Composite Fin-to-Keel Bulb Structural Laminate Design

A performance sailing yacht's carbon composite fin connects to a lead keel bulb through a small elliptical bolt pattern that must survive one of the harshest load cases a racing hull can experience: a hard grounding impact. Six titanium bolts carry the full deceleration load of the bulb into a laminated carbon structure, with almost no room for a heavy-handed, over-engineered solution — every gram in the connection zone works against the boat's performance.

The design challenge was as much philosophical as it was numerical: rather than treating the connection as a single worst-case calculation to be over-built and forgotten, each element of the load path — bolt sizing, bearing stress, insert pull-out, and the laminate itself — was validated individually against its own governing failure mode, so that the final architecture carried margin where it mattered and no more material than necessary everywhere else.

The guiding design philosophy was to size every element of the connection against the specific failure mode it was actually exposed to, rather than blanket over-design. A worst-case grounding scenario was converted into a bulb deceleration load, distributed across the six-bolt pattern, and carried through separate, purpose-specific checks: bolt tensile capacity, bearing stress in the composite sole, and pull-out resistance of the threaded inserts anchored in the lead bulb itself. Each check used a failure criterion appropriate to its own material and geometry — titanium tensile strength for the bolts, interlaminar shear-based bearing allowables for the composite, and lead alloy shear strength for the insert pull-out — so that the margin built into the design was deliberate and traceable back to a specific physical mechanism, not a single blanket safety factor applied blindly everywhere.

That philosophy carried directly into the laminate itself, where the central engineering problem was structural: transitioning from a thick, 48-ply solid sole under the bolted connection down to the fin's intrinsic 23-ply sidewall laminate, without creating a sharp discontinuity that would concentrate stress at the transition. The solution was an internal staircase ply-drop pattern, terminating groups of core plies at progressively trimmed distances from the sole perimeter rather than dropping the entire stack at once. This distributed the geometric discontinuity across multiple staggered termination lines instead of concentrating it at a single knife-edge, spreading the associated stress concentration over a wider zone of the structure.

The ply architecture itself was split by function: a 36-ply core block carrying the bulk of the bearing and bending load, bracketed by 12 shear web plies extending outward roughly 480 mm to carry the taper transition into the sidewall. The staircase pattern inevitably created small internal voids at each ply termination step, which were addressed with a structural epoxy filler paste loaded with milled carbon fibre, manually applied before laying up the shear web plies — ensuring the taper zone remained a continuous load path rather than a laminate with trapped resin-rich pockets.

Each of the six bolted connections was further reinforced with a laminated carbon fibre sleeve integrated directly into the fin structure around the bolt hole, distributing local bearing stress into the surrounding laminate rather than relying on the resin and fibre immediately adjacent to the hole alone. The complete connection specification — titanium bolts, stainless inserts, PTFE isolation sleeving, and structural sealant — was defined to prevent galvanic interaction between the titanium fasteners, the lead bulb, and the carbon structure, a consideration as central to the connection's long-term integrity as the load calculations themselves.