```html Seastead Structural Design Software Guide

Seastead Structural Design Software Stack

Open Source Solutions for Marine Aluminum Truss Design & Weight Estimation

Executive Summary: For your trimaran seastead design (80'×40' triangle, 3 NACA foil floats), the optimal open-source workflow combines FreeCAD (parametric CAD + FEM), Python scripting for automated truss generation, and CalculiX for structural validation. No single open-source tool fully automates truss design from float positions, but FreeCAD's Python API allows you to build this automation.

Primary Recommendation: FreeCAD Ecosystem

1. FreeCAD (v0.21 or v1.0)

Role: Primary CAD, Parametric Modeling, Weight Calculation

Why it's ideal for your seastead:

Specific Workflow for Your Design:

  1. Use Arch Structure tools to generate triangular truss from 3 points (Front, Left, Right)
  2. Parametrically link float positions to truss nodes using Sketcher constraints
  3. Assign aluminum extrusion profiles (e.g., 4"×4"×1/4" wall square tube for main members)
  4. Use Arch Schedule to auto-calculate total weight by volume × density
  5. Export to CalculiX for wave load simulation (19ft floats, 50% submerged)

Weight Calculation Method:

# FreeCAD Python console example import FreeCAD as App doc = App.newDocument() # Create truss member beam = doc.addObject("Part::Box", "Beam") beam.Length = 2400 # 80ft in mm beam.Width = 100 # 4 inches beam.Height = 100 # 4 inches # Calculate weight volume = beam.Shape.Volume # mm³ density = 2.66e-6 # kg/mm³ (aluminum) weight_kg = volume * density

Specialized Tools for Components

Component Software Purpose Output
NACA Foil Floats FreeShip Plus
or
JavaFoil		 Generate accurate 0012/0015 foil sections, calculate displacement .dxf/.stl → import to FreeCAD
Hydrodynamics Meshmagick
+
PyMesh		 Hydrostatics for 50% submerged 19ft floats Buoyancy center, GM stability
Truss Optimization Top3d (Python) Optimize aluminum member sizing for 80ft span Minimum weight structure
Thruster Placement Blender +
CFD Blender		 Visualize 6 RIM thruster flow interaction Flow simulation

Alternative: OpenSCAD for Parametric Truss Generation

OpenSCAD (Script-based CAD)

If you prefer code-over-clicks, OpenSCAD excels at generating truss structures from parameters:

// Seastead Truss Generator module triangle_truss(front, left, right, height) { // Front-Left leg beam(front, left, height); // Front-Right leg beam(front, right, height); // Left-Right back beam beam(left, right, height); // Cross bracing for(i=[0:0.2:1]) { interpolate_beam(front, left, right, i); } } // Calculate weight automatically total_volume = beam_count * beam_volume; aluminum_weight = total_volume * 0.0975; // lbs/in³

Pros: Version control friendly, exact parametric control
Cons: No FEA integration, manual structural validation needed

Marine Aluminum Specifications for Software Input

When setting up your material databases:

Alloy Density Yield Strength Use Case Corrosion Resistance
5083-H116 2.66 g/cm³ 215 MPa Floats (submerged) Excellent (marine grade)
6061-T6 2.70 g/cm³ 276 MPa Truss structure (above water) Good (anodize recommended)
6082-T6 2.70 g/cm³ 260 MPa Stabilizer wings Good

Automated Weight Estimation Script

Here's a Python approach combining FreeCAD API for your specific geometry:

