mises stress under frontal impact. Both converge at similar nodal intervals, reinforcing the validity of the analysis as a whole.