Open-Source Wave Simulation Options for Your Seastead Design

Your design is not a normal boat, and that matters a lot for software choice. It behaves more like a small-waterplane-area platform with three submerged buoyant bodies, large changes in immersion, unusual geometry, and likely strong nonlinear heave/roll/pitch behavior. That means many standard marine tools based on linear potential-flow / BEM assumptions will be useful for quick screening, but may miss important behavior once the legs/floats move significantly up and down through the free surface.

Short answer:
If you want the best balance of free, reasonably accurate, visualizable, and suitable for strongly nonlinear wave interaction, then the strongest candidates are:

DualSPHysics — best near-term choice for your use case.
OpenFOAM — potentially higher fidelity and more flexible, but much harder.
Project Chrono FSI/SPH — promising for coupled rigid-body dynamics, but less common in marine design workflows.

For quick early-stage screening:

Capytaine + Python for linear hydrodynamic coefficients and RAO-style studies.
MoorDyn only as an add-on if you later include moorings, tethers, or restraints.

Best Overall Recommendation

Recommended workflow in practice

Stage 1: Fast concept screening
Use Capytaine + Python for rough linear comparisons between hull/leg arrangements. This is not enough for final confidence, but very useful for design iteration speed.
Stage 2: Nonlinear wave/body simulation
Use DualSPHysics for the actual unusual-wave-response studies, immersion changes, accelerations, and visual understanding.
Stage 3: High-fidelity verification on critical cases
Use OpenFOAM on a small number of worst-case scenarios once you know which designs are promising.

That combination gives you:

Fast iteration early
Good nonlinear visualization
A path to higher-confidence CFD later

Key Technical Issue for Your Design

The main reason this is tricky is that your three buoyant legs are:

partly submerged,
slender in one sense but not really “thin” in another,
changing immersion substantially in waves,
attached to a large above-water structure,
likely to show strong nonlinear restoring and damping.

That means:

Linear BEM / radiation-diffraction tools can be misleading when immersion varies a lot.
Fully nonlinear CFD or particle methods are more appropriate if you want high confidence in heave/roll damping and extreme-wave behavior.
Visualization matters because seeing deck motions, slam, overtopping, and leg immersion changes is exactly the kind of thing that helps design intuition.

Comparison of Free Software Options

Software	Method	Good For	Weak For	Accuracy for Your Use	Visualization	Difficulty	Likely Time to First Useful Sim
DualSPHysics	SPH particle CFD, GPU-friendly	Nonlinear free-surface motion, large immersion changes, violent wave interaction, good visuals	Very fine resolution can be expensive; careful setup needed for accuracy	High potential if set up well	Good	Medium	1–3 weeks
OpenFOAM	Grid-based CFD, VOF free surface, RANS/LES options	High-fidelity hydrodynamics, detailed pressures, body motions, validation-grade studies	Much harder meshing/setup; longer run/debug cycles	Very high potential	Good via ParaView	High	2–8 weeks
Project Chrono FSI-SPH	Multibody + SPH/FSI coupling	Coupled rigid-body dynamics, unusual mechanisms, nonlinear body motion	Marine examples/community smaller than OpenFOAM / DualSPHysics	Potentially good	Moderate	High	2–6 weeks
Capytaine	Linear potential flow / BEM	Hydrodynamic coefficients, RAOs, early screening, frequency-domain work	Large immersion changes, nonlinear wave-body effects, slam, strong free-surface nonlinearity	Useful but limited	Limited native visuals	Low–Medium	2–7 days
MoorDyn	Lumped-mass mooring dynamics	Moorings/tethers/cables	Not a wave-body solver by itself	Not enough alone	Low	Low	1–3 days as add-on
WEC-Sim	Time-domain body dynamics using hydrodynamic coefficients, MATLAB/Simulink based	Wave energy converters, rigid body wave response from BEM inputs	Not open source in practical cost sense because MATLAB stack needed; still often linear/weakly nonlinear	Moderate for some cases	Moderate	Medium	1–2 weeks after software purchase
Blender	Animation / approximate physics	Visualization, concept art, post-processing	Engineering-grade wave hydrodynamics	Not suitable	Excellent for visuals only	Low	1–3 days for animation only
NEMOH	Linear BEM	Like Capytaine, legacy/open-source BEM workflows	Same nonlinear limits	Limited for your case	Limited	Medium	3–10 days
REEF3D	CFD/free-surface numerical wave tank	Wave-structure interaction, offshore/coastal cases	Smaller ecosystem; setup can still be technical	Potentially high	Moderate	High	2–6 weeks
Proteus	Multiphase CFD / finite element	Research-grade free-surface problems	Steeper learning curve; less marine-user convenience	Potentially strong but niche	Moderate	High	3–8 weeks

Detailed Comments on Each Option

1) DualSPHysics

Probably your best first serious tool.

