Introduction

In our previous article, we explored how foiling systems must be designed as integrated platforms. The next challenge is ensuring that these systems perform as expected in reality.

In foiling vessel design, simulation has reached an unprecedented level of sophistication.

Today, CFD tools can capture complex flow phenomena, multi-degree-of-freedom dynamics, and interactions between hull, foils, and propulsion systems. Entire concepts can be developed, tested, and optimized in a fully digital environment.

Yet, one fundamental question remains:

How do we ensure that what works in simulation will work in reality?

Bridging this gap—from numerical prediction to real-world performance—is where many foiling concepts succeed… or fail.

The Limits of Simulation

Numerical tools are essential, but they are not infallible.

Foiling systems operate in highly sensitive regimes, where small variations can have significant consequences:

• Ventilation and cavitation onset
• Transient instabilities (waves and manuevers)
• Coupling between hydrodynamics and control response
• Fluid and structure interaction

Even advanced simulations depend on assumptions:

• Boundary conditions
• Turbulence models
• Numerical resolution
• Simplifications of real operating environments

As a result, simulation should not be seen as a final answer, but as a powerful guide within a broader validation process.

Why Validation Is Critical in Foiling

Foiling platforms operate in a very dynamic equilibrium.

Their performance depends on:

• Precise lift generation
• Stable control of ride height and trim
• Continuous adaptation to changing conditions

This makes them inherently sensitive to:

• Sea state
• Speed variations
• Load distribution
• Control system behavior

Without proper validation, even well-designed concepts may encounter:

• Reduced efficiency
• Unstable behavior
• Limited operational envelope

Validation is therefore not a final step. It is an integral part of the design process.

A Structured Validation Approach

Ensuring that a foiling concept performs as expected requires a multi-stage approach.

1. Numerical Development

The process begins with CFD-driven design:

• Exploration of foil geometries and configurations
• Resistance and lift prediction
• Identification of critical regimes (e.g. ventilation risk)
• Multi-DOF simulations to capture dynamic behavior

At this stage, the objective is not only to optimize performance, but also to understand sensitivities and potential failure modes.

2. Prototype Development

Once a concept reaches sufficient maturity, physical validation becomes necessary.

Prototypes can take different forms:

• Scaled models
• Partial systems
• Full demonstrators (depending on project scope)

The goal is to:

• Observe real flow behavior
• Validate assumptions made during simulation
• Identify discrepancies early

This step often reveals phenomena that are difficult to fully capture numerically.

3. Experimental Testing

Testing environments such as towing tanks or controlled trials provide a critical intermediate step between simulation and open-water operation.

They allow for:

• Controlled variation of parameters
• Repeatable measurements
• Detailed observation of hydrodynamic behavior

Key aspects typically assessed include:

• Lift and drag characteristics
• Stability and trim response
• Onset of ventilation or cavitation

These tests help refine both the design and the simulation models.

4. Control System Integration

One of the defining elements of foiling vessels is the interaction between hydrodynamics and control systems.

Flight Control Systems (FCS) are responsible for:

• Maintaining stable ride height
• Adapting to external disturbances
• Ensuring safe and efficient operation

Validation must therefore include:

• Coupling between control logic and hydrodynamic response
• Transient behavior under changing conditions
• Robustness across the operating envelope

Ignoring this interaction can lead to significant performance gaps between theory and reality.

5. Sea Trials and Real-World Feedback

Ultimately, the true validation of a foiling concept happens on the water.

Sea trials introduce:

• Irregular wave conditions
• Variable loading scenarios
• Real operational constraints

This is where the full system is tested:

• Hydrodynamics
• Structure
• Propulsion
• Control systems

And, most importantly, their interaction.

Recent trials of autonomous hydrofoil platforms have shown how critical this phase is. Moving from simulation to real-world operation reveals behaviours that cannot be fully anticipated numerically, particularly in terms of control response, stability margins and interaction with unpredictable sea states.

As illustrated in recent autonomous hydrofoil trials in Valencia , real-world testing provides essential insights that refine both the design and the simulation models.

Data collected during sea trials is essential to:

• Confirm performance predictions
• Refine models
• Improve future designs

Closing the Loop: From Testing Back to Design

Validation is not a linear process. It is an iterative loop:

Simulation → Prototype → Testing → Feedback → Improved Simulation

Each stage informs the next.

This continuous feedback loop is what enables:

• Increased accuracy in predictions
• More robust designs
• Reduced development risk

Over time, it builds a deeper understanding of complex phenomena such as ventilation, stability limits, and system interactions.

From Concept to Reliable Performance

For shipyards and developers, the ultimate goal is not theoretical performance. It is reliable, repeatable performance in real operating conditions.

Achieving this requires:

• A rigorous validation methodology
• Integration across disciplines
• Continuous iteration between digital and physical environments

Foiling is a powerful technology, but only when it is validated as a system.

Conclusion

Simulation has transformed the way foiling vessels are designed. But it is validation that transforms concepts into real, working platforms.

From CFD models to sea trials, the path from idea to operation is defined by one key principle:

Performance must be proven, not assumed.