What Category A actually asks of the structure
Category A does not prescribe a single FreeCAD load case. It requires the craft to remain structurally sound in severe ocean conditions. In practice that means:
- Global hull-girder bending (hogging and sagging) and, for a catamaran, torsion and splitting of the two hulls.
- Local bottom and side plating under hydrostatic + wave + slamming pressure (ISO 12215-5 design pressures).
- Cross-deck / wing-deck / beam structure that keeps the hulls parallel under racking and pitch-connecting loads.
- Deck, cabin-top, and mast/engine foundation loads if applicable.
- Plate and stiffener buckling (aluminum is modulus-limited).
ISO 12215 remains the reference for design pressures and allowable stresses. FEM is useful to check 3-D load paths that formula scantlings miss (beam-to-hull joints, cut-outs, bridgedeck slamming).
Material to assign in FreeCAD
Typical hull alloys: 5083-H116/H321 or 6082-T6. Enter isotropic linear-elastic data, then compare von Mises (or principal) stress to welded allowable, not parent-plate yield.
| Property | 5083-H116 (typical) | Notes |
|---|---|---|
| Young’s modulus E | 70–71 GPa | Use 70 000 MPa in CalculiX |
| Poisson’s ratio | 0.33 | |
| Density | 2660 kg/m³ | Needed if you apply gravity / inertia relief |
| Parent Rp0.2 | ~215–270 MPa | Do not use this as allowable in welds |
| Welded / HAZ allowable (ISO 12215 style) | often ~0.5–0.6 × parent, check the table for your alloy and weld type | Fatigue and buckling govern many panels |
Create a FreeCAD material with those elastic constants. If you later run non-linear analysis, add a bilinear plastic curve with HAZ-reduced yield — only after linear screening looks acceptable.
Recommended FEM tests (in order)
1. Local panel pressure (ISO 12215-5 equivalent)
Purpose: Check plating and stiffeners under design bottom / side / deck pressure, including Category A slamming on the hull bottoms and bridgedeck.
Geometry: One representative bottom panel with attached stiffeners and surrounding structure (or a full half-hull if the mesh allows). Include welds as geometry or as reduced-thickness/HAZ strips if you model them.
Constraints: Fix or simply-support the panel edges the way they are supported in the real boat (girder, bulkhead). Do not over-constrain the middle of the plate.
Loads: Uniform or linearly varying pressure on the wetted face. Magnitudes should come from ISO 12215-5 for Category A (displacement, speed, deadrise, longitudinal position). Bridgedeck underside often sees high slam pressure — run that as a separate case.
Read: von Mises vs welded allowable; plate deflection vs span/200–span/100 as a serviceability check; first buckling eigenvalue if you add a buckling step.
2. Catamaran torsion / racking (critical for cats)
Purpose: The two hulls try to pitch and roll independently. Crossbeams and wing deck must take torsion and differential shear.
Geometry: Full catamaran or a long midship section including both hulls and all cross structure. Omit non-structural furniture.
Constraints: Prefer inertia relief in CalculiX if you can (unconstrained floating body). Otherwise a 3-2-1 support far from the highly stressed joints (e.g. three points on one hull keel with care not to create artificial hot spots).
Loads: Equal-and-opposite vertical forces or moments on the two hulls (one hull “up”, one “down”) sized from a design wave (order of displacement × g × a lever from ISO / class torsion formulae, or a 4 m+ equivalent wave). Add a splitting (athwartships tension) case on the crossbeams.
Read: Stress at beam–hull connections, bulkhead toes, and wing-deck plating; relative vertical deflection between bows; buckling of wide cross-deck panels.
3. Global hogging and sagging
Purpose: Hull-girder bending in a head sea.
Constraints: Inertia relief, or simple supports at stations that do not fight the bending (not both ends fully fixed).
Loads: Gravity + buoyancy pressure that is deliberately unbalanced (hog: extra buoyancy amidships; sag: extra buoyancy in the ends), or equivalent nodal forces from a still-water + wave bending moment taken from ISO 12215-5 / 12215-7 or a class rule. Include self-weight of structure and a realistic equipment mass.