Why it fits your case:

Handles nonlinear free-surface motion.
Handles large body motions and changing immersion better than linear BEM.
Works well with GPU acceleration, which you specifically have.
Can produce simulations that are visually understandable.
Has been used in wave-structure interaction work and violent hydrodynamics.

Main caution:

SPH can be very good, but you must be careful about:
- particle resolution,
- boundary treatment,
- wave generation quality,
- numerical damping,
- validation against simpler known cases.

For your specific seastead, it is especially good for:

seeing whether those 19 ft foil-shaped legs actually damp heave/roll,
studying wave height progression until things become unacceptable,
extracting rigid-body motion and accelerations at chosen points,
watching deck edge and living-space motions in a way humans can interpret.

Verdict: If you only choose one free tool first, choose DualSPHysics.

2) OpenFOAM

Best “serious CFD” option, but harder.

Why it fits:

Can model free-surface waves with VOF methods.
Can handle rigid body motion and detailed pressure loads.
Good for high-fidelity studies of extreme cases.
Strong open-source ecosystem and many academic/offshore users.

Why it may not be your first choice:

Meshing a geometry like yours is not trivial.
Body motion + waves + stability + moving boundaries becomes a fairly advanced setup.
Getting from “installed” to “trustworthy result” takes more time than with DualSPHysics.

Verdict: Excellent second-stage verification tool. Harder than DualSPHysics, but likely the strongest open-source high-fidelity path.

3) Project Chrono::FSI-SPH

Your understanding is roughly right: if you use Chrono FSI/SPH, you are not relying on the same kind of linear BEM assumptions as in potential-flow solvers. So yes, that avoids the exact concern you raised about large immersion changes.

Pros:

Strong for rigid-body dynamics and multibody systems.
Potentially nice if later you add articulated stabilizers, control systems, or docking/dinghy interactions.
Useful if structural and hydrodynamic interaction both matter.

Cons:

For your current need—wave-body response of an offshore platform-like object—there are generally more direct examples and community knowledge in DualSPHysics and OpenFOAM.
May take more experimentation to get a stable, believable marine setup.

Verdict: Worth considering, but I would put it below DualSPHysics and OpenFOAM for your first implementation.

4) Capytaine + Python

Capytaine is very useful, but only if you use it for the right question.

It is good for:

hydrodynamic coefficients,
added mass and damping,
frequency-response understanding,
comparing variants quickly.

It is not good for:

strongly nonlinear immersion changes,
large-amplitude motions,
wave slam or overtopping,
free-surface breakup,
final confidence on your strange geometry.

Verdict: Very good as a preliminary screening tool, not as your main final simulator.

5) MoorDyn

MoorDyn is not a replacement for wave-body hydrodynamics. It is mainly for moorings, lines, and cable dynamics. If your seastead is anchored or restrained later, it can become important. But by itself it will not answer your main question.

Verdict: Add later if needed. Not a primary solution.

6) WEC-Sim

Technically useful in some marine dynamic contexts, but you are right: it is not a practical open-source path if you need MATLAB, Simulink, and likely Simscape Multibody.

For a non-student, a rough estimate is:

MATLAB base license: commonly around USD $900–$2,350+ depending on license type and region
Simulink: often USD $300–$1,000+
Simscape / Simscape Multibody: often USD $500–$1,500+ each or more

Actual pricing in Anguilla may require a quote from MathWorks or a reseller, but realistically you should expect a multi-thousand-USD total if buying commercially.

Verdict: Not recommended for your “free/open-source” requirement.

7) Blender

Still true: Blender is not an engineering hydrodynamics tool. It is excellent for:

rendering,
animation,
post-processing imported simulation data.

It is not good for:

accurate wave-body interaction prediction,
marine stability analysis,
pressure/force validation.

Verdict: Use Blender only after the simulation, for nicer videos if desired.

8) Other free options worth mentioning

REEF3D

A respectable open-source CFD/wave-structure code. Not as mainstream as OpenFOAM, but potentially useful. If you find examples closer to offshore structures than in your other tools, it may be worth a look.

Proteus

Research-grade open-source multiphase CFD. Powerful, but probably not the fastest path to productivity for your project.

Kratos Multiphysics

This is another open-source multiphysics framework worth knowing about. It can do FSI and free-surface problems, but for your immediate goal it is probably not the quickest route compared with DualSPHysics or OpenFOAM.

Recommended Ranking for Your Exact Needs

Rank	Software	Why
1	DualSPHysics	Best mix of nonlinear wave handling, GPU use, free-surface visualization, and practical fit for unusual geometry.
2	OpenFOAM	Best high-fidelity open-source CFD path, but significantly more setup effort.
3	Capytaine + Python	Best fast screening tool, but not enough alone for confidence in this design.
4	Project Chrono FSI-SPH	Interesting and capable, but likely less direct than DualSPHysics/OpenFOAM for your immediate wave-stability questions.
5	REEF3D / Kratos / Proteus	Potentially strong, but not the easiest first choice unless you find excellent example cases.
6	MoorDyn	Important later if moorings matter.
7	Blender	For rendering only, not engineering simulation.

