```html Software Options for Seastead Structural Draft Design

Software for Drafting & Weighing Your Seastead Structure

For the kind of workflow you're describing — "given these float shapes and this triangle geometry, design the actual truss and spit out a bill of materials with weights" — there isn't one perfect off-the-shelf tool, but there are several good open-source building blocks. Below I've grouped them by what part of the job they do, then suggested a practical pipeline.

1. Parametric Geometry & Frame Modeling

Tool	License	What it does for you
FreeCAD (with the Ship and Arch/BIM workbenches)	Open source (LGPL)	Fully parametric 3D CAD. You can script the triangle frame, floats, and NACA foil legs in Python. The Ship workbench computes hydrostatics (displacement, waterplane area, CB, CF, GM) which is exactly what you need for a SWATH-like design. Material properties + volumes give you weights automatically.
OpenSCAD or CadQuery	Open source	Script-driven parametric modeling. Great for "change one number, regenerate the whole seastead." CadQuery (Python) is more suited to structural parts than OpenSCAD.
Blender + Sverchok	Open source (GPL)	Node-based parametric geometry. Good for visualization, less ideal for engineering BOM.

2. Truss / Frame Structural Analysis (Sizing Members)

Tool	License	Notes
PyNite (Pynite)	Open source (MIT), Python	3D finite element frame analysis in pure Python. You define nodes, members, loads, and material (e.g., 6061-T6 or 5083 marine aluminum). It returns forces, deflections, and reactions. Pair with a member-sizing loop and you can auto-pick tube sizes to meet stress/deflection limits, then sum weights.
OpenSees / OpenSeesPy	Open source (BSD)	Very powerful FEA, originally for earthquake engineering. Overkill for a first pass but handles nonlinear dynamic loads (wave slam, etc.).
anaStruct	Open source, Python	2D frame analysis — useful for quick sanity checks on individual trusses before building the 3D model.
CalculiX / code_aster	Open source	Full 3D FEA solvers. Good if you later want to analyze the foil legs as shells or check stress concentrations at joints.
FreeCAD FEM workbench	Open source	Wraps CalculiX with a GUI inside FreeCAD — nice if you're already modeling there.

3. Naval Architecture & Hydrostatics

Tool	License	Notes
FreeCAD Ship Workbench	Open source	Hydrostatics, displacement, tank capacity, GZ curves. Works on any hull geometry — including your three separate foils.
Delftship Free	Freeware (not FOSS)	Hull modeler with hydrostatics. Less suited to trimaran/SWATH but usable.
Capytaine	Open source, Python	Boundary element hydrodynamics — wave loads, added mass, motion RAOs. Very relevant for SWATH-type platforms where motions in waves are a key design driver.
OpenFOAM	Open source	CFD. Use later to check drag on the foil legs and thruster wash.

4. Bill of Materials & Weight Reporting

None of the above gives you a polished BOM automatically, but all of them expose geometry and material data via Python. A ~200-line Python script can:

Read the parametric model (FreeCAD or CadQuery).
Iterate over each member, get length and cross-section.
Look up the marine-aluminum alloy density (≈ 2700 kg/m³ for 5083/6061).
Compute mass per member, group by type (truss chord, diagonal, skin plate, foil rib, etc.).
Emit an HTML or CSV weight report.

5. Recommended Pipeline

Suggested open-source stack:

CadQuery (Python) — parametric geometry of triangle frame, floats, foil legs, stabilizers.
PyNite — 3D frame FEA; auto-size truss tubes against marine-Al allowables.
FreeCAD Ship workbench — hydrostatics and trim check on the three submerged foils.
Capytaine (later) — wave response / seakeeping.
Custom Python glue script to produce an HTML bill of materials with weights.

6. Commercial Tools (for reference)

Tool	Notes
Rhino 3D + Grasshopper + Karamba3D	Excellent parametric structural design; Karamba does exactly the "auto-size members, get weight" loop. Not free, but the cheapest commercial option for this kind of workflow.
SolidWorks / Inventor + Simulation	Great CAD + FEA but heavy licenses.
Maxsurf / Orca3D / ShipConstructor	Marine-specific; powerful but expensive and more focused on conventional hulls.
RISA-3D / SAP2000 / STAAD	Pro-grade frame analysis.

7. Rough Weight Estimate Shortcut (Before Writing Any Code)

For early trade studies you may not even need software. Marine-aluminum displacement platforms and aluminum catamarans typically come in at:

Deck/frame structure: 25–45 kg/m² of deck area for a well-trussed aluminum deck.
Foil-shaped buoyancy legs (monocoque): 60–120 kg/m² of wetted surface, depending on plate thickness (typically 5–8 mm for 5083).
Superstructure walls + roof (living module): 15–25 kg/m².

For your geometry (80 × 40 ft triangle ≈ 149 m² deck, three 19 ft × 10 ft foils ≈ ~18 m² wetted each) this gives a ballpark structural mass of roughly 7,000–11,000 kg before equipment, solar, batteries, water, stores, and people. Use this as a sanity check against whatever the software produces.

Tip: Because your design is a SWATH-style (small waterplane), the weight budget is dominated by how much reserve buoyancy you want. A 1-inch change in waterline on a 3-ft-chord foil moves only a small volume, so the design is sensitive to weight growth. Parametric software lets you re-solve displacement every time you tweak a truss member — very worthwhile.

8. If You Want a Starter Script

I can write you a Python starter (CadQuery + PyNite) that:

Takes inputs: triangle size, foil chord/length, truss depth, member spacing, alloy.
Builds the 3D geometry.
Applies self-weight + a live load on the deck + a wave-slam load case on the foils.
Iterates tube sizes until stresses are within allowables.
Writes seastead_bom.html with a per-member table and totals.

Just ask and I'll draft it.

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