Read: Deck in compression (hog) and bottom in compression (sag); stiffener tripping; overall deflection.
4. Bridgedeck slamming
Purpose: Category A cats slam the wet deck in steep seas. This often governs plate thickness and beam size.
Loads: High short-duration pressure on the underside of the wing deck (use ISO slamming pressure, not calm-water hydrostatic). Combine with case 2 if the slam is asymmetric.
Read: Plate yield, beam bending, and whether the slam punches through into the cabin sole.
5. Local foundations (engines, saildrive, mast, chainplates, keels)
Point or distributed loads from ISO / supplier data (engine torque, mast compression and stay tension, grounding on a keel). Clamp only the far structure. Check insert plates and weld ends.
6. Linear buckling of plates and wide flanges
After a static step, add a CalculiX buckling analysis on the same mesh for compressed decks, wing-deck, and slender hull sides. Eigenvalue < 1 under factored design load means the panel needs thickness, stiffener pitch, or curvature changes. Aluminum’s low E makes this more important than on steel.
Generic FreeCAD FEM setup
- Geometry: Solid or shell. For a whole cat, mid-surface shells (Part or Surface WB → thickness later in FEM) keep the mesh tractable. Keep fillets at beam-hull joints if those are hot spots.
- Analysis container: New Analysis → CalculiX. Solver type: static (and a second buckling analysis).
- Material: Mechanical material with E, ν, density as above. Assign to all hull solids/faces. Optionally a second material for HAZ strips.
- Element geometry: For shells, set thickness per region (bottom, sides, deck, beams). For solids, no thickness card.
- Mesh: Netgen or GMSH. Aim for ≥ 4–6 elements across a plate span between stiffeners; refine at weld toes and beam roots. Second-order tets/shells if the model still solves.
- Constraints: Displacement constraints or inertia relief. Never fully encastre a floating hull unless you are only testing a local panel.
- Loads: Face pressure (hydrostatic, slam), self-weight (gravity 9.81 m/s²), concentrated forces/moments at foundations. Units: N, mm, MPa or N, m, Pa — pick one system and stay in it.
- Job: Run CalculiX. In post, plot von Mises, max principal, displacement, and (buckling) mode shapes. Hide rigid-body modes if present.
- Acceptance (screening): Peak stress in parent plate below ISO allowable for that location; weld/HAZ below the reduced allowable; buckling factor ≥ 1.0 (preferably ≥ 1.5–2 with load factors); deflections that do not flood or jam doors. Re-mesh hot spots before you trust a peak.
How to turn ISO pressures into FreeCAD loads
Compute design pressures with ISO 12215-5 using Category A, your displacement, LWL, speed, deadrise, and area reduction factors. Apply those kPa values as Pressure loads on the corresponding faces (bottom, side, deck, wet deck). Do not guess a single “4 m wave” pressure for local plating — the ISO formulas already fold wave height, speed, and slamming into design pressure.
For global cases, convert rule bending moments / torsion moments into equivalent pressure patches or force pairs that produce the same moment at midship. Document the conversion; a notified body will ask.
What FreeCAD FEM will not do for you
- It does not automatically satisfy CE documentation. Keep a traceable load-case report.
- Linear static ignores residual weld stress, fatigue (Category A is long-range), and crack-like details.
- A coarse mesh under-predicts weld-toe stress. Refine or use hot-spot methods if you claim fatigue life.
- Hydrodynamics (true slamming CFD, green water) are outside CalculiX; you only apply equivalent static pressures.
Suggested run sequence on the current design
- Material + mesh quality check on a single bottom panel (fast).
- ISO bottom and bridgedeck pressure on that panel + stiffeners.
- Full-ship torsion + splitting.
- Hog / sag.
- Buckling on compressed decks and wing deck.
- Iterate thickness / stiffener spacing where utilization > 1 or buckling factor < 1, then re-run.
If utilization is already > 1 on ISO pressures with welded allowables, the design is not Category A ready regardless of mesh polish.