How Long to First Working Simulation?

Assuming:

you have a good GPU,
Claude Code / Cursor help with scripting and setup,
you are okay with Linux or Windows,
the first goal is a credible rigid-body wave-response sim, not publication-grade validation.

Software	Install / Setup	Geometry Prep	Wave Tank / Case Setup	Postprocessing	Total to First Useful Result
DualSPHysics	1–3 days	2–5 days	3–7 days	1–3 days	1–3 weeks
OpenFOAM	1–4 days	3–10 days	7–21 days	2–5 days	2–8 weeks
Capytaine	1 day	1–3 days	1–3 days	1–2 days	2–7 days
Chrono FSI-SPH	2–5 days	2–5 days	5–15 days	2–5 days	2–6 weeks

These ranges assume you are not building everything from source in a painful way and that AI coding help is effective.

How Hard Is It to Simulate a Different Seastead Model Later?

After the first model is working, future models get much easier if you build the workflow correctly.

If you do it well

The effort for a new model can drop to:

Capytaine: a few hours to 2 days
DualSPHysics: 1–5 days
OpenFOAM: 2–10 days

What makes future models easy

Parametric CAD or scripted geometry export
A standard wave tank template
A standard mass-property definition workflow
Reusable post-processing scripts for:
- heave, roll, pitch,
- acceleration at deck corners,
- leg immersion percentage,
- relative wave elevation near key points

Best practical approach

Build a pipeline like this:

Geometry export from CAD/STL/OBJ
Auto-calculate mass/inertia from a spreadsheet or Python script
Auto-generate simulation config from a template
Auto-run a standard set of wave cases
Auto-generate plots and video snapshots

With Claude Code or Cursor helping, this is very realistic.

Can These Tools Make Videos Like the Example?

Yes. The best options are:

DualSPHysics + ParaView for engineering visualization
OpenFOAM + ParaView for engineering visualization
Blender only for making nicer-looking cinematic renders from simulation output

If your goal is “understand how things go wrong,” then ParaView is usually enough and is much more directly tied to the actual simulation data.

Accuracy Notes

You said you want a high level of engineering accuracy. That is possible, but accuracy depends on more than software choice. You will also need:

correct mass and center-of-gravity assumptions,
correct inertia tensor,
realistic wave spectra and amplitudes,
sufficient domain size,
enough spatial resolution,
time step checks,
convergence testing,
comparison against at least one simpler benchmark case.

Even a powerful CFD/SPH solver can give wrong answers if the setup is poor. So I would strongly suggest:

First simulate a simpler known object in waves.
Then a simplified version of your seastead.
Then the full geometry.

Best Practical Plan for You

Plan A: Most practical

Capytaine + Python for fast early design comparison
DualSPHysics for nonlinear wave-motion studies and videos
OpenFOAM only for a few critical validation cases

Plan B: If you want only one tool

DualSPHysics

Plan C: If you want maximum technical depth and are willing to suffer more setup

OpenFOAM

Direct Answers to Your Specific Questions

Question	Answer
Is Chrono BEM accurate enough for this unusual model?	Probably not as your main final tool if immersion changes are large. Linear BEM is likely too limited for your key questions.
If we use Chrono::FSI-SPH, is Chrono not using BEM?	Correct. Chrono FSI/SPH is a different approach and avoids the linear BEM limitation you are worried about.
Can Blender do reasonably accurate simulation of this?	No, not for engineering hydrodynamics. Use it only for visualization after the real simulation.
Could WEC-Sim and MoorDyn handle this?	Partly, but not ideal for your nonlinear unusual geometry, and not free in practice due to MATLAB ecosystem cost.
Does DualSPHysics seem good for this?	Yes. It is one of the best free options for your needs.
Is OpenFOAM good?	Yes, very good, but harder and slower to get right.
Any better free software?	Not clearly better overall than DualSPHysics/OpenFOAM for your described goals. REEF3D and Kratos are worth a look but are not obviously easier or better.

Final Recommendation

My recommendation:

Start with DualSPHysics.
Use ParaView for visualization and videos.
Optionally add Capytaine + Python for quick early-stage comparisons.
Use OpenFOAM later if you need higher-confidence verification on selected cases.

That is the best match to your priorities:

free/open source,
good GPU use,
nonlinear wave interaction,
visual understanding,
reasonable path to engineering-quality results.

If You Want, Next I Can Help With

a step-by-step toolchain recommendation for Windows vs Linux,
a first DualSPHysics project layout for your seastead,
a parameterized geometry approach so future designs are easy to swap in,
a list of outputs to record like heave/roll/pitch/accelerations at deck corners,
or a proposed validation ladder so you know when to trust the results.