#!/usr/bin/env python3 # Seastead Weight Calculator # Saves to CSV for design iteration comparison class SeasteadDesigner: def __init__(self): self.aluminum_density = 0.0975 # lb/in³ self.water_density = 0.0361 # lb/in³ def calculate_truss(self, length_ft, width_ft, height_ft): """80ft x 40ft x 7ft triangular truss""" # Convert to inches L, W, H = length_ft*12, width_ft*12, height_ft*12 # Estimate members: 3 perimeter chords + cross bracing perimeter = L + 2*((W/2)**2 + L**2)**0.5 # Approximate cross_members = 20 * W # 20 cross beams at 40ft width total_length = perimeter + cross_members # Assume 4"x4"x1/4" square tube (0.25 lb/in) weight = total_length * 0.25 # Add gusset plates (15% of member weight) weight *= 1.15 return weight # lbs def calculate_floats(self, length_ft, chord_ft, width_ft): """3x NACA foil floats""" # Approximate NACA 0015 volume (0.15 * chord * span * width) volume_per_float = 0.15 * (chord_ft*12) * (length_ft*12) * (width_ft*12) total_volume = 3 * volume_per_float # 5083 aluminum, 3/16" hull thickness estimate surface_area = 2 * (chord_ft * length_ft) + 2 * (width_ft * length_ft) hull_volume = surface_area * 144 * 0.1875 # in³ weight = hull_volume * self.aluminum_density # Buoyancy check (50% submerged) displacement = 0.5 * total_volume * self.water_density return weight, displacement def total_weight(self): truss = self.calculate_truss(80, 40, 7) floats, buoy = self.calculate_floats(19, 10, 3) living = 14*45*7 * 0.5 * self.aluminum_density # Shell estimate total = truss + floats + living + 2000 # Hardware allowance return { 'truss': truss, 'floats': floats, 'living_quarters': living, 'total': total, 'buoyancy': buoy, 'safety_factor': buoy / total }

Recommended Workflow

  1. Phase 1 - Geometry: Use FreeCAD to model the 80ft triangle frame with parametric float positions
  2. Phase 2 - Structure: Apply Arch Workbench truss tools, assign 6061-T6 aluminum to frame, 5083 to floats
  3. Phase 3 - Validation: Export to CalculiX (built into FreeCAD FEM) to verify 7ft height can handle wave loads
  4. Phase 4 - Optimization: Use Python macros to iterate tube sizes (4"x4" vs 6"x6") and minimize weight while keeping stress < 100 MPa
  5. Phase 5 - Documentation: Generate BOM (Bill of Materials) with Arch Schedule showing each extrusion length and weight
Important Limitation: Fully automated truss design (topology optimization) from just float positions requires expensive commercial software like Altair OptiStruct or ANSYS. The open-source approach requires you to define the truss topology (Warren, Pratt, Howe) manually, then optimize member sizes parametrically.

Installation Commands (Ubuntu/Debian)

sudo apt update sudo apt install freecad calculix-ccx python3-pip pip3 install numpy scipy meshio # For custom calculations # Optional: FreeCAD weekly builds for latest Arch workbench features sudo snap install freecad --edge

Weight Budget Estimate (Preliminary)

Based on your specifications, initial estimates using the above methodology:

Component Estimated Weight Notes
Main Truss (80'×40') ~4,200 lbs 4" sq tube, 7ft height, marine aluminum
3 NACA Floats ~2,800 lbs 19ft×10ft×3ft, 3/16" 5083 plate
Living Space Shell ~1,800 lbs 14'×45'×7', insulated panels
Stabilizers (3×) ~240 lbs 10ft span, 6061-T6
Solar/Roof ~600 lbs ~5kW array + mounting
Thrusters/Hardware ~800 lbs 6 RIM drives + cabling
Total Structure ~10,440 lbs Before payload/fresh water
Payload Capacity ~8,000 lbs Based on 50% submersion buoyancy

Note: This assumes efficient truss design. Actual weight may vary ±20% based on connection details and safety factors for open ocean conditions.

Next Steps

  1. Download FreeCAD 1.0 (latest stable)
  2. Work through the "Arch Tutorial" specifically for truss structures
  3. Create a spreadsheet linking float positions (X,Y,Z) to truss member lengths
  4. Use the Python console to automate weight calculation as you adjust the 80ft geometry
  5. Validate with basic hand calculations: Aluminum trusses typically weigh 8-15 lbs/ft² of deck area for marine applications
Pro Tip: For the "small waterline area" stability you mentioned, ensure your FreeCAD model includes the 3 stabilizer wings as hydrofoils. The software can calculate the center of buoyancy shift as the vessel heels, critical for a 40ft wide trimaran with 19ft floats.
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