Free!Ship is a powerful, open-source hull design and naval architecture software that allows anyone to create complex vessel designs for free. Built on subdivision surfaces, it provides an intuitive way to model smooth, hydrodynamically efficient hulls without expensive commercial licenses.
This guide will take you from a blank canvas to a fully realized, hydrostatically sound vessel design. Understanding the Free!Ship Subdivisions Method
Traditional CAD software relies on NURBS surfaces, which require managing complex networks of control points. Free!Ship uses a subdivision surface approach.
The Control Mesh: You manipulate a low-resolution cage (mesh) of points, lines, and faces.
The Smooth Hull: The software automatically interpolates these points to create a perfectly smooth, continuous hull curvature.
Crease Lines: You can assign sharp edges (like hard chines) by tells the software not to smooth specific lines.
This approach ensures that your hull remains fair (smooth and free of bumps) even if you move control points drastically. Step-by-Step: Modeling Your First Hull 1. Initial Setup and Main Dimensions
When you start a new project, Free!Ship prompts you for basic parameters. Input your target length, beam (width), and draft. The software will generate a standard, default hull shape based on these dimensions to give you a starting point. 2. Shaping the Profile and Plan Views
Switch to the profile (side) and plan (top) views to establish the silhouette of your boat. Use the Transform tools to scale or move entire sections.
Align the bow and stern profiles to match your desired aesthetic and functional goals (e.g., a plumb bow for modern racing sailboats or a raked bow for traditional cruisers). 3. Refining the Cross-Sections
Switch to the body plan view to shape the stations (cross-sections).
For a round-bilge hull: Keep the mesh faces connected smoothly without constraints.
For a hard-chine hull: Select the longitudinal edges where the chine should be, right-click, and set them to Crease. This creates sharp transitions typical of plywood or aluminum boats. 4. Adding Detail (Decks and Transoms)
Extrude or add new faces to close off the deck or form a flat transom at the stern. Ensure all points at the centerline (Y = 0) are constrained to the center plane to maintain perfect symmetry. Analyzing Your Design
A beautiful hull is useless if it capsizes or creates massive drag. Free!Ship includes built-in naval architecture tools to analyze performance in real time. Hydrostatics Calculations
As you manipulate control points, Free!Ship updates your hydrostatic data instantly. Pay close attention to:
Displacement: The total weight of water displaced by the hull. This must match your target vessel weight.
Longitudinal Center of Buoyancy (LCB): This dictates how the boat trims. For most displacement hulls, the LCB should sit slightly aft of midships. Block Coefficient ( Cbcap C sub b ): Measures hull fullness. A low Cbcap C sub b (0.4–0.5) indicates a sleek racing hull, while a high Cbcap C sub b (0.7+) indicates a high-capacity cargo vessel or barge. Resistance and Powering
Free!Ship includes basic resistance prediction algorithms (such as the Delft Series for sailboats or Holtrop for motor vessels). These tools estimate the effective horsepower required to achieve your target hull speed, helping you optimize the underwater shape before finalizing production. Exporting for Production
Once your hull is fair and your hydrostatics meet your design goals, you need to export the data for construction or further CAD detailing.
IGES/DXF: Export the 3D surface lines or mesh to commercial CAD software like Rhino, AutoCAD, or SolidWorks for structural modeling.
Offsets Table: Free!Ship can generate a traditional table of offsets. This text file lists the precise X, Y, and Z coordinates of your hull lines, allowing traditional boatbuilders to loft the boat full-scale on a floor.
2D Plates Development: For steel, aluminum, or plywood chine boats, Free!Ship features a Plate Development tool. It flattens 3D curved surfaces into 2D templates that you can send directly to a CNC laser or plasma cutter. Tips for Success
Keep the mesh simple: Use the fewest control points possible. Too many points create unfair bumps that are difficult to smooth out.
Check fairness frequently: Use the built-in visual inspection tools, like zebra striping or curvature plots, to find hidden lumps in your surfaces.
Save iterations: Always save backups before executing major mesh splits or transformations.
If you want to take your design to the next stage, let me know:
What type of boat are you trying to design? (e.g., kayak, sailboat, motor yacht)
What construction material will you use? (e.g., plywood, fiberglass, steel)
Do you need help understanding a specific hydrostatic metric? Saved time Comprehensive Inappropriate Not working
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